16 OUTLINES OF ENTOMOLOGY. 



ments, and are termed the spiracles or stigmata. They vary in number 

 from two in some water insects to eleven, usually nine, pairs, and are often 

 quite conspicuous, especially in larvae. In their perfect state insects 

 breathe mainly through the first four or five pairs of orifices, and conse- 

 quently most species may be easily suffocated and killed by a sharp 

 pinch or continued pressure on the thorax. Many insects have large 

 vesicles connected with the trachea which they innate when about to 

 fly, thus lessening their specific gravity and enabling them to continue 

 long on the wing with less muscular exertion than would otherwise be 

 necessary. The breathing organs of aquatic insects are termed branchia* 

 They are analogous to the gills of fishes and present many very sin- 

 gular forms. 



The organs of Nutrition are few in number and simple in structure 

 They consist principally of the mouth, already described, by which the 

 food is seized and masticated, and, internally, of a large, long tube in 

 which digestion takes place. This tube is the alimentary. canal, and 

 occupies a central position in the body of the insect. In the more 

 lowly species it is simply a straight duct or sac extending the whole 

 length of the body ; but in the highly organized forms it is contracted 

 at intervals so as to form several chambers, each of which has a dis- 

 tinct function. It terminates in a convolution of minute tubes that are 

 supposed to represent the liver and the small intestines of higher ani- 

 mals. The sesophagus or gullet opens into the first chamber or cavity 

 of the central canal, which is analogous to the crop. This in turn com- 

 municates with a smaller cavity, which is ridged internally or covered 

 with hard, tooth-like points, and performs the office of a gizzard, from 

 which the food passes into the largest alimentary division, representing 

 the true stomach. In the latter are secreted the gastric and pan- 

 creatic fluids, which, mingling with the comminuted food, prepare it to 

 nourish the system. The digestive powers of insects are enormous in 

 proportion to their size, many species of herbivorous larvae being 

 capable of digesting more than twice their own weight of leaves in the- 

 course of a day. 



The organs of secretion, aside from those that elaborate the fluids 

 necessary to digestion, consist mainly of the salivary, odoriferous and 

 poison glands. The salivary glands, so called, are greatly developed in 

 such insects as the silk-worm, and the bee and wasp. They consist of 

 two tubes, running parallel to the anterior portion of the alimentary 

 canal. In the silk- worm and other larvae of the same family these 

 glands secrete the gum which, when drawn out through a little pointed 

 tube beneath the mouth, becomes the beautiful and valuable fiber 

 known 



