HEXAPODA. 
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 Hies 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 cesophagns , 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 larv®. 
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, //, and Plate II, 1. This extends from near 
the caudal end of the abdomen through the thorax into 
the head. That part of this system that lies in the abdomen 
Fig. 82.—Dia¬ 
gram of a 
part of the 
heart of a 
May-beetle. 
