ANATOMY AND PHYSIOLOGY 9 1 



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 there 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 for- 

 mer. 



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 in- 

 sects, 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 

 286). The blind cave-beetles of the genus Anophthalmus 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 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 this does not prove that 

 the colors are of no assistance to the insect, though it does show that 

 they are not the sole attraction the odor also being an important 

 guide'. The honey bee is able to distinguish color patterns, according 

 to the experiments of C. H. Turner. 



Problematical Sense Organs. As all our ideas in regard to the 



