320 MEDICAL ENTOMOLOGY 



He thinks that this is mainly due to the fact that weakly animals cannot 

 escape the attacks of the flies, whereas well-cared for animals resent 

 their attentions and usually run away ; such animals are usually kept in 

 stalls during the heat of the day when the imagines are most in evidence. 

 Warbles are less common during wet and cold weather. 



Up to the time of the publication of Had wen's paper, it was believed 

 that Hypoderma lineata, Villers, was the common Warble fly of North 

 America ; it would now appear that this is not the case, Hypoderma 

 bovis being the common species. 



Hypoderma bovis, de Geer, is about half an inch long and in general 

 appearance simulates a honey bee ; it is a very hairy insect. On the front 

 part of the thorax the hairs are yellow, in the middle they are black and 

 shiny, while on the posterior part they are whitish. On the base of the 

 abdomen the hairs are whitish yellow, in the middle portion they are 

 dark, and at the apex orange red. 



Hypoderma lineata, Villers, another North American species, is about 

 the same size as bovis. The hairs on the thorax are yellowish white, 

 reddish and brownish black ; in addition there are four narrow light 

 longitudinal lines on the thorax from which it gets its name lineata , 

 a white horizontal band runs across the posterior end of the thorax. 

 The hairs of the terminal segment of the abdomen are lemon yellow 

 in colour. 



The egg of Hypoderma bovis is about 1 mm. long and is of a yellowish 



white colour. It consists of two distinct parts, the egg itself and the 



pedicel or clasping base ; the latter consists of two lips. 



a ^. y s * According to Hadwen, who has studied the egg and 



The egg. 



the method by which it is laid, the pedicel end comes 

 out of the ovipositor first ; in some way or other the lips are opened and 

 adapted to the hair close to its root, the sticky substance accompanying 

 the eggs causing them to adhere to the hairs. This method of oviposition 

 is in marked contrast to that in Gastrophilus, in which the egg is lightly 

 fixed to the end of a hair. The eggs of Hypoderma are laid singly. 

 Hadwen shows that the favourite sites for the fly to lay her eggs are 

 in the region of the hock, the back of the knee and occasionally as 

 high as the stifle and along the flanks. Rarely the fly will lay an egg 

 near the jaw. 



When about to oviposit the fly strikes the animal in a clumsy manner 

 it never planes over it as has been described by several authors settling 

 for a moment while it glues on its egg at the root of the hair. It is the 

 pertinaceous way in which it repeats the process, buzzing round an4 



