ARTHROPODS. CLASS INSECTS 123 
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 pests, others, especially in the 
early stages of their existence, are 
of great benefit. 
120. Familiar examples—Owing ie. 75.—Horse-fly ( Therio- 
to the widely different habits and plectes). 
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. 
