ARTHROPODA. 293 



joints, on the abdomen, upon which they chiefly rely in 

 locomotion. The wings are expansions of the crust, 

 stretched over a net- work of horny tubes. The venation, 

 or arrangement of these tubes (called veins and veinlets), 

 particularly in the fore-wings, is peculiar in each genus. 

 In many Insects, the abdomen of the female ends in a 

 tube which is the sheath of a sting, as in the Bee, or of an 

 ovipositor, or " borer," as in the Ichneumon, by means of 

 which the eggs are deposited in suitable places. 



Cephalization is carried to its maximum in this class, 

 and we have animals of the highest instincts under the 

 articulate type. The " brain " is formed of several gan- 

 glia massed together, and lies across the upper side of the 

 throat, just behind the mouth. The main cord lies along 

 the ventral side of the body, with a swelling for each seg- 

 ment ; besides this, there is a visceral nerve representing, 

 in function, the sympathetic system of Yertebrates. The 

 digestive apparatus consists of a pharynx, gullet (to which 

 a crop is added in the Fly, Butterfly, and Bee tribes), giz- 

 zard, stomach, and intestine. There are no absorbent ves- 

 sels, the chyme simply transuding through the walls of 

 the canal. The blood, usually a colorless liquid, is driven 

 by a chain of hearts along the back, i. e., by a pulsating 

 tube divided into valvular sacs, ordinarily eight, which 

 allow the current to flow only towards the head. As it 

 leaves this main pipe, it escapes into the cavities of the 

 body, and thus bathes all the organs. Although the blood 

 does not circulate in a closed system of blood-vessels, as in 

 Yertebrates, yet it always takes one set of channels in go- 

 ing from the heart, and another in returning. Respira- 

 tion is carried on by tracheae, a system of tubes opening 

 at the surface by a row of apertures (spiracles), generally 

 nine on each side of the body. 



The sexes are distinct, and the larvae are hatched from 

 eggs. As a rule, an Insect, after reaching the adult, or 



