DIPTERA. 63 
heads and necks extended, their tails trembling, and held in 
a line with the body, to rush to the nearest river or pond, while 
such as are not attacked disperse. It is asserted that the buzzing 
alone of the Cstrus terrifies a bullock to such an extent as to 

Fig. 45.—Bot-tly (istrus bovis). 
render it unmanageable. As for the insect, it simply obeys its 
maternal instinct, which commands it to deposit its eggs under the 
skin of our large ruminants. 
Let us now explain how the eggs of the Cstrus deposited in 
the skin of the bullock accommodate themselves to this strange 
abode. The mother insect makes a certain number of little 
wounds in the skin of the beast, each of which receives an egg, 
which the heat of the animal serves to bring forth. It is a 
natural parallel to the artificial way which the ancient Egyptians 
invented of hatching the eggs of domestic fowls, and which has 
been imitated badly enough in our day. 
Directly the larva of the Bot-fly is out of the egg and lodged 
between the skin and the flesh of its host, the bullock, it finds 
itself in a place perfectly suitable to its existence. In this happy 
condition the larva increases in growth, and eventually becomes a 
fly in its turn. Those parts of the animal’s body in which the 
larvee are lodged are easily to be recognised, as above each larva 

