96 INTRODUCTION TO ZOOLOGY. 



It is unnecessary here to dwell on the nervous system of 

 insects; their general character is given in that of the class 

 (page 57). In different families of insects, the ganglions, or 

 nervous centres, whence nerves are sent to the several organs, 

 are different in their number, and in the amount of con- 

 centration which they present (Fig. 70); and, as might 



Fig. 70. NERVOUS SYSTEM OF INSECTS. 



naturally be expected, they undergo modifications, according 

 to the changing form and powers of the same insect, in its 

 different stages of development. 



With regard to the external senses, insects differ from the 

 higher animals in the possession of two processes appended to 

 t'he head, and which, in 'the Butterfly, resemble delicate horns 

 terminated by a Imob. The entomologist * calls them antenna / 



Fig. 70. A, Nervous system of an Ear- wig. B, Of a Grasshopper. C, Of a 

 Stag-beetle. D, Of a Field-bug (Pentatoma). a, Brain. b, c, The Optic nerves. 

 d, Thoracic ganglions. e, Abdominal ganglions. 



* Entomology is that department of Natural History which treats of 

 insects. 



