1878.] the Simple and Compound Eyes of Insects. 



269 



of those in the centre, and the radius of curvature is very short at the 

 extreme periphery. 



In most insects the acuity of vision determined in this manner 

 diminishes very rapidly at the periphery of the field. In the centre 

 of the field it enables them to perceive, as distinct, objects which 

 subtend one degree. In JEscJina grandis the sharpness of vision is 

 much greater, as the adjacent facets make an angle of only eight 

 minutes with each other. This is the least angle I have measured in 

 any insect, but I have no doubt, from the nature of the curve forming 

 the meridians of the eye in the great dragon flies, that a small part of 

 the centre of the field has a much greater acuity of vision than this ; 

 in the wasp the angle subtended by the smallest visual perceptions is 

 twice as great as in Mschna ; and in the bee it is half a degree. 



The direction of the visual line, or the line perpendicular to the 

 compound cornea in the centre of the field of most acute vision, varies 

 in different insects. In the predaceous kinds it is directed forwards in 

 the plane of the body, or forwards and outwards, making an angle of 

 30° between the visual lines of the two eyes. In the pollen feeders it 

 is directed downwards as well as forwards and outwards. 



The size of the corneal facets varies in different insects from 

 2000 ^° tto °f an ^ ncn ^ diameter. Their size, except in a few 

 insects, is dependent on the size of the insect, the largest insects 

 having the largest and the smallest the smallest corneal facets. 

 From this it follows that the vision of large insects is more perfect than 

 that of small ones, except where the curvature of the cornea is very 

 flat. This corresponds with the manner in which the insects fly. For 

 instance, the small Diptera fly round in small circles, and seldom leave 

 the place in which they first attain their adult condition, except when 

 borne away by currents of air, whilst the larger species take long 

 flights when disturbed or in search of food. The experiments of 

 Muller and others have shown that the direction and length of the 

 flight of insects depends largely on the visual powers of the insect. 

 The forward flight of Tabanus and of many flies corresponds with the 

 direction of their visual line, and the same may be said of the lateral 

 movements of the large dragon flies. 



The mimicry of insects, especially that between the Diptera and the 

 Hymenoptera is sufficiently close to be a protection or advantage to the 

 unarmed insect, and is such that it would render the one indistinguish- 

 able from the other, or the two insects would be scarcely to be dis- 

 tinguished under conditions of vision equal to those with which the 

 insects appear to be endowed except at very close quarters. 



In the extreme periphery of the cornea the adjacent facets make 

 an angle of from 30' in wasps and some other Hymenoptera, to 12° 

 in many insects. In the microrhabdic eye of Tipula the curvature 

 of the common cornea approaches the segment of a hemisphere. 



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