]86 ELEMENTARY ZOOLOGY 



commissures lying close together and running from the 

 head to the next to the last abdominal segment, which 

 bears a series of segmentally disposed ganglia, each 

 ganglion being composed of two ganglia more or less 

 nearly completely fused. There is, in addition, a lesser 

 system called the sympathetic system, which comprises a 

 few small ganglia and certain nerves which run from 

 them to the viscera. The function of the nervous system 

 of insects reaches a very high development among the 

 so-called "intelligent insects" and certain extraordi- 

 narily complex and interesting instincts are possessed by 

 many forms. The social or communal habits of the ants, 

 bees, and wasps and the habits connected with the deposi- 

 tion of the eggs and the care of the young exhibited by 

 the digger wasps and other insects are of extreme 

 specialization. The organs of special sense are highly 

 specialized, the sense of smell (fig. 48) reaching in par- 

 ticular a high degree of perfec- 

 tion. One of the compound eyes 

 (figs. 49 and 50) may contain as 

 many as 30,000 distinct eye- 

 elements or ommatidia, but the 

 sight is probably in no insect 

 very sharp or clear. Among 

 insects there are organs of hear- 

 ing of two principal kinds. In 

 FIG. 50. -Part of cornea, snow- one kind the organ for taking up 



ing facets, of the compound ^ ie sound-waves is a PTOUD of 

 eye of a horse-fly (Therioplec- & 



tes sp.). (Photo-micrograph vibratile hairs usually situated on 

 by Geo. O. Mitchell.) the antennJE> as j s the case with 



the mosquito; in the other kind, it is a stretched mem- 

 brane or tympanum such as is found in the fore leg of a 

 cricket or katydid or on the first abdominal segment of 

 the locust (fig. 51) 



The sexes are distinct in insects, and there is often a 



