THE ANATOMY OF INSECTS. 



part of the digestive system, but are organs of offence, the secretion 

 being acrid or otherwise offensive. 



Insects take the greater part of their food during the larva or 

 nymph state ; for it is in this period that they acquire their growth. 

 In some cases, as with the May-flies arid the Bombycid moths, no 

 nourishment is taken during the adult state. With certain larvae 

 (larvae of the higher Hymenoptera, Pupipara, and Ant-lions), the 

 stomach ends blindly, and does not communicate with the intestine. 

 In the Ant-lions the rectum is transformed into a silk gland ; and 

 the silk of which the cocoon is made is spun from the anus. 



The Adipose Tissue. 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 ali- 

 mentary 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 face of the abdominal wall, in the 

 vicinity of the nervous system, and at the sides of the body. It also 

 abounds in the pericardial sinus. In a full- 

 grown larva of Corydalus cornutus 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 in- 

 sects it usually exists in much less quantity 

 than in larvae. 



The Circulatory System. In insects the 

 circulatory 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 its appendages, where the blood 

 fills the space not occupied by the internal 

 organs. The only blood-vessels that exist in 

 these animals lie just beneath the body-wall, 

 above the alimentary canal (Fig. 36, //). They 

 extend from near the caudal end of the ab- 

 domen through the thorax into the head. 

 That part of this system that lies in the ab- 

 domen is usually termed the heart, and con- 

 sists of a series of chambers corresponding to 

 the segments of the body (Fig. 40, d). The 

 number of these chambers varies, but it is rarely more than eight. 



Fig. 40. Heart of May-beetle 

 (af ter S traus- D u re khe i m ). a , 

 lateral aspect of aorta; , 

 interior of heart showing 

 valves; c, ventral aspect of 

 heart and wing-muscles the 

 muscles are represented as 

 cut away from the caudal 

 part of the heart; </, dorsal 

 aspect of heart. 



