ARTHROPOD A — INSECTS 317 



CLASS E, INSECTA 



These six-footed arthropods have a body consisting of head, 

 thorax and abdomen, with the skin hardened by chitin. To 

 this hardened skin, as in all other Arthropoda, the muscles are 

 attached. The head bears normally a pair of feelers or antennae, 

 a pair of mandibles and two pairs of maxillae, and usually a pair 

 of compound, sessile eyes (the house fly has 4000 facets {i.e. 

 indi\ddual eyes) to each eye; the dragon fly 28,000). The 

 thorax has a pair of unbranched legs to each of its three seg- 

 ments and usually two pairs of wings. The abdomen of seven 

 to eleven segments has no appendages. Since the wings are 

 merely expansions of the chitinous skin stretched over chitin- 

 ous tubes, — the veining, these are often preserved in the fossil 

 state notwithstanding their extreme delicacy. These tubes 

 consist of a tube within a tube, the inner carrying air, the outer 

 blood. 



Respiration is by trachece, — a system of air-tubes held open 

 by elastic, spiral threads, penetrating all parts of the body and 

 terminating upon the surface of the body in several button- 

 hole-like openings, the stigmata ; by the expansion of the abdo- 

 men air is drawn in through the stigmata and forced throughout 

 the body ; the contraction of the abdomen forces the air out 

 again. Thus air is taken directly to all parts of the body in- 

 stead of indirectly as in the vertebrates, where the blood acts as 

 carrier of oxygen from the lungs. There is accordingly no need 

 for either arteries or veins, though there is an elongate tubular 

 heart situated in the dorsal part of the abdomen, which aids the 

 movements of the body in transferring the blood and with it the 

 digested food particles to all parts of the body. 



The sexes are separate. Development is usually accompanied 

 by metamorphosis. For example, the house fly lays 100 to 200 

 eggs in decaying animal matter, such as a manure pile ; these in 

 warm weather hatch into larvae ("maggots ") which, living upon 

 this putrid matter, become fully grown in about one week, when 



