34 



AN INTRODUCTION TO ENTOMOLOGY. 



The Nervous System. — The central part of the nervous system, as 

 already indicated, consists of a ganglion in the head above the oesoph- 

 agus, and of a series of ganglia (typically one for each segment of 

 the body) lying on the floor of the body-cavity, and connected by 

 two longitudinal cords. In the head, one of these cords passes on 

 each side of the oesophagus, from the brain to another ganglion in 

 the head below the oesophagus, thus forming a nervous collar about 

 the alimentary canal. From each ganglion nerves arise, which sup- 

 ply the adjacent parts ; and from the thoracic ganglia nerves extend 

 to the legs and wings. This series of ganglia is really a double one ; 

 but each pair of ganglia are more or less closely united on the mid- 

 dle line of the body, and often appear as a sin- 

 gle ganglion. Fig. 44 gives a general view of 

 the nervous system of Corydalus cornutus as 

 represented by Leidy. From the brain (a) two 

 large nerves extend to the compound eyes, 

 and a smaller pair to the antennae ; the sub- 

 cesophageal ganglion (b) supplies the mouth- 

 parts with nerves ; and each of the thoracic 

 and abdominal ganglia supplies its segment of 

 the body. 



In Corydalus (Fig. 44) the eighth and ninth 

 pairs of abdominal ganglia are united, and 

 drawn cephalad into the seventh abdominal 

 segment. The same thing is presented by 

 the larva of Cossus (Plate I., 10). This is an 

 illustration of what has been termed ceph- 

 alization of the nervous system. In the adults 

 of insects of the higher orders this cephaliza- 

 tion of the nervous system is carried to a 

 great extent. In some cases the abdominal 

 ganglia are fused into a common mass and drawn cephalad into the 

 thorax. Between this and the form presented by Corydalus every 

 gradation exists. With the higher insects the nervous system un- 

 dergoes marked changes during the life of the individual. In a 

 caterpillar it is of the form shown in Plate I.; in the pupa state 

 it becomes somewhat shortened ; and in the adult the abdominal 

 ganglia are all or nearly all united with the thoracic ganglia into a 

 common nervous mass.* 



Fig. 44.— Nervous system of 

 Corydalus. 



* See figures by Newport, Cycl. Anat. and Phys. II. pp. 963-965. 



