IN SECT A. 21 



the rods and cones of the vertebrate eye) are the nerve- 

 end cells, because the ultimate tibrils of the optic nerve 

 terminate in them, and because the cells are always pig- 

 mented, either by a diffuse fluid — retinal purple — or by 

 pigment in granules, or both. The discovery that the 

 retinulae contain a true retinal purple was made by Ley dig 

 in 1864. The retinulae with portions lying back of them 

 constitute, according to Hickson, the retina of insects. 



The experiments of Plateau,^ however, point to the 

 conclusion that insects cannot distinguish the forms of 

 objects, or, at best, can distinguish them very poorly. 

 The conclusions reached in this paper are extremely 

 interesting. The experiments were made with certain 

 Diptera, Hymenoptera, Lepidoptera, Odonata (Dragon- 

 flies), and Coleoptera, in a room which only admitted 

 light through two orifices. One of these orifices was 

 large enough for the insect to pass through, while the 

 other consisted of many small openings, no one of which 

 would allow the passage of the insect. In each experiment 

 the intensity of the light of the two orifices was deter- 

 mined. If the insects could distinguish the form of the 

 two openings, it was assumed that they would fly to the 

 one which was large enough to allow them to escape. Re- 

 peated experiments proved that they did not take into 

 account this difference in form, but flew to the orifice 

 which was most luminous. These experiments also tended 

 to prove that the ocelli of the Odonata, Hymenoptera, and 

 of many Diptera are only rudimentary organs, and of 

 almost no use to their possessors. In these experiments 

 the animals were placed under such conditions that they 

 could use no sense excepting that of vision, and could not 

 be guided either by the color or odor oT bodies. Certain 



^ " Recherches experimentales sur la Vision chez les In- 

 sectes," Bulletins de D Academie Royale de Belgique, 3me serie, 

 t. X., No. 8, 1885. 



