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, with 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 larve do not succeed in 
entering the host, but perish upon the way. In some cases 
the flies are viviparous, and bring forth the larve already 
hatched. The more mature larve 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 larvee 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 larve 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 larve or grubs, 
when 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 burrow 
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 larve of Bot- and Warble-flies enter the ground,and how 
the latter leave the tumors they have produced. Hewrites 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 
