278 ANATOMY. 



closely contiguous to both the main stems ; each diverges from the 

 main stem in its course to about half its length, and then approaches 

 it again as far as the spot where the main stem passes into the first 

 thoracic ganglion, and then rejoins it. A delicate auxiliary branch of 

 this exterior connecting nerve originates from it closely beyond its 

 middle, passing to the first radiating nerve of the first thoracic gang- 

 lion, which it joins. The second connecting nerve (the same, i, i.) 

 originates in the same manner from the first ganglion of the thorax as 

 the first does from the cerebellum, and unites itself at a right angle 

 with the first radiating nerve of the second thoracic ganglion. At 

 their point of union a small ganglion is formed, from which two new 

 radiating branches proceed, distributing themselves between the thoracic 

 muscles. The third auxiliary connecting nerve (the same, k, &.) springs 

 from the posterior end of the second thoracic ganglion, and passes into 

 the third ganglion, forming an arch near the main stem, from which 

 from two to three small nerves originate, and distribute themselves 

 to the muscles. An auxiliary nerve connecting the third thoracic 

 ganglion with the first abdominal one is not to be detected. 



187- 



If we turn back from this general inspection of the auxiliary connect- 

 ing nerves of these ganglia, which, as far as I know, have not hitherto been 

 observed in any other insect, and certainly do not exist in many, par- 

 ticularly the larvae of Lepidoptera, as may be adduced from Lyonet's 

 accurate anatomy of the caterpillar of the great willow moth, to the 

 diiferences of the chief form of the nervous system, we may adopt the 

 following as a very general law : 



The ventral cord has as many ganglia as there are freely moveable 

 divisions of the body. 



This law is everywhere confirmed. The caterpillars of the Lepi- 

 doptera, whose similar segments have an equal motion, have as many 

 ganglia as segments. In the Diptera, in which the three segments of 

 the thorax are united into one, we find but a single large ganglion ; 

 lastly, in the larvae whose thick fat bodies exhibit no distinct segments, 

 the ganglia entirely disappear, and instead of a ganglionic we here find 

 a simple thoracic cord, from which the fine nerves pass off on each side. 

 We will inspect this in greater detail in the several forms of the 

 nervous system and their transformation during the metamorphosis. 



A simple short ventral cord, destitute of ganglia, is found in many 



