THE NERVOUS SYSTEM. 15 



in the fly, probably owing to their being imbedded in the sub- 

 stance of the cephalic ganglion. No one who has once seen them 

 can doubt their far-advanced type above the ordinary ganglia of 

 insects, and the superior intelligence manifested by the insects 

 possessing them is not difficult to perceive. In fact, long before 

 I detected these organs in the fly, I felt almost sure I should 

 eventually find them, from the fact that many of the acts of this 

 insect bear evidence of some memory at least ; for instance, the 

 manner in which flies will avoid a person who is pursuing them 

 is clearly more than an ordinary reflex act. 



Nevertheless, it must be conceded that most of the acts of 

 insects are probably entirely reflex, or the result of impressions 

 from without, and this coincides with the comparative bulk of the 

 organs of sensation over that of the higher nerve centres. 



M. Faivre, * however, thinks otherwise, from the fact that in 

 his experiments on the Dytiscus, that insect always gyrated to 

 the left when the right side of the supra-cesophageal ganglion 

 was removed, and to the right when the left was operated on, from 

 which he argues the existence of voluntary power. He sup- 

 poses the supra -CDSophageal ganglion to be analogous to the 

 brain of vertebrates; but M. Faivre clearly removed the great 

 organs of sensation, and the rotatory direction pursued by the 

 insect was probably entirely due to its seeking the direction in 

 which the light proceeded, and so turning away from the 

 injured side. 



The thoracic ganglion gives off ten pairs of nerves, besides one 

 single one distributed to the abdomen, and the great trunk 

 which unites it to the cephalic ganglia. It is the homologue of 

 the thoracic and abdominal ganglia of the typical homogan- 

 gliata, and is, therefore, like the cephalic, a compound gan- 



' r M . Faivre Bur le Cerveau des Dytisques, Ann. Sc. Nat. Serie IT. 

 Tom viii. 245. 



