108 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



tubes (Fig. 278). 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 above the mouth. The main nerve 

 cord lies along the ventral side of the body, and bears 

 several large ganglia ; besides this, there is a visceral 

 nerve representing, in function, the sympathetic system 

 of vertebrates. The digestive apparatus consists of a 

 pharynx, gullet (to which a crop is added in the fly, 

 butterfly, and bee tribes), gizzard, stomach, and intestine 

 (Figs. 239, 240, 241). There are no absorbent vessels, 

 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 toward 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 vertebrates, yet it always takes one set of 

 channels in going from the heart, and another in return- 

 ing. Respiration 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 (Figs. 

 276, 277, 278). 



The sexes are distinct, and the larvae are hatched from 

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



