INSECTS. 



101 



shape. The tracheae 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 oesophagus is called 

 the cerebral ganglia. Fig. 112, #z, shows the brain; and 

 from here nerves pass to the various sense-organs. The 

 ganglia below is called thoracic (Fig. 112, JV), and sends 



nerves to the wings, 

 legs, and other 

 parts. 



Development. 

 Most insects are 

 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 

 stage is called the 

 larva grub, mag- 

 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 



FIG. 116. Larvae of insects. 



FIG. 117. Cocoons. A, showing inside of co- 

 coon, containing the remains of a chrysalis- 

 skin. 



