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 LIBRARY 
 
 New York State Veterinary College 
 
 ITHACA, NEW YORK 14850 
 
 SF 
 626 
 
 27 
 
 Hadwen , S 
 
 Warble flies, Hypoderma Lineatiam, Villers, 
 and Hypoderma Bovis, DeGeer, Ottawa, 1919- 
 
3r 
 
 ^7 
 
 LIBRARY 
 
 NEW YORK STATE 
 COLLEGE OF VETERINARY MEDICINE 
 
 ITHACA, N.Y. 
 
23 
 
 The Principal Differences Between the two, Flies. 
 
 H. bovis. 
 
 1. Length 14 mmr Distance between eyes 
 
 just in front of ocelli 1-9 mm., greatest 
 diameter of eye 1-9 mm. 
 
 2. Yellow hair on anterior part of thorax. 
 
 3. Wing veins dark brown. 
 
 4., Alulae have a reddish brown border. 
 
 5. Under part of abdomen and thorax nearly 
 : black; 
 
 6. Legs clean with few hairs. 
 
 7. Colouring of tail end, orange yeUow. 
 
 H. lineatum. 
 
 Length 12-7 mm. Distance between eyes 
 just in front of oceUi 1-9 mm., greatest 
 diameter of eye 1-6 mm. 
 
 Anterior part of thorax black and shining. 
 
 Wing veins nearly black. 
 
 Alulae are uniformly white. 
 
 Under part of abdomen and thorax hghter. 
 
 Legs rough and hairy. 
 
 Colouring of tail end, reddish orange. 
 
 Methods of Oviposition. 
 
 1 
 
 Lays in sunshine, principally from 10 a.m. 
 to 4 p.m., but has been seen both earlier 
 and later. 
 
 2. Causes cattle to "gad," both old and 
 
 young. 
 
 3. Lays its eggs principally when the animals 
 
 are running. 
 
 4. The eggs are laid mostly on the outside of 
 
 the hind quarters and on the legs above 
 the fetlocks. 
 
 5. The fly is a clumsy insect and strikes at the 
 
 animals blunderingly. 
 
 6. The eggs are laid singly at the roots of the 
 
 hairs and are rarely visible without lifting 
 the hairs. 
 
 Lays principally in the shade of the animals, 
 from about 10 a.m. to 4 p.m. 
 
 Causes annoyance especially to young 
 
 animals. 
 Lays its eggs mostly when the animals are 
 
 recumbent. 
 The eggs are laid on aU parts which the fly 
 
 can reach when resting on the ground, 
 
 i.e., while the animals are lying down. 
 
 Even when the animals are standing, the 
 
 fly is able to lay eggs on those hairs which 
 
 are close to the ground, namely, on the 
 
 heels." 
 The fly is much more gentle and dehberate 
 
 in its movements. 
 Several eggs are attached to a single hair and 
 
 are often visible without lifting the 
 
 hair. 
 
 Skin Lesions. 
 
 Hypodermal rash. An irregular scattering 
 of round raised lumps, especially on the 
 outside of the hind quarters, appearing 
 suddenly, anaphylactic in nature and 
 rarely tending to suppurate. Very Uttle 
 exudation of serum. 
 
 Hypodermal rash. Diffuse irregular swell- 
 ings, very irritable, appearing suddenly, 
 anaphylactic in nature, often large and 
 ending in the production of pus, and a 
 marked dermatitis. The lesions follow 
 the parts of the animal which come in 
 contact with the ground, i.e., legs, 
 flanks and sternum. A large quantity 
 of exudate mats the hairs. 
 
 Larva. 
 
 The segment in front of the spiracular seg- 
 ment imarmed. 
 
 The segment in front of the spiracular seg- 
 ment armed. 
 
 Summary. 
 
 Hypoderma lineatum lays its eggs as early as April 15, but the usual 
 laying period is during the month of May. At Agassiz, they have never been 
 captured later than May 30. H. bovis begins to lay in the early part of June 
 and continues up to the beginning of August. Between the last appearance of 
 H. lineatum and the first of H. bovis there is usually a period of ten days when 
 the cattle are immune from attack of either species. H. bovis frightens the 
 cattle much more than H. lineatum. The eggs take about a week to hatch; 
 the larvse bore through the skin in the coarser, porous parts, taking several 
 hours in the process; at this stage they are about 1 mm. long. The lesions 
 
24 
 
 resulting from this penetration are caused partly by anaphylactic reactions and 
 partly by bacterial invasion, those produced by H. lineatum being the more 
 severe. 
 
 For the skin lesions the name of hypodermal rash has been proposed. 
 
 At this point there is a hiatus in the Ufe history. It is not positively known 
 how the larvae reach the oesophagus, where they are subsequently found; most 
 likely they travel in the loose connective tissues under the skin up to the region 
 of the throat and into the oesophagus where the muscles bifurcate. Passing 
 down the oesophagus they follow the submucosa and are almost always found 
 lying along the long axis of the canal. Whilst in the oesophagus, small oedematoup 
 swellings are found surrounding the grubs; these are sterile and are anaphy- 
 lactic in character; the exudate contains large numbers of eosinophilic leucocytes. 
 
 The earliest record made at Agassiz of larvse in the oesophagus was on 
 August 15, when a larva 3 '4 mm. was found and several slightly larger. 
 Continental observers have recorded smaller larvae than this. 
 
 H. lineatum makes its appearance in the backs of cattle about December 
 15, and H. bovis about a month later. The larvae at this time have grown to 
 about 1"5 cm., and are similar in size to, those which are found in the neural 
 canal and under the skin. At this stage it is difficult to separate the larvae 
 of the two species, but Bishopp has, recently, discovered good distinguishing 
 marks between them.' The life histories overlap at this period making it difficult 
 to follow the migration, but in the latter part of the season (the middle of March) 
 the last larvae to leave the oesophagus are at the paunch end. They pass out under 
 the pleura and go to the neural canal, either up the crura of the diap'hragm, or 
 up the posterior border of the ribs entering the canal by the posterior foramen. 
 The larva evidently makes use of the canal as an easy means of access to the 
 lumbar region, the part of the animal which is best suited for passing its last 
 ^tages within the host. 
 
 The larvae follow connective tissue exclusively and no larvae have been 
 discovered in muscular tissue. 
 
 The mature larvae leave the animals' backs from the early part of the year 
 up to the first days of July. 
 
 The periods for the two species have not been fully worked out; but, 
 judging from what records there are of the pupal period and the time of year 
 the flies are on the wing, H. lineatum begins to emerge in February and finishes 
 about May 1. H. bovis begins about May 1 and ends approximately on 
 July 1. The average pupal period for H. bovis is 32 ' 5 days, and for H. lineatum 
 a little less. The duration of the life of the flies is short, seeing that they cannot 
 feed. This hfe history applies to Agassiz, British Columbia; doubtless in other 
 countries variations wiU be noticed, but the period spent by the larvae within 
 the host must be of the same duration, seeing that animals' temperatures are 
 the same the world over. 
 
 Prevention. — Animals should be housed during the heat of the day to prevent 
 the flies laying upon them. Warble grubs should be squeezed out as early as 
 possible during the year. This method will lessen the damage to animals and 
 their hides. If the total eradication of the pest is attempted, co-operative 
 measures must be undertaken. 
 
 Acknowledgments. 
 
 I am indebted to Miss E. B. Cramp for assistance in compiling this 
 paper, and to Dr. E. M. Walker for the loan of three of the figures, which were 
 published in the Canadian Entomologist. 
 
Cornell University 
 Library 
 
 The original of tiiis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924000297642 
 
Plate 1. 
 
 \ 
 
 ^IK/^ 
 
 Fig. 1. — Hypoderma lineatum, Villers. 
 
 *{/f. 
 
 Fig. 2. — Natural size. 
 
 Fig. .3. 
 
DEPARTMENT OF AGRICULTURE-CANADA 
 
 HEALTH OF ANIMALS BRANCH 
 
 SCIENTIFIC SERIES, No. 27 
 
 WARBLE FLIES 
 
 HYPODERMA LINEATUM, Villers 
 
 and 
 HYPODERMA BOVIS, De Geer 
 
 By 
 
 S, HADWEN, D.V.Sc, F.E.S. 
 
 Chief Animal Pathologist 
 OTTAWA, ONTARIO 
 
 1 
 \ 
 
 Published by the authority of the Minister of Agriculture 
 
 JULY, 1919 
 
 OTTAWA 
 
 J. DB LABROQUERBE TACH^, 
 
 PHINTER TO THE KING'S MOST EXCELLENT MAJESTY 
 
 1919 
 
 69548—1 
 
To the Honourable, 
 
 The Minister of Agriculture. 
 
 Sir, — I have the honour to submit to you a report on Warble FHes, by 
 S. Hadwen, D.V.Sc, F.E.S., Chief Animal Pathologist, Biological Laboratory, 
 Ottawa, and request that same be printed as Btilletin No. 27. 
 
 I have the honour to be, sir. 
 
 Your obedient servant, 
 
 F. TORRANCE, 
 Veterinary Director-General. 
 Ottawa, September 8, 1919. 
 
* INTRODUCTION. 
 
 The investigations here described were started in 1911, at Agassiz, British 
 Columbia; since that time a number of papers on Warble flies have been 
 published by the author. 
 
 As these papers are now out of print, at the suggestion of Doctor Torrance, 
 Veterinary Director General, the subject matter has been summarized and 
 enlarged to form the present bulletin, which includes many of the original 
 illustrations: 
 
 The object of the experiments was to discover expedients to reduce the 
 number of Warble flies and, consequently, the damage they occasion. In this 
 connection, it is interesting to note that, owing to . representations made by 
 . the War Ofiice to the Board of Agriculture and Fisheries, an attempt is now 
 being made in England to eradicate Warble flies in certain districts. The 
 advisory committee chosen consists of Sir Stewart Stockman, Professor 
 Carpenter, Mr. Fryer, Professor Macdougal, and Mr. Seymour-Jones. 
 
 The amount of damage caused, annually, in Canada, is very great. Figures 
 obtained from the principal tanners show that 27-5 per cent of our hides are 
 injured every year. In addition to the hides, how much milk is lost, how much 
 flesh, must be left to the- imagination, as there is no reliable method to compute 
 these losses. 
 
 It. is to be hoped, however, that, when the reader has scanned through 
 these pages, he will gain an idea of the magnitude of the problem that is before us. 
 
 To conclude, mention should be made of those -who have, in the past, 
 contributed most to our knowledge of these flies. Bracy, Clark's remarkable 
 observations were published in 1796, 1826, and 1827. After him came Brauer, 
 Hiurichsen, Koorevaar, Curtice, Schaupp, Carpenter, Hewitt, and Glaser; 
 among whom, in America, Schaupp, and Doctor Cooper Curtice of the Bureau 
 of Animal Industry, deserve special notice. 
 
 Finally, the lines quoted by Bracy Clark will not be out of place as they 
 show that long ago the annoyance and damage which the flies occasion was 
 well understood. 
 
 "I will put the brizein'a tail, shall set him gadding presently." — Old Elizabethan drama. 
 "The herd hath more annoyance by the brize than by the tiger." — Shakespeare. 
 
 The psychic effect of non-biting flies has also been noted: — 
 
 "Un avorton de mouohe en cent lieux le haroele; 
 
 Tantot pique I'Sohine, et tantot le museau, 
 
 Tantot entre au fond du naseau, 
 _ La rage alors se trouve k son fatte montfee. 
 
 Les plus £k craindre sent souvent les plus petits." — La Fontaine. 
 
WARBLE FLIES. 
 
 DESCRIPTION OF HYPODERMA LINEATUM 
 
 Hypoderma lineatum is a fly 12-7 mm. in length, with distinctive tail 
 colouring of reddish orange. The under surface of the thorax and the anterior 
 part of the abdomen are black, the wing veins nearly black and the alulae 
 uniformly white. The lines on the anterior part of the thorax are bare and 
 prominent; and the legs are rough and hairy. The distance between the eyes 
 is 1 -9 mm., the same as in H. bovis, which is the larger fly. (Plate I, figs. 1-3.) 
 
 Seasonal Prevalence. 
 
 H. lineatum can withstand comparatively cold weather and occasionally 
 has been seen ovipositing on cloudy days. In 1915, at Agassiz, the first flies 
 were taken on April 15, and several more on April 22, 23 and 24. The period 
 during which H. lineatum was active was eighteen days, and flies were either 
 seen or caught on nine of those days. The only correspondingly early record 
 seen in European literature is by Imms, who examined specimens in the British 
 Museum and says that the specimens in the collection have dates varying from 
 the end of April to the beginning of June. The deficiency, both in Canadian and 
 in European literature, is probably due to the fact that the earher fly, H. lineatum, 
 causes so little annoyance to cattle, that collectors have not looked for it. 
 
 OVIPOSITION AND ITS PSTCHIC EFFECT ON AnIMALS. 
 
 H. lineatum irritates cattle much less than H. bovis; in fact^ when gentle 
 animals are being attacked, the disturbance among them is often very shght 
 
 Fig. 4. — H. lineatum ovi-positing on the spotted cow in the foreground. 
 
 Note contented look of animals. ' 
 
 and may pass unnoticed by the casual observer. On the other hand, wild 
 (range) animals, free to go where they please, exhibit a good deal more annoyance. 
 Among gentle cattle the young ones will sometimes run or gallop, but the older 
 
 4 
 
animals, though they may occasionally lift their tails and perhaps run a short 
 distance, are often so indifferent to the irritation caused by egg-laying that 
 they -will merely stamp or whisk their tails. These remarks only apply if egg- 
 laying takes place on standing animals, for when animals are in this position, 
 it is necessary for the fly to grasp the hairs to oviposit, with th« exception of 
 those hairs, which are about the heels, which can be reached with the ovipositor 
 while the fly is resting on the ground. The fly has a habit of settling under the 
 shadow of the cow's heel and it can often be seen running backwards and lifting 
 up its ovipositor, attaching its eggs to the coronet. Other favourite places for 
 egg-laying are in the regions of the fetlocks of the fore and hind legs, and also 
 on the knees and hocks. Flies are seldom seen ovipositing higher than this on 
 
 Fig. 5. — Fly ovipositing on cow's tail. 
 
 a standing animal, though in fig. 5 a fly may be seen ovipositing quite high up 
 on a cow's tail. The fly alights on the animal quite gently and since it lays a 
 number of eggs in sequence on a hair it must necessarily cause the animal but 
 shght annoyance in order to do so. 
 
 Seeing that oviposition takes place principally when the animals are 
 resting, the insect has a much "better opportunity for laying where it likes. 
 The line of contact of the animal's body with the soil is where most eggs will be 
 found. From before backwards, the line extends from the sternum, along the 
 flank to a point about six inches below the ischium. The udder is also a favourite 
 place for egg-laying, and another spot is just behind the elbow. In this case, 
 the fly runs up the hoof which is tucked in under the elbow; when it reaches 
 the top it backs up and pushes its ovipositor among the hairs. H. lineatum 
 invariably lays with its head pointing in the same direction as the hairs. (Fig. 5.) 
 
 In searching for the fly among recumbent animals, it is well to look under- 
 neath them in the shady parts. A fly -^ill often remain with an animal for 
 
 69548—2 
 
6 
 
 fifteen or twenty minutes, and in some cases a good deal longer. The only 
 irritation noticeable in such a case may be an occasional raising or shifting of 
 the body, or perhaps the tail may switch a little, but the annoyance is not 
 sufficient to make the animal rise. The time of egg-laying is during the warmest 
 part of the day, usually from 10 a.m. to 4 p.m. 
 
 The Egg. 
 
 The egg is cyUndrical, yellowish white, with a pedicel by which it is attached 
 to the hair. The number of eggs on a single hair varies, as many as fourteen 
 having been found. (Figs. 6, 7.) 
 
 Fig. 6. - Fig. 7. 
 
 Eggs of H. lineaium. 
 
 The eggs are often in full view; this is because eight or ten eggs in sequence 
 take up a considerable space on the hair. 
 
 The time taken for eggs to hatch is about seven days, the shortest period 
 being four or five days. Eggs, which are removed from an animal, hatch 
 readily under varying conditions; they have been hatched with and without 
 moisture, and with and without heat. 
 
 The Larva. 
 
 Larvae have been seen in the act of emerging from the egg on several 
 occasions. In one experiment, eggs were taken from an animal together with 
 a portion of the underlying skin. This was done at 11.45 a.m. A larva hatched 
 at 12.15 p.m., a second at 2, and a third at 4.45. It is interesting to note that the 
 egg next to the skin hatched first and the other two in order. This is probably 
 due to the fact, that those nearest the skin had been laid first, and also that 
 those nearest the body derive most heat. 
 
Method of Penetration. 
 
 Upon emerging from the egg the larva crawls actively along the hair to 
 the skin. It is apparently aided in this by a sticky exudate with which it is 
 covered, which seems also to serve the purpose of preserving it from drying out. 
 Upon reaching the root of the hair, it begins to work with its mouth parts. In 
 no instance was a larva observed to penetrate the skin otherwise than by way 
 of the hair follicle. The adherence of the larva to the hair keeps it in proper 
 position for penetration and provides it with a point d'appui. 
 
 Hewitt in 1914, who was the first, to observe this penetration, saw several 
 larvae of H. bovis penetrate the skin, and Carpenter, Hewitt and Redden fnention 
 
 |HpH^^v^jr J^HVIk %i 
 
 
 V f t* .*• 
 
 ■ M- :^ 
 
 
 -t 
 
 Fig. 8. — Larva of H. Kneaium penetrating a 
 hair fotiicle. 
 
 Fig. 9. — Enlargement of larva shown in Fig. 8. 
 
 the difl&culty of watching the larvae on a live animal. They say, "it took the 
 larva,e about six hours to get into the skin; possibly the hair follicles may have 
 facilitated entrance." The actual penetration of the skin by the larvae of 
 H. lineatum was not observed by them. Since that time the penetration of 
 the skin by the larva has been amply verified; and, at Agassiz, larvae have been 
 discovered in the act of burrowing into the skin. 
 
 Seeing there has been so much controversy on this subject, all the first 
 experiments are here republished in full, just as they were made in 1915: — 
 1. Twelve larvae, which had just hatched, were placed on a calf's back. They 
 worked actively, but none of them succeeded in going through the hide during 
 the period of observation^ 10.30 a.m. to 11.15 a.m. The larvae worked so slowly 
 and there was so much difficulty in keeping the calf quiet, it was decided to 
 abandon this method. 
 
 2. A small piece of skin was removed from an animal and five larvae placed 
 upon it. Two of these made determined efforts to pierce the skin, and about 
 1 p.m. had succeeded in getting half way in. The skin was then placed in an 
 
 69548— 2i 
 
incubator at 88°F.; at about 2.30 the larvae were apparently dead. Another 
 experiment was made on the following day and of twenty-three larvae placed 
 on the skin at 3 p.m., one had just disappeared from sight at 9.30 the following 
 morning; by 2 p.m., the same day, one larva had completely penetrated the 
 hide, and three others were three-quarters of the way in. 
 
 3. A third experiment was made to try and discover the natural penetration 
 of the skin by larvae hatched on the cow. A small piece of skin, on which eight 
 eggs were attached to a hair in the centre, was cut off a cow's udder. Six of 
 the eggs were hatched and two unhatched. There was a sUght amount of 
 subcutaneous inflammation. The hairs around this spot were cUpped off, leaving 
 only the hair to which the eggs were attached. A larva was found close to 
 this point actively working and penetrating the skin. (Figs. -8 and 9.) The 
 experiment commenced at 3.45 p.m.; at 10.20 p.m. the larva was nearly three- 
 quarters of the way in, but at 8 the following night, it had not disappeared from 
 sight. Repeating the previous experiment, a piece of skin underlying three 
 hairs, to which twelve eggs were attached, was removed from a cow. Nine of 
 the eggs had hatched. At the foot of one of the hairs a droplet of clear serum 
 was exuding and kept increasing in size; evidently some force was expressing it. 
 On the piece of skin being grasped with strong forceps and bent double, a larger 
 flow of serum resulted and eventually two larvae were pressed out. One of these 
 was extremely active and endeavoured to re-enter the hair follicle from which 
 it had come. • 
 
 The larvae were no doubt placed at a disadvantage in the experiments 
 where the skin was removed; for, in the first place, it is probable that the skin 
 movements of the living animal may aid larvae to penetrate; secondly, when 
 working on skin which has been removed from the living animal less serum 
 exudes, though the piece of skin be kept constantly moistened underneath; 
 thirdly, putrefactive changes occur. Soon after the larvae have gone under the 
 ■ skin there is an outpouring of serum from the openings through which they 
 have entered. It may be that several larvae go through the same opening, seeing 
 that when they descend the same hair to which the eggs are attached they all 
 land at the same point. But this has not been proven. The reaction on the part 
 of the animal against this penetration is much more pronounced in' the old 
 animals than in the young, no doubt owing to the fact that the former have 
 become sensitive to the larval secretions and excretions during previous infesta- 
 titos. The swellings are thus reactions against parasites and account for the 
 smaller number which liiature in the old animals as compared with the young. 
 These skin swellings are irregular in shape and sometimes attain large dimensions; 
 when the swelling subsides it will be found that portions of the skin have become 
 necrotic and a scab has formed. When this scab is lifted off, one or more conical 
 pits will be discovered filled with yellow pus. This pus has been examined and 
 is found to consist almost entirely of a variety of a white blood cell, known as 
 the eosinophile. This cell has the special function of attacking parasites, 
 including worms, and no doubt plays a very important part in the destruction 
 of larvae. 
 
 The name of hypodermal rash has been proposed for these skin lesions. 
 They are naturally seen in the regions where the eggs are laid, and figs. 11-14 
 show where they may be found. 
 
 Following the penetration very Uttle is known about the migration of the 
 larva. However, early in August, larvae have been found in the walls of the 
 oesophagus, but by what route they have reached this point is not known; 
 most hkely they travel in the loose connective tissues to the region of the throat 
 and into the oesophagus where the muscles bifurcate. 
 
Migrations of the Larvae Through the Tissues. 
 
 The young larva having penetrated the skin, arrives in time at the gullet. 
 (Fig. 10.) ^ 
 
 Note.— Hadwen, S., and Bruce, E. A., 1916, Observations on the Migration of Warble larvaj through 
 the tissues. Dom. Dept. of Agr. BuUetjn No. 22. 
 
 1 
 t 
 
 m 
 
 
 1 
 
 1 
 
 •'"^^^ 
 
 a*. 
 
 Fig. 10. — Larva in oesophagus. Mucous membrane incised to show larva. The shadowy 
 outline of another larva may be seen at the point marked +. 
 
 What route it follows has not been ascertained, though it would seem highly 
 probable that it makes its long journey exclusively through the connective 
 tissues. It has been suggested that the larvse might reach the anterior part 
 of the body by the blood stream, but this seems unlikely. It has been proved 
 by observation that the larvse follow the connective tissues very closely, 
 and a number of experiments were made with larvse, which had been extracted 
 from cows' gullets. These larvae were dropped into small pockets made under 
 the skin. In one case a pocket was made on the right hand quarter of a calf 
 and twelve larvse were dropped in; subsequently, four of these larvse reached 
 the animal's back. 
 
 In another experiment made on January 25, 1914, twenty-five larvse were 
 placed in a pocket on the left hind quarter of a November calf, and on February 
 2nd, several of the grubs had reached the back, and later, eleven of them pierced 
 the skin. 
 
 * In another experiment, November 17, 1914, with a three weeks' old calf,- 
 five larvse were slipped under the hide on the outside of the knee; twenty days 
 later, two of the larvse had punctured the skin on the calf's back. The calf 
 was killed and the leg skinned. A gelatinous track was found at the point 
 where the larvse had been introduced, viz., on the outside of the knee, at a point 
 over the tendons of the extensor muscles. Following this track it was found to 
 extend in towards the radius, passing round it posteriorly, upwards under the elbow 
 joint to the scapula. A larva was found about .two-thirds the way up this 
 bone, near the posterior border, that is to say, under the sub-scapularis and above 
 the serratus magnus. The track was plainly visible in the white fibrous tissue. 
 
10 
 
 Plate II. 
 
 Fig. 11. — Lesions ot the skin caused by the 
 
 extrance of Warble larvse. Note the lesions 
 
 on the tail. 
 
 Fig. 12. — The same animal two days later. 
 
 Fig. 13. — Lesions on cow's udder. 
 
 Fig. 14.— Circular spots after the larv^ have 
 entered. An exudation of serum has matted 
 the hairs. 
 
.11 
 
 These experiments show that the larvse choose the easiest and the most 
 non-vascular tissues in their migrations, and it would seem probable that even 
 when they are newly hatched, they would have the same habits that they adopt 
 in their later migrations. 
 
 If, then^ this is correct, in all probability they gain entra,nce to the oeso- 
 phagus in the region of the throat. 
 
 In a large number of gullets, which were examined at regular intervals 
 throughout the winter of 1914-1915, it was found that the larvse move up and 
 down the gullet continuously, that the greatest number were found in the gullets 
 during December, and that they gradually diminished in numbers until the 
 month of March when they disappeared from this region.- Hiurichsen, Koore- 
 vaarin Europe, Curtice, in America, and recently. Carpenter, in If eland, have 
 obtained very similar results. 
 
 The last larvae to leave the gullet were at the junction of the oesophagus and 
 the paunch, where the muscle fibres are widely separated, and have a radiating 
 arrangement. This is undoubtedly the place where the larvse would start on 
 their journey to the back. They were here found going in different directions. 
 At this point there is a reflection of the pleura over the end of the gullet, beneath 
 which they could readily pass up the crura of the diaphragm, or else take a 
 longer course along the posterior borders of the ribs. 
 
 On finding that the larvse were disappearing from the oesophagus, attention 
 was next directed to tracing them in their subsequent migrations. Apart from ■ 
 finding the larvse leaving the oesophagus at its junction with the paunch, very 
 little is known about their route to the neural canal, except that gelatinous tracks 
 were noted by the author, which were very likely caused by them. Other observers 
 have occasionally encountered larvse between the gullet and the neural canal. 
 Curtice, found some under the pleura in the region of the eleventh rib, and 
 Ransom has a larva which was discovered under the pericardium. 
 
 Direct evidence has been secured that the larvse make use of the posterior 
 foramen to gain> entrance to the neural canal. In the neural canal itseK, a 
 considerable number of larvse have been found, as many as seven larvse have been 
 . counted in one animal; also several larvse were discovered just emerging from 
 the posterior foramen. The larvse are found in the soft areolar tissue, which 
 surrounds the dura mater. The presence of the larvse in the neural canal 
 causes a gelatinous infiltration and a greenish pigmentation. The fat also 
 exhibits grey degenerate areas, which is evidence that the larvse cause irritation 
 and that there is a reaction against them. 
 
 All stockmen are familiar with the appearance of the mature larva. When 
 it first reaches the back, it is whitish, about 1.5 cm. long; . when, however, the 
 , larva is mature and ready to leave the back it measures about 2.5 cm. and has 
 changed to a grayish black colour. 
 
 Suggestions for the Complicated Life History Within the Body. 
 
 There are various reasons which suggest themselves as to why the larvse 
 select different portions of the body, in which to pass certain stages of their 
 existence. In penetrating the skin, for example, it will be noticed that they 
 choose the coarser and more porous parts to penetrate. The gullet where they 
 are next found, appears to be admirably suited to them as a habitat during the 
 autumn months. The parts they live in are only sUghtly vascular, hence the 
 reaction against them is not as great as it would be in other tissues, but the 
 reason for their constantly being on th^ move is evidence that there is such a 
 reaction; and if a number of gullets are examined, it will be seen that the larvse 
 are often surrounded by oedemas. 
 
 In other cases, one can see that the larva has just moved away from the 
 inflammatory area, or else is just beginning, to do so. 
 
12 
 
 Plate III. 
 
 Fig. 15. — The under side of a cow's hide showing (on the right) a partial dissection which 
 reveals a young larva lying horizontally and (on the left) an unopened Warble. 
 
 Fig. 16. — The same specimen shown in Fig. 15, the Warble on the left having, however, beea 
 partially dissected so as to reveal the position of the larvae. 
 
Plate IV. 
 
 Fig. 17. Mmm^mi^^mDe Gcer. 
 Bypoaenna bovis, 
 
 
 
 
 
 ^»»'^pf 
 
 
 iW^'^fe^,^^ 
 
 
 *-™ -"" ■■; 
 
 
 
 
 
 w; 
 
 
 <ii<i-- 
 
 Fig. 18. Natural size. 
 
 Fig. 19. 
 
13 
 
 _ .These swellings have been carefully studied and the principal cell encountered 
 m them is the eosmophile. 
 
 The migration from the gullet to the back is evidently quite a short process, 
 as larvae taken from the gullet, the neural canal, and the back of the same 
 animal_ all proved to be of the same length. The final position taken by the 
 larvae is in connective tissue, very similar to that found in the gullet. Here 
 agam, there is a bodily reaction against them, which is manifested by eodematous 
 swelhngs. Very probably this reaction would end in their destruction since 
 they have now taken up a fixed position. But, shortly after reaching the back, 
 they bore through the skin, the result being that air gains entrance into the 
 cavity in which they lie. Other factors also come in, such as the introduction 
 of bacteria; consequently, the defence on the part of the tissues against the 
 larvae will be modified. 
 
 During the approach of the larvae to the back and the period in which they 
 are boring through the skin, the animals suffer a good deal of uneasiness, which 
 ip evinced by their constantly licking the region of the loins; and, if a cow stable 
 be visited at this period, it will be noticed that the hair has been well licked in 
 the regions that can be reached by the animal's tongue. After the larva has 
 finished boring through the skin, the oedema of the skin subsides to a consider- 
 able extent and the irritation and licking are not so pronounced. 
 
 The Position of the Labv^ under the Hide. 
 
 The opening through the hide is obhque and the larva lies with its ventral 
 surface upwards. The caudal end is bent and protrudes through the opening, 
 which gives the impression from an outside view that the grub is placed per- 
 pendicularly beneath the hide, instead of horizontally, as is actually the case. 
 (Figs. 15 and 16.) The advantage of this position is that the effect of the skin 
 movements is reduced to a minimum. 
 
 For a somewhat similar reason, the second stage larvae lie longitudinally in 
 the oesophagus. The contraction of the longitudinal muscles would simply 
 cause the larva to shorten its body as it does naturally. If it lay transversely, 
 the circular muscles would bend the body sideways. 
 
 Time Taken by H. lineatum to Emerge from the Pupa. 
 
 According to continental observers, the pupal period is about thirty days. 
 In Canada, it has been proved that with artificial heat this period may be 
 shortened considerably and a fly has emerged in as brief a period as thirteen 
 days and several others in fourteen days. 
 
 DESCRIPTION OF HYPODERMA BOVIS. 
 
 Hypoderma bovis is a fly 14 mm. in length, with yellow hair on the anterior 
 part of the thorax, dark brown wing veins, and a reddish brown border to alulae. 
 The under part of the abdomen and thorax are nearly black; the tail end orange 
 yellow and the legs are clean with few hairs. The latter qualification is a 
 distinctive feature for H. bovis. The distance between the eyes in front of the 
 ocelli is 1-9 mm. and the greatest diameter of the eye is 1-9 mm. (Plate 4, 
 figs. 17-19.) 
 
14 
 
 Seasonal Prevalence. 
 
 H. bovis appears in the early part of June and continues up to the beginning 
 of August. For the years 1912, 1914 and 1915, the average period during which 
 H. bovis was active was fifty-six days. In 1915, careful records were kept and 
 
 Fig. 20. — Penetration of the skin by the 
 larva of H. bovis. Lesions on outside of 
 cow's leg. 
 
 Fig. 21. — Lesions oni;hindquarters."S|Note 
 large swelling von left leg behind the 
 udder. 
 
 on twenty-eight days out of the season of fifty-five days, flies were either seen 
 or caught attacking the animals. This means that during the season there were 
 twenty-eight days which were favourable to the fly. 
 
 These observations coincide closely with the pupal period and with the 
 time the last larvae emerge from the backs of cattle. 
 
 Method of Oviposition. 
 
 H. bovis lays in. sunshine, generally when the animals are running, on the 
 outside of the hind quarters and on the legs above the fetlocks; if the animal be 
 quiet, the eggs are deposited low down, but higher, as soon as the animal moves 
 (Figs 20 and 21.) 
 
 Egg-laying has been frequently observed and, whenever witnessed, never 
 occupied more than a few minutes. It may be described as a frenzied process. 
 Flying about level with the stifle joint, the insect strikes twenty or thirty times 
 rapidly, leaves the animal for fifteen minutes or so, returns and repeats the 
 attack. Sometimes it follows behind, catches up for a moment and drops back 
 to the same position. 
 
 As the fly strikes, it hangs on with its legs to a hair for a second, cementing 
 an egg to the root of the hair, rendering the egg invisible unless the hair be 
 raised. An egg, deposited in the natural way, is firmly attached. The groove 
 
15 
 
 seems to be squeezed on in such a way that it opens and grasps the hair; and the 
 sticky material which makes the egg adhere so firmly must be inside the groove 
 when the egg is extruded. 
 
 As only one egg is laid at a time on a hair, the fly has no object in finding 
 the exact spot on which it laid a previous egg, and, as the cattle are running, 
 
 Fig. 22.— Warble-fly egg attached to ha^T. Much enlarged. 
 
 it is perhaps unable to do so. The eggs are thus deposited in an irregular manner 
 as can be seen by referring to the illustrations. The eggs were first discovered 
 in 1912, and the following experiments were carried out.i , , ^^ 
 
 A day was chosen when the animals were gallopmg about, and the first 
 thing was to capture a fly. This done, all the animals were, housed except the 
 one on which the experiment was to be made. The latter was securely tied to 
 a fence and closely watched as the fly was liberated. The insect, after sunning 
 itself, usually attacked low down on the legs. Whenever it struck, an examina- 
 tion was made; so close was the egg to the base of the hair that it was often 
 difficult to pass a thin pair of scissors between the egg and the skin. (Figs. 22-24.) 
 
 Later on, it was found that the best way to observe the method of attack 
 was to put a number of animals in a small enclosure and watch the fly at work. 
 
 Experiments were also made in anotTier way. Fhes, with then- wings 
 clipped were placed on cows' backs and watched for considerable periods. 
 Though the flies laid a number of eggs they did not attach them to the hau-s; 
 presumably, because the. wings had been clipped. The eggs which they laid 
 did not appear to be at all sticky, probably on account of the groove not being 
 
 "^^'^To secure a number of fertile eggs, flies were caught as gently as possible, 
 and held by the net on an animal's flank or back. The flies rested on the animal 
 and pushed their ovipositors under the hairs at intervals. The rapidity with 
 which they work is. surprising, and a large number of eggs may be obtained m 
 this manner in a comparatively short time. 
 
 iFor the penetration of the skin by the larva, see p. 8. 
 
16 
 
 Fig. 23. — Ovipositor with egg. 
 
 Fig. 24.— Larva inside egg much eiJarged. 
 
17 
 
 In these experiments the animals seem to feel little or no irritation during; 
 tne process; though, on occasions, when the fly buzzed loudly or flew about a 
 good deal, the animals showed uneasiness. 
 
 The Psychic Effect of Oviposition on Animals. 
 
 , . The terror-inspiring effect of H. bovis has mystified people for a very long 
 tune ihe fear and terror that it inspires cause cattle to lose their heads com- 
 pletely and the results are often disastrous. 
 
 
 
 3 
 
 
 Fig. 25.— Cow being chased by fly. Note terriEed look of eyes. 
 
 Wild cattle naturally get more panicky, and are less tolerant than gentle 
 cattle. They are also more agile than a fat bullock grazing in a small pasture. 
 Gentle animals on an ordinary farm usually find ways and means of escaping 
 from the attacks of the fly by running into the shelter or shade of buildings and 
 bushes, or into the water. On the prairies, it is quite a different matter, as there 
 are so few trees, and the only chance the animals have is to make for a lake, 
 or river, or else to dust themselves. 
 
 The following story illustrates a common incident where cattle are used 
 as work animals. One day in Alberta, a cowboy seemed to be vastly amused 
 about something; and the cause being asked, he said: "Well, I came riding by 
 a little lake just now, and I saw some cattle standing in it with their yokes on; 
 a disconsolate settler was sitting on the bank and said that he was waiting for 
 the animals to come out with the plough, and that he would not get much 
 ploughing done if the flies did not stop chasing his cattle into the water." 
 
 ' When an animal is attacked by a fly for the first time, it allows the fiy to 
 oviposit without showing any greater sign of discomfort than the tickhng of a 
 straw would occasion. If the fly is persistent, the flrst noticeable sign is that 
 of annoyance; the animal will stamp or kick and switch its tail vigorously. 
 The next thought the animal has is apparently to move away from the annoying 
 insect, which it does by walking to another place. If a fly continues to pester it, 
 it begins to trot and finally to gallop. When an animal is running in this manner, 
 it wiU be noticed that it frequently turns its head to try and see if the fly is still 
 pursuing it,^ and to do so pauses in its flight. If, at this moment, it finds that 
 the fly is stiU pestering it, the flight will begin again with redoubled energy and 
 with additional terror. 
 
 This, in brief, reveals the mental process the animal goes through, and it is 
 easy to see when the fly's methods are studied, what an annoying and aggravating 
 insect it is. ' 
 
18 
 
 The mental process produced by the Warble fly on cattle is similar to that 
 produced on a bald-headed man by house flies; the feeling of annoyance, followed 
 by anger, is exactly the same, but it should be remembered that H. hovis is 
 rougher in its movements and is a much larger fly than M. domestica. 
 
 A number of experiments have been made on the terror producing effect 
 of the fly. In 1915, a test was carried out on two calves, which had been kept 
 
 Fig. 26.^ — Calves attacked by Hypoderma bovis 
 trying to get into a barrel of water. 
 
 in doors until the season for H. bovis, and were closely watched when they were 
 first liberated. They went through all the phases, which are described above. 
 There are some other results which follow fly attacks; for instance, if the 
 flies are plentiful and the animals are kept constantly on the move, they naturally 
 become tired; then they seem to regain their senses somewhat, and try to find 
 ways and means of evading the fly, such, as in fig. 26, in which two calves are 
 doing their best to get inside a barrel. If the animals become positively 
 exhausted, then they sometimes become quite indifferent to the attacks of the 
 fly and lie on the ground in a fit of stubborness, refusing to move. In other 
 cases, they are plainly endeavouring to hide and make themselves as unobtrusive 
 as possible. In this way, they certainly appear to some extent to escape the 
 attentions of the fly, as it undoubtedly prefers to attack standing animals. , 
 
 The origin of a stampede. 
 
 The terror that the fly produces is contagious, and it suffices that one animal 
 should be attacked to set the whole herd running. In this connection it was 
 interesting to watch the behaviour of the experimental animals which were used. 
 Each animal had different habits and when a Warble fly appeared in the barn 
 yard the alarm spread through the whole herd. They immediately acted upon 
 it; the animal which was attacked of course galloped away; the others all 
 dispersed to favorite corners, or tried to get as far away from the point where 
 the insect was seen or heard. 
 
 On a number of occasions, the offending fly was captured and held prisoner 
 for a time; the animals would then quiet down and return to their food, or some 
 shady or favorite spot, and all that was necessary to upset this tranquil scene 
 was to liberate the fly again. 
 
 This is a most convincing method of demonstration, and, on a number of 
 occasions, doubting persons have quickly become interested in the fly. Many 
 people have an idea there must be a large number of flies attacking the animals 
 at one time, but in all experiments, and on all the days spent in studying these 
 insects the largest number captured in a single day was six. 
 
19 
 
 It cannot be denied that other flies annoy animals to a very great extent. 
 However, the annoyance is of a totally different character; for instance, though 
 the Tabanidae draw much blood and cause painful bites, yet cattle are able 
 to drive them off by switching their tails and, generally speaking, these flies 
 cause them but Uttle annoyance. The same thing applies to the Simulidae 
 and to the Mosquitoes. Cattle become exasperated, it is true, when there are 
 a large number of flies about and will frequently dash among bushes or dust 
 themselves, to get rid of them; but, the fear is not contagious and only one or 
 two animals are affected at a time. 
 
 Treatment. 
 
 Many remedies are recommended for killing or extracting the grubs after 
 they have bored through the skin of the back. The practice of killing the larvse 
 under the skin by injecting or applying mixtures is unscientific, for if the larva 
 is nearly mature its chitinous cuticle cannot be absorbed, and may be found 
 months later under the hide which has completely healed above. 
 
 The best method, undoubtedly, is to squeeze out the larvs as early as 
 possible, softening the skin first; in this way the wound rapidly heals. In some 
 cases, it may be necessary to enlarge the opening by a shght cut, especially in 
 regions where the skin cannot be properly grasped. The flexibility of the skin 
 also varies according to the condition of the animal. 
 
 Prevention. 
 
 Though many attempts have been made to exterminate Warble flies in 
 various countries and locahties, so far these efforts have met with little success. 
 
 The squeezing out of the grubs seems to be a simple matter, but though 
 many cattle breeders perform this operation each year, their animals are quite 
 heavily infested the following season. No doubt the explanation hes in the fact 
 that they overlooked some of the larvse, either early or late, or it may be that 
 their neighbours' cattle are not similarly treated. As a single fly is capable of 
 laying several hundred eggs, it is quite easy to see how the pest survives from 
 year to year. One point which has never been cleared up is the distance which 
 Warble flies can travel, but it seems likely that they do not go any great 
 distance. 
 
 The mixtures apphed to prevent egg-laying have proved useless in the 
 past and many of them have been injurious to the hides and to the animals. 
 The older authors recommended farmers to apply dressings to the animals' 
 backs and, in England, a common practice was to smear them with train and 
 fish oil. Seeing that the eggs are not laid on the back, the practice was decidedly 
 useless and costly. 
 
 According to Ransom, in the United States of America, arsenical dips appear 
 to have some beneficial effects, but no definite experiments have been published 
 as yet. It has been said that squeezed out larvae do not hatch, but if the larvse 
 be sufl&ciently mature they will do so; therefore it is wise to destroy them after 
 they have been taken out. The eradication of the fly is theoretically possible 
 by the "squeezing-out" method; this would apply to organized districts where 
 every animal could be treated. Among wild animals such as are found on the 
 Western Canadian ranges it would be ahnost impossible. In settled districts 
 squeezing out the larvae and housing the animals during the hot weather should soon 
 bring about a noticeable diminution of Warbles. The animals should be housed from 
 6 a.m. to 6 p.m. 
 
 Distribution. 
 
 In certain districts in Canada the flies are worse than in others; strange to 
 say, in some parts there appear to be none at all. No explanation can be given 
 for this fact; it may be that some climatic variation is responsible for this, in 
 the same way that Austraha and South Africa have escaped the fly. Generally 
 
20 
 
 speaking, Hypoderma bovis and lineatum appear to have an equal distribution 
 all over Canada. How far north these parasites occur has not been ascertained. 
 Bishopp states that H. bovis is commoner in the northern parts of the "United 
 States of America than H. lineatum, and that with regard to H. bovis it "must 
 have some well marked chmatic barriers which have prevented its general 
 dissemination throughout the country." 
 
 The Economic Aspect op • Wahble Flies. 
 
 In 1912, Doctor Torrance, the Veterinary Director General for Canada, 
 addressed a circular letter to the tanners in the different provinces throughout 
 the country to find out what damage was caused by Warble flies. Many letters 
 were received in answer and the evidence collected shows that Warbles damage 
 hides to a very great extent in the Dominion. 
 
 As the letters came from sound business men, who had no interest in the 
 matter save to get hides free from Warbles, the information is to be considered 
 most accurate. 
 
 The evidence showed that the man who undoubtedly loses is the farmer; 
 the tanner does not want warbled hides at any price, and several of the writers 
 testified that they only bought hides during the autumn season when the hides 
 are not grubby. 
 
 The uses a grubby hide can be put to are very Hmited, and for some purposes, 
 such as the manufacture of belts for machinery, they are practically useless. 
 (Fig. 27.) . 
 
 Some interesting facts were brought out concerning the grades of hides 
 .most affected by Warbles, which can be explained by a knowledge of the life 
 history of the parasite. 
 
 In Canada 25 per cent to 30 per cent of all hides are damaged by Warbles. 
 
 The length of the Warble season from the tanners' point of view extends 
 from late January to early July, the worst period being the month of April. 
 This statement tallies absolutely with the records kept at Agassiz. 
 
 Nearly all tanners agree that the rough, long-haired, ill-kept animals are 
 most "warbley" and that well-fed, sleek animals are not so badly affected. 
 The reasons are not far to seek : first, weak animals are always more parasited 
 than the strong and cannot fight the fly as well as the more robust; secondly, 
 dairy cattle and well bred animals are often housed during the heat of the day, 
 whereas, ill-kept beasts are at the mercy of the fly all day long. 
 
 One of the tanners mentioned the fact tliat northern hides were as a rule 
 thinner and more hairy than southern hides, and the latter, being thicker, may 
 afford the animals some protection from the penetration of the larv£e. 
 
 Young animals are more parasited than the old, and, therefore, their skins 
 will naturally be more damaged. 
 
 The tanners view a wet season with favour, as the following year will be a 
 better one for skins. The reason for this is quite apparent as the fly is only 
 active in fine weather. 
 
 One form of damage to the hide which the tanners have not apparently 
 connected with Warble flies is one which has been mentioned before and called 
 hypodermal rash; the scab formation, and pits made by the larvae in penetrating 
 the skin must necessarily destroy the grain of the leather. 
 
 Systemic Effect on Animals. 
 
 The systemic effect on the animals is very hard to gauge though badly 
 warbled animals certainly do not look as healthy as clean animals; whether an 
 animal's growth be stunted or not is difficult to prove and definitely controlled 
 experiments are needed to decide this point. The same thing applies to milk 
 
121 
 
 ?u^l^!l *^^* *^® ^^^^* ^° ^^^^^ ^^^^ ^^^^^ °" ^° animal is proved by the fact 
 that they are sensitive to the protein contained in the parasites. 
 
 |W-;-| 
 
 
 'l :■ -"i 
 
 ^g;;;7:::i;|:;:;^:KJ'fe 
 
 ^^H^H^^^^^g?'' .. 
 
 
 
 
 iiwefe>«.^8^P|ipM 
 
 
 IgMM^^MBJ^^^^^^^i^i^ 
 
 
 'S^^^^^^^^^^^^hHhH^^^s^^^^j 
 
 
 ^:;i»>'^<-^^pSi^^^^>^^ 
 
 Fig. 27. — ^Actual size of leather, 3i in. x 3j in. 
 
 The poisonous quahties of warble juice have been proved by a number of 
 experiments. (Figs. 28 and 29.) 
 
 When the juice derived from the larvse is injected into a sensitive animal, 
 that is, one which has been harbouring the parasites, the reaction termed anaphy- 
 laxis is frequently produced. The reaction occurs in two forms, one, which 
 is rapidly fatal, kills the animal in a few moments; the other, appearing in 
 twenty minutes to half an hour, causes grave symptoms followed by recovery. 
 The amount of material which is necessary to produce the effect appears to be 
 very small; in one case, a full grown cow was killed with the juice of three and 
 one half larvse. In other cases, larger amounts were used; but, as stated before, 
 the degree of sensitiveness in the animal appears to be more important than the 
 size of dose which is administered. In the rapidly fatal attacks, the most 
 marked symptoms noted werei those of asphyxia. In the non-fatal cases, 
 cedemas occurred in many parts of the body and also a marked wrinkling of the 
 skin. Blood may be passed from the natural openings. Cases of anaphylaxis 
 have also been diagnosed in animals, which had not received any injection. These 
 cases occurred'in dairy animals and were probably produced through injury 
 to the warbles on their backs. In two cases, there was a definite history of 
 such injury. A number of reports from Veterinarians have also been received 
 describing^ similar cases in cattle. 
 
 Note. — A full description of this reaction was published in the Journal of the American Veterinary 
 Association, Vol. 4, No. 1, April, 1917. 
 
22 
 
 Fig. 28. — Non-fatal reaction following injection of Warble juice. 
 
 Fig. 29. — The same animal shown in Fig. 28, Enls^rgement of eye. 
 
23 
 
 The Principal Differences Between the two. Flies. 
 
 H. bovis. 
 
 1. Length 14 mm." Distance between eyes 
 
 just in front of ocelli 1-9 mm., greatest 
 diameter of eye 1-9 mm. 
 
 2. Yellow hair on anterior part of thorax. 
 
 3. Wing veins dark brown. 
 
 4., Alulae have a reddish brown border. 
 
 5. Under part of abdomen and thorax nearly 
 
 black. 
 
 6. Legs clean with few hairs. 
 
 7. Colouring of tail end, orange yeUow. 
 
 H. lineatum. 
 
 Length 12-7 mm. Distance between eyes 
 just in front of ocelK 1-9 mm., greatest 
 diameter of eye 1-6 mm. 
 
 Anterior part of thorax black and shining. 
 
 Wing veins nearly black. 
 
 Alulae are uniformly white. 
 
 Under part of abdomen and thorax hghter. 
 
 Legs rough and hairy. 
 
 Colouring of tail end, reddish orange. 
 
 Methods of Oviposition. 
 
 1. Lays in sunshine, principally from 10 a.m. 
 
 to 4 p.na., but has been seen both earher 
 and later. 
 
 2. Causes cattle to "gad," both old and 
 
 young. 
 
 3. Lays its eggs principally when the animals 
 
 are running. 
 
 4. The eggs are laid mostly on the outside of 
 
 the hmd quarters and on the legs above 
 the fetlocfa. 
 
 5. The fly is a clumsy insect and strikes at the 
 
 animals blunderingly. 
 
 6. The eggs are laid singly at the roots of the 
 
 hairs and are rarely visible without lifting 
 the hairs. 
 
 Lays principally in the shade of the animals, 
 from about 10 a.m. to 4 p.m. 
 
 Causes annoyance especially to young 
 animals. 
 
 Lays its eggs mostly when the animals are 
 recumbent. 
 
 The eggs are laid on all parts which the fly 
 can reach when resting on the ground, 
 i.e., while the animals are lying down. 
 Even when the animals are standing, the 
 fly is able to lay eggs on those hairs which 
 are close to the groimd, namely, on the 
 heels." 
 
 The fly is much more gentle and dehberate 
 in its movements. 
 
 Several eggs are attached to a single hair and 
 are often visible without Ufting the 
 hair. , * 
 
 Skin Lesions. 
 
 Hypodermal rash. An irregular scattering 
 of round raised lumps, especially on the 
 outside of the hind quarters, appearing 
 suddenly, anaphylactic in nature and 
 rarely tending to suppurate. Very httle 
 exudation of serum. 
 
 Hypodermal rash. Diffuse irregular swell- 
 ings, very irritable, appearing suddenly, 
 anaphylactic in nature, often large and 
 ending in the production of pus, and a 
 marked dermatitis. The lesions follow 
 the parts of the animal which come in 
 contact with the ground, i.e., legs, 
 flanks and sternum. A large quantity 
 of exudate mats the hairs. 
 
 Larva. 
 
 The segment in front of the spiracular seg- 
 ment unarmed. 
 
 The segment in front of the spiracular seg- 
 ment armed. 
 
 Summary. 
 
 Hypoderma lineatum lays its eggs as early as April 15, but the usual 
 laying period is during the month of May. At Agassiz, they have never been 
 captured later than May 30. H. bovis begins to lay in the early part of June 
 and continues up to the beginning of August. Between the last appearance of 
 H. lineatum and the first of H. bovis there is usually a period of ten days when 
 the cattle are immune from attack of either species. H. bovis frightens the 
 cattle much more than H. lineatum. The eggs take about a week to hatch; 
 the larvse bore through the skia in the coarser, porous parts, taking several 
 hours in the process; at this stage they are about 1 mm. long: The lesions 
 
24 
 
 resulting from this penetration are caused partly by anaphylactic reactions and 
 partly by bacterial invasion, those produced by H. lineatum being the more 
 severe. 
 
 For the skin lesions the name of hypodermal rash has been proposed. 
 
 At this point there is a hiatus in the Hfe history. It is not positively known 
 how the larvae reach the oesophagus, where they are subsequently found; most 
 likely they travel in the loose connective tissues under the skin up to the region 
 of the throat and into the oesophagus where the muscles bifurcate. Passing 
 down the oesophagus they follow the submucosa and are almost always found 
 lying along the long axis of the canal. Whilst in the oesophagus, small cedematous 
 swellings are found surrounding the grubs; these are sterile and are anaphy- 
 lactic in character; the exudate contains large numbers of eosinophiUc leucocytes. 
 
 The earhest record made at Agassiz of larvae in the oesophagus was on 
 August 15, when a larva 3 '4 mm. was found and several slightly larger. 
 Continental observers have recorded smaller larvae than this. 
 
 H. lineatum makes its appearance in the backs of cattle about December 
 15, and H. boms about a month later. The larvae at this time have grown to 
 about 1"5 cm., and are similar in size to those which are found in the neural 
 canal and under the skin. At this stage it is difficult to separate the larvae 
 of the two species, but Bishopp has, recently, discovered good distinguishing 
 marks between them.' The Hfe histories overlap at this period making it difficult 
 to follow the migration, but in the latter part of the season (the middle of March) 
 the last larvae to leave the oesophagus are at the paunch end. They pass out under 
 the pleura and go to the neural canal, either up the crura of the diap'hragm, or 
 up the posterior border of the ribs entering the canal by the posterior foramen. 
 The larva evidently makes use of the canal as an easy means of access to the 
 lumbar region, the part of the animal which is best suited for passing it^ last 
 stages within the host. 
 
 The larvae follow connective tissue exclusively and no larvae have been 
 discovered in muscular tissue. 
 
 The mature larvae leave the animals' backs from the early part of the year 
 up to the first days of July. 
 
 The periods for the two species have not been fully worked out; but, 
 judging from what records there are of the pupal period and the time of year 
 the flies are on the wing, H. lineatum begins to emerge in February and finishes 
 about May 1. H. bovis begins about May 1 and ends approximately on 
 July 1. The average pupal period for H. bovis is 32 '5 days, and for H. lineatum 
 a little less. The duration of the life of the flies is short, seeing that they cannot 
 feed. This hfe history applies to Agassiz, British Columbia; doubtless in other 
 countries variations will be noticed, but the period spent by the larvae within 
 the host must be of the same duration, seeing that animals' temperatures are 
 the same the world over. 
 
 Prevention. — Animals should be housed during the heat of the day to prevent 
 the flies laying upon them. Warble grubs should be squeezed out as early as 
 possible during the year. This method will lessen the damage to animals and 
 their hides. If the total eradication of the pest is attempted, co-operative 
 measures must be undertaken. 
 
 Acknowledgments. 
 
 I am indebted to Miss E. B. Cramp for assistance in compiling this 
 paper, and to Dr. E. M. Walker for the loan of three of the figures, which were 
 published in the Canadian Entomologist.