THE ANATOMY OF INSECTS. 35 



In addition to the central nervous system described above, there 

 is what has been termed the visceral nervous system. This consists 

 of two parts, — the oesophageal nerves, and the respiratory nerves. 



There are two sets of oesophageal nerves, — the unpaired and the 

 paired. From each of the nerve-cords that connect the brain with 

 the subcesophageal ganglion there arises a nerve, which extends 

 cephalad upon the oesophagus; these unite to form the frontal 

 ganglion (Fig. 44, c).* From this ganglion a recurrent nerve passes 

 caudad through the oesophageal collar between the aorta and the 

 oesophagus. A short distance caudad of the brain, this nerve, in 

 some insects, enters a small ganglion, from which branches extend 

 to the sides of the alimentary canal ; in other insects it forms a 

 number of plexuses in the muscular layer of that organ without 

 apparent ganglia. These ganglia and nerves comprise the unpaired 

 system. The paired oesophageal nerves arise on either side from 

 the caudal aspect of the brain, and swell out at the sides of the 

 oesophagus to form ganglia (Fig. 44, d) which also supply nerves 

 to the walls of the alimentary canal. 



The respiratory nerves are not represented in the figure of Cory- 

 dalus ; but they are well shown in that of the larva of Cossus (Plate 

 I.). They consist of the following parts: In each segment of the 

 thorax and abdomen there is a short longitudinal cord between the 

 two cords of the central nervous system. Each of these cords en- 

 larges into a ganglion, from which branches extend laterally to the 

 sides of the body, where they supply the tracheal trunks and the 

 muscles of the spiracles. 



The Organs of Special Sense. — It is probable that insects possess 

 the five senses known to us ; and perhaps they have others the na- 

 ture of which we cannot conceive. Even in the case of the five 

 senses, the range of perception may be very different from ours. 

 Thus Lubbock has shown that ants perceive the ultra-violet rays, 

 which are invisible to us. There is, however, a great variation in 

 the degree of development of the different senses in different in- 

 sects ; for example, some are furnished with wonderful eyes, while 

 others are blind. It is probable that in many cases the great devel- 

 opment of one sense is correlated with a slight development of some 

 other. As an illustration, we find that in the Dragon-flies and Ci- 

 cadas, which are essentially directed by sight, the antennae are rudi- 



* See William C. A'ruuss, On the Nervous System of the Head of the Larva of Co- 

 rydalus cornutus : an extract from a thesis in Entomology presented to the faculty of 

 Cornell University for the Baccalaureate in Science: Tivchr, vol. IV. pp. 170-184. 



