58 VETERINARY PARASITOLOGY 
horses which have been at grass during the months in 
which the fly is active. 
The eggs are laid by the adult female, and glued to 
the hairs of the fore-limbs of the horse, or in some 
position from which they may be easily reached by the 
mouth. The eggs are provided with “‘ opercula,”’ or 
lids, which open and liberate small maggot-like larve. 
These cause irritation, and are licked off by the horse, 
and on reaching the stomach attach themselves by 
means of their hooked mandibles to the mucous mem- 
brane of the cardiac end of the stomach. They then 
undergo two moultings, and finally develop into the 
so-called ‘ bots.” 
Each bot is pyriform in shape, tapering towards the 
head, and is composed of eleven segments, the first eight 
of which carry a double row of stiff spines directed back- 
wards. The anterior tapered end carries two smooth 
antennz, while the posterior end has two pores or aper- 
tures for breathing. This bot is a dirty white in colour. 
Bots may migrate, and so reach the brain, larynx, 
pharynx, or bladder. The larve remain in the horse’s 
body for about nine months, and pass out with the 
feeces in the spring. After excretion, they shrivel and 
take on the chrysalis stage. They then undergo three 
moults, and finally emerge as adult perfect insects. 
Another variety of bot which is fairly common in this 
country is the Gastvophilus hemorrhoidalis. In this 
case the fly lays a number of brownish eggs around the 
muzzle and lips of the horse. These bots are of a dirty 
green colour, and may be frequently seen after the horse 
has passed feces, attached to the inner rim of the anus. 
Its habitat is identical with that of Gastrophilus equt, 
but it remains in the rectum for a little while instead of 
passing out at once. 
In Russia Gastvophilus pecorum is the common bot, 
and is occasionally seen in Russian ponies brought over 
