ARTHROPODS. CLASS INSECTS 



139 



FIG. 81. Horse-fly (Therio- 

 plectes). 



as those of the Crustacea, are comparatively large, and are 

 frequently composed of a great number of simple eyes 

 united together, upward of four 

 thousand forming the eye of the 

 common house-fly. 



These insects are widely distrib- 

 uted throughout the world, where 

 they inhabit woods, fields, or houses 

 as best suits their needs. Their 

 food is varied. Some suck the 

 juices of plants, others attack ani- 

 mals, and, while many are trouble- 

 some posts, others, especially in the 

 early stages of their existence, are 

 of great benefit. 



130. Typical forms. Owing to 

 the widely different habits and 

 structure of the members of this group, we shall briefly 

 consider two examples, the mosquito and the house-fly, 

 which will give us a fairly good idea of the characteristics 

 of alL The eggs of the mosquito are laid in sooty-look- 

 ing masses on the surface of stagnant pools. Within a 

 very short time the young hatch, and, owing to their pecul- 

 iar swimming movements, are known as " wrigglers." They 

 are then active scavengers, devouring vast quantities of 

 noxious substances and performing a valued service. They 

 frequently rise to the surface, take air into the tracheal 

 system, which opens at the posterior end of the body, and 

 descend again. After an increase in growth and many in- 

 ternal changes resulting in a chrysalis-like stage, they rise 

 to the surface, split the shell, and, using the latter as a float, 

 carefully balance themselves and soon fly away. 



The house-fly usually lays its eggs in decaying vegetable 

 matter, and the young, maggot-like in form, are active 

 scavengers. They too undergo deep-seated changes during 

 the next few days, finally transforming into the adult 



