THREE SPECIES OF BOT 
FLIES COMPARED 
Common Bot Fly 
(Gasterophilus intestinalis) 
Adult. —Largest of the three 
species; about the size of a honey 
bee. Body is covered with black 
and yellow hair. 
mottled. 
are 
Wings 
Female hovers about animal; 
darts from place to place as she 
cements egg after egg to hairs on 
the forelegs (a favorite place), the 
mane, the shoulders, the belly, the 
neck, and the flanks. Fly takes but 
a second to attach each egg. Sev- 
eral eggs may be fastened to the 
same hair; many may be attached 
to the mane and flanks. 
One female can lay 500 eggs. 
Egg.—Yellow. Ready to hatch 
in about 7 days. Eggs hatch any 
time (within 2 or 3 months) the 
animal licks or bites itself at the 
spot where eggs are attached; heat 
from the animal’s mouth stimu- 
lates hatching. 
Larva.—Newly hatched larvae 
are taken into animal’s mouth 
Lar- 
vae burrow into tongue. They con- 
when it licks or bites itself. 
tinue to burrow for 3 to 4 weeks, 
then pass into the animal’s stom- 
ach, attach themselves, and remain 
until mature. The total period for 
larval development is 10 to ll 
months. 
Mature larvae pass out of ani- 
mal with feces. 
4 
Throat Bot Fly 
(Gasterophilus nasalis) 
Adult.—Smaller than common 
bot fly and more rapid in flight. 
Wings have no markings. 
Female poises in midair, usualiy 
near the animal’s forelegs, then 
darts at the animal to attach eggs 
on hair under the jaws; cements 
1 to 4 eggs to a hair during each 
attack; flies away after each attack; 
returns in a few minutes. _ 
One female can lay 500 eggs. 
Egg.—Yellowish. Hatches in 
about 6 days. Does not require 
moisture or friction for hatching. 
Larva.—Newly hatched larvae 
crawl into animal’s mouth. Usually 
they lodge in the pockets between 
the molars. 
In 20 to 30 days larvae pass 
from the mouth into the stomach, 
then into the forward end of the 
small intestine, where they attach 
The 
total period for larval development 
is 10 to 11 months. 
Mature larvae pass out of animal 
and remain until mature. 
with feces. 
Nose Bot Fly 
(Gasterophilus haemorrhoidalis) 
Adult.—Smallest of boi flies; 
Most hairs 
on body are dark. Wings have no 
most rapid in flight. 
markings. 
Female darts at the lips of the 
animal; deposits one egg at the 
base of the hair, then darts away. 
A few seconds later she strikes 
