82 INSECTS AFFECTING DOMESTIC ANIMALS. 
The newly hatched larva (fig. 37, ¢) is a slender, worm-like creature, 
so transparent that the internal organs are plainly visible. It grows 
rapidly at first, its food consisting, probably, of the mucous secretions of 
the mouth and esophagus. When two days old it presents the appear- 
ance Shown in fig.37,d. As soon as it reaches the stomach it fixes itself 
to the walls by the hooks next the mouth and soon undergoes consider- 
able change in its form and appearance. The body becomes more con- 
ical, but instead of the head end being widest this becomes more slender, 
while the tail end broadens. The spiracles at first exposed in two flap- 
like projections from the last segment are drawn more within the body 
and are protected by the development of a horny plate. The spines on 
the segments following the head become more pronounced. When full 
grown the larva is three-fourths of an inch to one inch in length and of 
the appearance shown in the accompanying figure (fig. 37, g). At this 
time they occur in large clusters upon the walls of the stomach, gener- 
ally more especially numerous at the pyloric portion, where they serve 
to retain the contents of the stomach. From the fact that the food 
of the horse does not become completely reduced to fluid, this obstruc- 
tion may be considerably greater than if only fluid matter had to pass 
the pylorus. This growth has occupied from late in the fall, through 
the winter, till late in the spring, and when fully developed the bots 
loosen their hold and are carried through the intestines and, escaping 
with the excrement, burrow into the ground to pass the pupa stage. 
This lasts for several weeks, thirty to forty days, according to some 
authors, when the fly, which has been already described, issues and is 
ready to proceed with the business of providing for another generation 
of bots. 
The larva of this species has been found in the stomach of the dog, 
though it can not be considered a normal habitat for it, and it is very 
doubtful whether the young larve could survive in the stomach of 
- @ carnivorous animal. Railliet (Comptes Rendus des séances de la 
Société de Biologie, 1894) cites records by Colin and others of such 
occurrences, aid details experiments which show the possibility of the 
survival of nearly mature larve that are swallowed with fragments of 
the stomach walls attaching to the stomach of the dog and remaining 
alive and healthy fifteen days after the ingestion. 
PREVENTION. 
In dealing with bots in horses, by far the most important point is to 
prevent the introduction of the larve, and while we have no oppor- 
tunity, as in the case of the ox bot-fly, to completely exterminate the 
pest, it is certain that proper attention to preventive measures would 
in afew years greatly reduce the numbers of the insect and procure _ 
comparative freedom. The better care usually accorded horses makes 
it possible to deal with it in some respects more easily than the species 
infesting cattle. The most vulnerable point of attack lies in the con- 
spicuous position of the eggs. No horseman, probably, can overlook 
