128 



AN INTRODUCTION TO ENTOMOLOGY 



abdominal ganglion; and each element consists of a median nerve 

 extending from the ganglion of its origin caudad between the two 

 connectives, a pair of lateral branches of this median nerve, and one 

 or more ganglionic enlargements of each lateral branch. Frequently 

 the median nerve extends to the ganglion of the following segment. 

 A simple form of this system exists in the larva of 

 Cossus ligniperda (Fig. 122); and a more compli- 

 cated one, in Locusta viridissima (Fig. 145). 



From each lateral branch of the median nerve a 

 slender twig extends to the closing apparatus of the 

 tracheae. 



d. THE PERIPHERAL SENSORY NERVOUS SYSTEM 



Immediately beneath the hypodermal layer of the 

 body-wall, there are many bipolar and multipolar 

 nerve -cells whose prolongations form a network of 

 nerves; these constitute the peripheral sensory 

 Fig. I45- p ar t O f nervous system or the subhypodermal nerve-plexus. 



the ventral chain The fine nerves of this system are branches of 

 of ganglia of Lo- . ......... , 



custa viridissima larger nerves which arise in the central nervous sys- 



and of the ven- tern; and the terminal prolongations of the bipolar 

 nervous sy s- nerve-cells innervate the sense-hairs of the body-wall. 



tem; g, ganglion Figure 146 represents a surface view of a small 

 of the central ,. , , . , ., 



nervous system; P ar t * the peripheral sensory nervous system or the 



n, nerve; c, con- silkworm, Bombyx mori, as figured by Hilton ('02) ; 

 dian nerve of the the bases of several sense hairs are also shown. The 



sympathetic sys- details of this figure are as follows: h, h, h, the bases 



tem; gs, gang- r , 1-1 n 



lion of the sym- o * sense -hairs; s, s, s, bipolar nerve-cells; m, m, m 



pathetic system multipolar cells ; w, w, , nerves. All of these struct- 



(From Berlese). ., -. i r^r 1 



ures are united, forming a network. Of especial 



interest is the fact that the terminal prolongation of each bipolar 

 nerve -cell enters the cavity of a sense -hair and that the other pro- 

 longation is a branch of a larger nerve which comes from the central 

 nervous system. 



The peripheral sensory nervous system is so delicate that it can 

 not be seen except when it is stained by some dye that differentiates 

 nervous matter from other tissues. For this purpose the intra vitam 

 methylen blue method of staining is commonly used. 



