ARTIIROPODA. 
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 Vertebrates. 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 
Vertebrates, 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 
