LESSON 37.] NUTRITION IN INSECTS. 131 



rated by the pylorus. Sometimes this intestine is wanting, and in 

 that case the stomach joins the colon. 



606. The last division of the alimentary canal is THE COLON ; it 

 is usually divided from the preceding portion of the intestine by a 

 valve, which completely closes its aperture. The colon is subject to 

 great variety of form ; it is found cylindrical, clavate (club-shaped), 

 sac-shaped or longitudinally folded. The situation of the colon is 

 always determinate found at the apex of the abdomen, and sur- 

 rounded by the last segments. 



607. THE CCECUM. In many insects a blind, sac-shaped appen- 

 dage is found in connection with the colon; this is the co3cum. 

 Like the intestine to which it is attached, this is also subject to great 

 variation in form ; this portion of the canal is peculiar to the carniv- 



'orous tribes, although occasionally found in the butterflies. The ex- 

 act function of this organ is not known in any animal, but (in con- 

 junction with the colon) in the higher animals it is supposed to be as 

 a second stomach. 



608. THE BILIARY VESSELS. These are narrow, filiform tubes, 

 which open into the duodenum, if present, or the ileum, where the 

 former is wanting ; the ends not connected to the intestines are either 

 free and closed, or pass into each other, and thus form one vessel, 

 which pierces the intestine at both ends. 



609. THE SALIVARY GLANDS. These are various in form, num- 

 ber, and the situation where their secretion is poured. In some in- 

 sects they are tubular, in others vesicular, and in another series lobu- 

 lated. In some the secretion is emptied into the mouth, in others 

 the proboscis, and lastly, directly into the stomach. Some insects 

 have one pair of these glands, some two, and others (like the higher 

 animals) three pairs. Illustrations of them will be hereafter given. 



610. Before proceeding further, it becomes necessary to show the 

 principles of classification of insects, which will render that portion 

 of the subject very simple, which, without its aid, would be involved 

 in great confusion. As the simplest, but by no means the best, the 

 Linnsean system will be adopted. 



611. This naturalist selected the structure of the wings, thence 

 called the Alary (wing) system, as the basis of his classification ; he 

 had seven orders, the last being wingless. 



In addition to the names of the orders, and translation of the 

 Greek compound names, an illustration of the structure of the wings 

 in each order is appended. 



