BOT-FLIES. 211 



bumble-bees. Their very small mouth has only abortive 

 mouth-parts; the feelers are small and are almost hidden in 

 little depressions or pits. The female fly differs from the 

 male by possessing a more pointed abdomen, ^vith a very ex- 

 tensive ovipositor. As is the case with most parasites a 

 large number of eggs are deposited; the necessity for this is 

 self-evident, as many of the young larvae do not succeed in 

 entering the host, but perish upon the way. In some cases 

 the flies are viviparous, and bring forth the larva already 

 batched. The more mature larvcC or grubs are thick and 

 fleshy; instead of feet they are provided with rows of hooks 

 or spines, which they utilize in moving about. They breathe 

 through one or two scaly plates at the end of the body. 

 Those larvae that live in the stomachs of their hosts have a 

 mouth furnished with horny hooks, which they use to cling 

 to the lining membrane; those, however, that live in tumors 

 under the skin have no such hooks, but possess instead 

 fleshy tubercles; they seem to live on the pus caused by 

 their irritating presence. The younger larvie are quite differ- 

 ent from the older ones, and as most of them lead a different 

 mode of life this is but natural. All these larvae or grubs, 

 w^hen mature and ready to transform, leave their hosts, 

 drop to the ground, and burrow in the soil, where they con- 

 tract into peculiar puparia, inside of which the final changes 

 take place. Because such grubs have to burrow in the soil 

 few city-horses are troubled with bots, as the grubs which 

 drop to the ground with the excrement can not burro^w 

 through the pavement, and are killed by exposure and by 

 other means before they have found such a suitable shelter. 

 Dr. Williston, our great authority on flies, has observed how^ 

 the larva of Bot- and Warble-flies enter the ground, and how 

 the latter leave the tumors they have produced. Rewrites in 

 the Standard Natural History: 



"They have the peculiar ability to contract either end into 

 an elongate cylindrical form, which not only serves them in 

 their egress, but also to bore into the ground. A few days 

 before they are ready to emerge they begin to enlarge the 

 opening by this expansion and contraction; when they have 

 enlarged it sufficiently, a ring-like contraction of the body 



