114 ENTOMOLOGY 



pigment cells elongate around the ommatidium and their pig- 

 ment granules absorb from the cone cells and rhabdom the 

 excess of light. If the light is weak, they shorten, and absorb 

 but a minimum amount of light. 



Origin of Compound Eye. The compound eye is often 

 said to represent a group of ocelli, chiefly for the reason that 

 externally there appears to be a transition from simple eyes, 

 through agglomerate eyes, to the facetted type. This plausi- 

 ble view, however, is probably incorrect, for these reasons 

 among others. In the ocellus, a single lens serves for all 

 the retinulse, while in the compound eye there are as many 

 lenses as there are retinulse. Moreover, ocelli do not pass 

 directly into compound/ eyes, but disappear, and the latter arise 

 independently of the former. 



Probably, as Grenacher holds, both the ocellus and the com- 

 pound eye are derived from a common and simpler type of 

 eye are " sisters," so to speak, derived from the same 

 parentage. 



Perception of Light through the Integument. In vari- 

 ous insects, as also in earthworms, blind chilopods and some 

 other animals, light affects the nervous system through the 

 general integument. Thus eyeless dipterous larvae avoid the 

 light, or, more precisely, they retreat from the rays of shorter 

 wave-length (as the blue), but come to rest in the rays of 

 longer wave-length (red), as if they were in darkness (see 

 page 350). The blind cave-beetles of the genus Anophthal- 

 mus react to the light of a candle (Packard). Graber found 

 that a cockroach deprived of its eyesight could still perceive 

 light; but Lubbock found that an ant whose eyes had been 

 covered with an opaque varnish became indifferent to light. 



Color Sense. Insects undoubtedly distinguish certain col- 

 ors, though their color sense differs in range from our own. 

 Thus ants avoid violet light as they do sunlight, but probably 

 cannot distinguish red or orange light from darkness; on 

 the other hand, they are extremely sensitive to the ultra-violet 

 rays, which make no sensible impression upon us. Honey 



