138 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



If the caterpillar is of a kind that spins a coccoon when it is 

 ready to change into a chrysalis, the silk glands will be found as 

 a pair of long, smooth, rather thick, whitish cords lying one 

 on each side of the alimentary canal and running forward to 

 the mouth. Another pair of smaller, shorter tubes, not ex- 

 tending farther back than about the beginning of the ventri- 

 culus are the salivary glands. The silk glands are, indeed, 

 only an enlarged and modified second pair of salivary 

 glands. 



Respiratory System. In taking out the adipose tissue and 

 alimentary canal there will be noted many dark little thread- 

 like processes which are in reality fine tubes, called trachea. 

 By tracing them to their origin they will be found to arise 

 from larger tracheae, which in turn are given off from main 

 longitudinal trunks. There are two or four of these trunks, 

 one or two on each side of the body, and from them arise 

 not only the branches that by repeated subdividing extend 

 to all parts of the body, but short strong lateral trunks 

 that run to small openings called spiracles, or stigmata, in the 

 sides of the body. In most caterpillars nine pairs of spiracles 

 will be found, one pair on the prothoracic segment and the 

 others on the abdominal segments, one pair to each. The 

 spiracles and tracheae are the organs of the respiratory system of 

 the caterpillar, and similar organs, although varying much in 

 number and arrangement, will be found in all insects, except 

 a few very small and thin-skinned ones, which respire directly 

 through the skin. The spiracles show on the outside of the 

 body as small blackish spots, but are actually small openings 

 in the body-wall, provided usually with valves or fringes of 

 hairs to keep out foreign particles. They allow air to pass into 

 the interior system of tracheae, and carbon dioxide to pass out. 

 The tracheae, although thin-walled and delicate, especially 

 the finer ones, are lined with a thin chitinous membrane in 

 which are spiral thickenings which hold them open and give 

 them a certain necessary elasticity. When the insect contracts 

 certain muscles lying as longitudinal and circular bands along 

 the inner side of the body-wall, the pressure of the body con- 

 tents forces the tracheae to close and expels the gas in them out 



