THE INTERNAL ANATOMY OF INSECTS 95 



two longitudinal cords: one of these masses, the brain, lies in the 

 head above the alimentary canal ; the others are situated, one in each 

 segment, between the alimentary canal and the layer of muscles of the 

 ventral side of the body; the two cords connecting these masses, or 

 ganglia, pass one on each side of the oesophagus to the brain. The 

 reproductive organs (r) lie in the cavity of the abdomen and open near 

 the caudal end of the body. The respiratory organs are omitted from 

 this diagram for the sake of simplicity. We will now pass to a more 

 detailed study of the different systems of organs. 



I. THE HYPODERMAL STRUCTURES 



The active living part of the body-wall is the hypodermis, already 

 described in the discussion of the external anatomy of insects. In 

 addition to the external skeleton, there are derived from the hypo- 

 dermis an internal skeleton and several types of glands. 



a, THE INTERNAL SKELETON 



Although the skeleton of an insect is chiefly an external one, there 

 are prolongations of it extending into the body-cavity. These 

 inwardly directed processes, which serve for the attachment of 

 muscles and for the support of other viscera are termed collectively 

 the internal skeleton or endo-skeleton. The internal skeleton is much 

 more highly developed in adult insects than it is in the immature 

 instars. 



Sources of the internal skeleton. — The parts of the internal skele- 

 ton are formed in two ways : first by the chitinization of tendons of 

 muscles; and second, by invaginations of the body- wall. 



Chitinized tendons. — Chitinized tendons of the muscles that move 

 the mouth-parts, of muscles that move the legs, and of other muscles 

 are of frequent occurrence. As these chitinized tendons help support 

 the internal organs they are considered as a part of the internal 

 skeleton. 



Invaginations of the body-wall or apodemes. — The second and more 

 important source of the parts of the internal skeleton consists of 

 invaginations of the body- wall. Such an invagination is termed an 

 dpodeme. The more important apodemes, if not all, arise as invagina- 

 tions of the body-wall between sclerites or at the edge of a sclerite on 

 the margin of a body-segment; although by the fusion of sclerites 

 about an apodeme, it may appear to arise from the disc of a sclerite. 



