INSECTS. 



339 



FIG. 303. 



The diptera are almost as numerous as the coleop- 

 tera ; there are more than twenty thousand species of 

 flies, most of which are without special interest, except- 

 ing that they frequently become pests. 



Mosquitoes and gnats abound in all parts of the world, 

 as well in the frozen regions of the North as at the 

 equator. They prefer marshy localities, and sometimes 

 render such neighbor- 

 hoods almost uninhab- 

 itable. They deposit 

 their eggs on the sur- 

 face of water, and the 

 larvae are aquatic. 



The horse-fly and 

 ox-fly are common 

 during the summer in 

 woods and pastures, 

 where they torment 

 cattle to such an ex- 

 tent by their bites as 

 sometimes to drive 

 them nearly mad. The gad-fly or bot-fly, of which there 

 are several species, have no organs for perforation, but 

 deposit their eggs on the skin or in the nostrils of large 

 mammals, especially the ox, horse, and sheep. The 

 Iarva3 may develop in the nostrils, or the eggs, taken 

 into the mouth by the tongue of the animal, are intro- 

 duced into the digestive canal, where they develop, and 

 which the insects leave at the period of their last trans- 

 formation, allowing themselves to fall on the ground. 



HORSE BOT-FLY. a, a horse-hair with eggs 

 of bot-fly ; b, one egg magnified ; c, larva ; 

 d, pupa ; e, perfect insect, female, a little 

 larger than life. 



