636 INSECTS AT HOME. 



parts of the mouth are quite undeveloped, the whole of the 

 structures being represented by two or three little tubercles. 

 In the perfect state, therefore, these Flies are quite harm- 

 less, but in their larval condition they are hurtful to various 

 animals. In fact, all our cattle are attacked by these insects, 

 which are parasitic upon or in them, and not even the thick- 

 skinned camel and rhinoceros are exempt from them. 



One of these insects, the common Bot-fly {Gasterophilus 

 equi) is shown on Plate XX. Fig. 3. Its life history is briefly 

 as follows : — 



The mother insect flies about a horse, and Avith its ovipo- 

 sitor places an egg on the hair of some part which the horse is 

 sure to lick. The egg is thus taken up by the tongue and con- 

 veyed into the stomach, where it is hatched. The larva then 

 attaches itself to the inner membrane of the stomach, and 

 appears to feed upon the secretions. 



The number of these larvae which will inhabit the stomach 

 of the horse is really wonderful. I have seen tlie interior of that 

 organ so covered with them that the surface of the stomach 

 was completely hidden by their depending bodies. In form 

 they are nearly cylindrical, but the body is suddenly narrowed 

 towards the head, which is armed with a number of little 

 hooks, by which the grub attaches itself to the interior of the 

 stomach. They remain within the stomach of the horse 

 throughout the winter, and in the spring they are full-fed and 

 loosen their hold of the stomach. They are then carried 

 through the intestines and fall to the ground, into which they 

 at once burrow and there assume their pupal form. The pro- 

 cess of transformation lasts about three weeks, ' and at the 

 expiration of that time the perfect fly ajjpears. 



Opinions are greatly divided as to the effect produced on the 

 horse by these larvae. Some of our best authorities, Mr. 

 Youatt among the number, believe that they do no harm, 

 while some go so far as to think that their presence is bene- 

 ficial. I cannot but think, however, that as these larvae derive 

 all their nourishment from substances within the stomach of the 

 horse, they must abstract a corresponding amount from the 

 nourishment of the horse, and that, whether they cause incon- 

 ^^enience or not to the animal, their presence must be in some 

 way hurtful to it. 



