36 



THE STUDY OF INSECTS 



an oesophagus, sometimes a crop, sometimes a gizzard, a stomach, a small 

 intestine, and a large intestine. 



The adipose tissue, or fat. — On opening the body of an insect, espe- 

 cially 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 Corydalus 

 cornutus the adipose tissue is often greater in bulk 

 than all of the other organs found inside of the mus- 

 cular walls of the body. In adult insects it usually 

 exists in much less quantity than in larvae. 



The circulatory system. — In insects the circula- 

 tory system is not a closed one, the blood flowing in 

 vessels during only a part of its course. The greater 

 part of the circulation of this fluid takes place in the 

 cavities of the body and of its appendages, where it 

 fills the space not occupied by the internal organs. 



Almost the only blood-vessel that exists in insects 

 lies just beneath the body-wall, above the alimentary 

 canal (Fig. 67, h). It extends from near the caudal 

 end of the abdomen through the thorax into the 

 head. That part of it that lies in the abdomen is 

 the heart; the more slender portion, which traverses 

 the thorax and extends into the head is the aorta. 

 On each side of the heart, there is a series of 

 triangular muscles extending from the heart to the 

 lateral wall of the body. These constitute the dorsal 

 diaphragm or the wings of the heart. 



The heart is a tube, which is usually closed at its 

 posterior end; at its anterior end it is continuous 

 with the aorta. The heart is divided into chambers 

 (Fig. 69). The number of these chambers varies 

 greatly in different insects; in some, there is only 

 one, in others, as in the cockroach, there are as many 

 as thirteen, but usually there are not more than 

 eight. The blood is admitted to the heart through slit-like openings, the 

 ostia of the heart; usually there' is a pair of ostia in the lateral walls of each 

 chamber. Each ostium is furnished with a valve-like structure which 

 closes it when the chamber contracts. 



When a heart consists of several chambers, they contract one after 

 another, the wave of contraction passing from the caudal end of the heart 

 forwards. As the valves between the chambers permit the blood to move 

 forward but not in the opposite direction, the successive contractions of 

 the chambers cause the blood received through the ostia to flow toward 

 the head, into the aorta. The blood flows from the open, cephalic end 

 of the aorta and passes in quite definite streams to the various parts of 

 the body-cavity and into the cavities of the appendages. These streams, 

 like the ocean currents, have no walls but flow in the spaces between 

 the internal organs. After bathing these organs, the blood returns to the 

 sides of the heart, which it enters through the ostia. 



Fig. 69. — Heart of May- 

 beetle (after Straus Durck- 

 heim): a, lateral aspect of 

 aorta; b, interior of heart 

 showing valves; c, ventral 

 aspect of heart and wins- 

 muscles — the muscles are 

 represented as cut away 

 from the caudal part of the 

 heart; d, dorsal aspect of 

 heart. 



