260 TEXT-BOOK OF ENTOMOLOGY 



both with or without ocelli, in addition to the compound eyes, and with the 

 same results. 



From repeated experiments on flies, bees, etc., butterflies and moths, dragon- 

 flies and beetles, Plateau concludes that insects with compound eyes do not notice 

 differences in form of openings in a half-darkened room, but fly with equal readi- 

 ness to the apparently easy and apparently difficult way of escape ; that they are 

 attracted to the more intensely lighted opening, or to O7ie with apparently 

 greater surface ; hence he concludes that they cannot distinguish the form of 

 objects, at least only to a very slight extent, though they readily perceive objects 

 in motion. 



One result of his experiments is that insects only utilize their eyes to choose 

 between a white luminous orifice in a dark chamber, or another orifice, or group 

 of orifices, equally white. They are guided neither by odorous emanations nor 

 by differences of color. He thinks that bees have as bad sight and act almost 

 exactly as flies. 



From numerous experiments on Odonata, Coleoptera, Lepidoptera, Diptera, 

 and Hymenoptera Plateau arrives provisionally at the following conclusions : 



1. Diurnal insects have need of a quick strong light, and cannot direct their 

 movements in partial obscurity. 



2. Insects with compound eyes do not notice differences of form existing be- 

 tween two light orifices, and are deceived by an excess of luminous intensity as 

 well as by the apparent excess of surface. In short, they do not distinguish the 

 form of objects, or if they do, distinguish them very badly. 



Lubbock, however, does not fully accept Plateau's experiments with the win- 

 dows, and thinks they discern the form of bodies better than Plateau supposes. 



How far can insects see? It is now supposed that no insects can perceive 

 objects at a greater distance than about six feet. On an average Lepidoptera 

 can see the movements of rather large bodies 1.50 meters, but Hymenoptera only 

 58 cm., and Diptera 68 cm.; while the fire-fly (Lainpyris) can see tolerably well 

 the form of large objects at a distance of over two meters. 



Until further experiments are made, it seems probable, then, that few if any 

 insects have acute sight, that they see objects best when moving, and on the 

 whole except dragon-flies and other predaceous, swiftly flying insects, such as 

 certain flies, wasps, and bees, which have very large rounded eyes insects are 

 guided mainly rather by the sense of smell than of sight. 



Relation of sight to the color of eyes. It appears from the observations of 

 Girschner that those Diptera with eyes of a uniform color see better than those 

 with brightly banded or spotted eyes. Thus those flies (Asilidse, Empidae, Lep- 

 tidfe, Dolichopida?) whose predaceous habits requires good or quick sight have 

 uniformly dark eyes, as have also such flies as live constantly on the wing, 

 i.e., the holoptic Bombyliidae, Syrphidse, Pipunculidse, etc., whose eyes are also 

 very large. 



Those flies whose larvfe are parasitic on other animals have eyes of a uniform 

 color that they may readily detect the most suitable host for their young ; such 

 are the Boml>yliid;<>, Conopidte, Pipunculidse, and Tachinidse. 



Certain flics which live in the clear sunlight, as many Dolichopidre, some 

 Bomliyliida', and certain Tabanidse (Tabanus, Chrysops, Hnematopota), and 

 which are often easily caught with the hand, have eyes spotted or banded with 

 bright or metallic colors. This is also a sexual trait, as the males of some horse- 

 flies visiting flowers have eyes of a single color, the spots and bands surviving 

 only on the lower and hinder parts of the eye, while their voracious blood-suck- 

 ing females have the entire eye spotted or banded (Kolbe). 



The color-sense of insects. Insects, as Spengel first suggested, appear to be 



