SLIME SLUGS, MYRIAPODS AND INSECTS 141 



cords or connections, and the knots, the ganglia, or nerve cen- 

 ters. The ganglion in the head above the esophagus is called 

 the brain, and from it nerves run to the eyes and antennae. 

 From the head ganglion under the esophagus nerves run to the 

 mouth-parts. From the ganglia in the thoracic segments 

 nerves run to the legs and to the strong thoracic muscles that 

 move the legs. In insects with wings, nerves run from these 

 .ganglia also to the wing muscles. From the ganglia in the 

 abdomen nerves run to the various body organs such as ali- 

 mentary canal, dorsal vessel, tracheae, muscles, etc. Thus 

 although the head ganglia of an insect may be looked on as 

 the most important nerve centers of the body, and one is 

 called the brain, by analogy with the brain of vertebrate 

 animals, yet really each ganglion is a little brain for its own 

 part of the body, and there is a good deal more independence 

 about the control of the different parts of the insect's body than 

 there is in the vertebrate's body. 



Although the ganglia and connecting longitudinal cord, or 

 commissure, seem to be single knots and a single thread, they 

 are in reality all double, each ganglion consisting of a pair 

 fused together on their inner faces, and the connective com- 

 missure also is composed of two cords lying so close together 

 as to seem but one. 



In most insects there are not as many ganglia as we find in 

 the caterpillars, the reduction in number being brought about 

 not so much by the loss as by the fusion of ganglia. The 

 typical six or seven abdominal ganglia may be fused to form 

 but two or three or even one, and the three thoracic ganglia 

 are also often fused to form a single one. In certain highly 

 specialized insects, indeed, all the abdominal and thoracic 

 ganglia join to form one large thoracic nerve center, or "body 

 brain," as it has been called. Only in young insects and in 

 adults belonging to generalized or primitive species, are there 

 separate ganglia for most of the segments of the body. 



Besides the central nervous system, most insects have also a 

 sympathetic nervous system, which usually consists of a very 

 small ganglion just in front of the brain, and one or two small 



