100 ENTOMOLOGY 



the ommatidium and their pigment 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. In diurnal insects the 

 pigment is adapted to absorb an excess of light; in nocturnal insects, 

 on the contrary, it is adapted to permit a maximum amount of light 

 to reach the retinal cells. 



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 plausible view, however, is probably incor- 

 rect, for these reasons among others. In the ocellus, a single lens serves 

 for all the retinulae, while in the compound eye therfc are as many lenses 

 as there are retinulae. Moreover, ocelli do not pass directly into com- 

 pound eyes, but disappear, and the latter arise independently of the 

 former. 



Probably, as Grenacher holds, both the ocellus and the compound 

 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 various 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 307). The blind cave-beetles of the genus Anophthalmus react to 

 the light of a candle (Packard). Graber found that a cockroach de- 

 prived 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 colors, 

 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. Honey bees frequently select blue 

 flowers: white butterflies (Pieris) prefer white flowers, and yellow 

 butterflies (Colias) appear to alight on yellow flowers in preference to 

 white ones (Packard). In fact, the color sense is largely relied upon by 

 insects to find particular flowers and by butterflies to a large extent to 

 find their mates. To be sure, insects will visit flowers after the brightly 

 colored petals have been removed or concealed, as Plateau found, but 



