HEX A POD A. 71 



And the rapidity of their action is even more wonderful than their 

 strength. This rapidity is best illustrated by the muscles that move 

 the wings. Every one has observed gnats and other flies poising in mid 

 air by a movement of the wings so rapid that the eye cannot follow it. 

 Physicists have been able, however, to count these vibrations by de- 

 termining the pitch of the musical note produced in this way. And 

 they tell us that certain gnats vibrate their wings 15,000 times per 

 second. 



The Alimentary Canal. The typical position of this is represented 

 in the diagram (Fig. 80); and on Plate III, illustrating the anatomy 

 of a cockroach, its form in that insect is shown. In larvae it is a 

 nearly straight tube, extending from one end of the body to the other. 

 But in adult insects it is usually much longer than the body, and is 

 consequently more or less folded. It is composed of parts differing 

 in form and use. To these parts names have been given similar to 

 those used to designate the corresponding parts in higher animals ; 

 thus we distinguish a pharynx, an oesophagus, sometimes a crop, some- 

 times a gizzard, a stomach, a small intestine, and a large intestine. 



The Adipose Tissue, or Fat. On opening the body of an insect, 

 especially of a larva, one of the most conspicuous things to be seen is 

 fatty tissue, in large masses. These often completely surround the 

 alimentary canal, and are held in place by numerous branches of the 

 tracheae with which they are supplied. Other and smaller masses of 

 this tissue adhere to the inner surface of the abdominal wall, in the 

 vicinity of the nervous system, and at the sides of the body. In a full- 

 grown larva of Corydalis cornuta I have found the adipose tissue to 

 be greater in bulk than all of the other organs found inside of the 

 muscular walls of the body. In adult insects it usually exists in much 

 less quantity than in larvaa. 



The Blood-vessels. In insects all parts of the body cavity that are 

 not occupied by the internal organs are filled with blood. 

 Thus the alimentary canal is completely surrounded with 

 blood, and all the spaces between the muscles are filled *"" 

 by this fluid. This is a very different arrangement from 

 what occurs in our own body, where the blood is con- 

 tained in a system of tubes, the arteries and the veins. 

 We find, however, that insects are not entirely deprived of 

 blood-vessels. For there is one which lies above the ali- 

 mentary canal, just within the middle line of the back. 

 See Figure 80, h, and Plate II, i. This extends from near P art of the 



heart of a 



the caudal end of the abdomen through the thorax into May-beetle, 

 the head. That part of this system that lies in .the abdomen 



