8 yJNA TO J/]' AND MORPHOLOG Y OF INSECTS. 



meshes of which the blood circulates, although there are 

 special cavities, blood, or more properly lymph, sinuses in 

 various parts of the body. The blood of insects is in fact 

 equivalent to the lymph of vertebrates, which it closely re- 

 sembles in its physical and microscopic characters. 



The somatic nervous system* consists of two parts — a 

 cephalic nerve-centre or brain, which lies in front of the mouth ; 

 and a double ganglionated cord, which is situated on the 

 ventral aspect of the alimentary canal. The dorsal vessel, or 

 heart, is placed immediately beneath the skin of the back, 

 hence the ventral aspect is known as the neural, and the dorsal 

 as the haemal surface of the animal. 



The whole body is segmented transversely into a number of 

 annuli or somites, and these are grouped, in the adult form or 

 imago, into three distinct regions, known respectively as the 

 head, thorax, and abdomen. 



The somites of the head and thorax usually each possess a 

 pair of ventral appendages, jaws or limbs, and one or two 

 of the thoracic somites are generally provided with dorsal 

 appendages — wings. A somite with its appendages is spoken 

 of as a metamere ; the terms 'somite' and 'metamere' are 

 synonymous when all appendages are wanting. 



The external surface of the body is covered and protected 

 by a cuticle, impregnated more or less completely b}' a horny 

 substance called chitin, which renders it very dense and 

 elastic, and usually forms a complex exoskeleton. The cuticle 

 often consists of many super-imposed laminae, and is formed 

 by an underlying la3'er of cells known as the hypodermis, 

 which represents the external cellular epithelium of the mollusca 

 and the vertebrata, and which corresponds with the epiblast 

 of the embryo (see Fig. i). 



The cells of the hypodermis are usually short hexagonal 

 prisms, cemented together by their edges. In the larva the 

 size of these cells increases with its growth, and in the adult 

 larva they are generally very much larger than the correspond- 



* See Chap. IV., Sec. 7. 



