DIPTERA 867 



The sheep bot-fly, CEstrus ovis. — This species is viviparous; the 

 female fly deposits larvae, which have hatched within her body, in 

 the nostrils of sheep. The larv^ pass up into the frontal sinuses and 

 into the horns when they are present. Here they feed upon the mucus. 

 They are very injurious to sheep, causing vertigo or the disease known 

 as "staggers." When full-grown they pass out through the nostrils 

 and undergo their transformations beneath the surface of the ground. 



The ox-warble-flies, Hypoderma hovis and Hypoderma linedtum. — 

 If during the later winter months the backs of cattle be examined by 

 rubbing the hand over them, there will be found present in many 

 cases small lumps or swellings in each of which there is an opening 

 through the skin; these swellings are known as warbles, and each 

 contains a maggot, which when full grown measures nearly or quite 

 25 mm. in length. 



The maggots that produce these warbles are the larvas of flies, 

 which for this reason are known as warble-flies. Two species of 

 warble-flies both of which were introduced from Europe, infest cat- 

 tle in this countr}% and are very serious pests. 



The warble-flies when attempting to oviposit anno\" cattle, 

 which have an instinctive fear of them and run about in an eftort to 

 escape them; this leads to decreased milk yield. The larvae as para- 

 sites injuriously affect the health of the cattle. And the holes in the skin 

 through which the larvce escape from the warbles very seriously 

 reduce the value of the hide when made into leather. A careful 

 estimate made by the Department of Agriculture of Canada showed 

 that the annual loss in value of hides in Canada due to warbles is 

 between 25 and 30 per cent, of the total value of the hides. 



Our two species of warble -flies have much in common. The 

 adults measure from 12 to 14 mm. in length and are bumblebee-like 

 in appearance. They attach their eggs to hairs of cattle, usually on 

 the hind legs, more rarely on the flanks. The newly hatched larva 

 crawls down to the hair follicle where it penetrates the skin. Later 

 the second instar of the larva is found in the wall of the oesophagus. 

 The exact course of the migration from the hind legs to the oesophagus 

 has not been determined; but it is believed that the larv® travel in 

 the loose connective tissues under the skin to the region of the 

 throat and into the oesophagus where the muscles bifurcate. This 

 part of their migration occupies about four months. They remain 

 in the oesophagus about three months, and then migrate to the 

 lumbar region. This part of the migrations of the larvae is better- 

 known than the earlier part. Hadwen ('19) states as follows: "The 

 last larvae to leave the oesophagus are at the pai'nch end. They pass 

 out under the pleura and go to the neural canal, either up the crura 

 of the diaphragm, or up the posterior border of the ribs entering the 

 canal by the posterior foreamen. The lar\"a evidently makes use of 

 the canal as an easy means of access to the limibar region, the part of 

 of theanimal which is best suited for passing its last stages within the 

 host. The larvce follow connective tissue exclusively , no larvee have been 

 discovered in muscular tissue." 



