790 PARASITIC DISEASES. 



DIPTEEA. 



As remarked at tlie commencerQent of this chapter, some of 

 the flies are apt to prOve troublesome as internal parasites. 

 With those dipterous or two-winged insects which, as external 

 parasites, occasion suffering to animals, we have here little or 

 nothing to do ; but since some of these forms of insect life play 

 the double part of attacking their victims from within as well 

 as from without, it is desirable to speak of such of them as 

 fairly, in one phase of their life, come under the general class 

 of internal parasites. 



In this relation there is a particular group of insects that 

 distinguishes itself above all the others. This is the so-called 

 bot-producing family, comprising various forms of gadflies 

 ((Est7'ida;). 



As enemies of the horse, ox, and sheep, the gadflies have 

 acquired notoriety from the earliest times. Thus they originally 

 obtained their family title from the ancient Greeks, who called 

 the gadfly of cattle the Oistros {Olarrpo'?, from Otw, I impel ; 

 Latin (CEstrus). 



When any person became unduly excited they said he had a 

 fit of the Oistros. One can readily see the force of this expres- 

 sion after noticing how outrageously excited and furious a herd 

 of cattle becomes when attacked by gadflies. As, however, it is 

 not with the gadflies, viewed in the light of external enemies or 

 parasites, that we have here to deal, we may dismiss this part 

 of the subject by observing that, in the case of cattle, the gad- 

 flies have the ingenuity to select as their victims young beasts 

 from two to three years old. The hide of an older beast is more 

 difficult to pierce. 



Since different species of gadfly attack different animals, and 

 several kinds of fly, in the larval state, infest one and the same 

 animal, it is desirable to speak of the forms belonging to our 

 various domestic animals separately. In the first place we will 

 consider those of the horse. 



The common gadfly of the horse {CEstrus equi) attacks the 

 animal whilst grazing late in the summer ; its object being not 

 to derive sustenance, but to deposit its eggs on the coat ; and 

 this it accomplishes by means of a glutinous material causing 

 the ova to adhere to the hairs. The parts of the animal selected 



