INSECTS. 101 
shape. The trachez connect with the exterior at various 
parts of the sides of the body, the openings (Fig. 114) being 
called spiracles, stigmata, or breathing-holes. The tubes 
extend over a greater part of the body (Fig. 115), bringing 
fresh air in contact with the blood in the tissues. This is 
even accomplished in the wings, so that they serve as 
lungs as well as organs of flight. Breathing seems to be 
accomplished by a contraction and expansion of the ab- 
dominal segments. 
Nervous System —The nervous system of insects is 
made up of a chain of ganglia that is connected by a 
double nervous cord, and occupies the ventral portion of 
the body. The portion above the cesophagus is called 
the cerebral ganglia. Fig. 112, m, shows the brain; and 
from here nerves pass to the various sense-organs. The 
ganglia below is called ¢horacic (Fig. 112, V), and sends 
nerves to the wings, 
legs, and other 
parts. 
Development, — 
Most insects are 
Fic. 116,——Larve of insects. produced by eggs, 
while some appear 
directly in the adult form. The changes through which 
the former pass are called metamorphoses. The butter- 
flies, beetles, etc., 
pass through three 
changes from the 
egg to the perfect 
insect. The first - 3 ; ae ieee 
: 1G. 117.—Cocoons. A, showing inside of co- 
shine 15 called the coon, containing the remains of a chrysalis- 
larva—grub, mag- skin. : 
got, or caterpillar 
(Fig. 116). In this form it is worm-like, has numbers of 
feet, eats voraciously, changes its skin (moults) frequent- 
ly, and finally in many instances spins a silken case or 
