77o 



NA TURE 



[December 9, 1922 



intimate than any biologist can have with any 

 animals or plants, with abundance of direct, not 

 merely circumstantial evidence, have no need for 

 the traditional biology of biological teachers. They 

 are in a position to construct, and for all practical 

 purposes have already constructed, a biology of their 

 own. The traditional teaching has with them no 

 influence whatever except as a waste of time, and 

 ought to be, and before long is sure to be, eliminated 

 from a curriculum which has outgrown it. 



G. Archdall Reid. 

 9 Victoria Road South, Southsea, 

 November 19. 



Divided Composite Eyes. 



It is not uncommon to find among insects in- 

 stances where each composite eye is divided into 

 two portions, so that in appearance there seem to be 

 four eyes instead of two. In sections, however, it is 

 seen that both parts are connected with the same 

 ganglion. 



Sometimes the reason for the division is obvious, as 

 in the case of certain beetles which have a prominent 

 sort of " armoured belt " carried horizontally round 

 the head. Here half the eye is above and half below 

 the belt, thus giving a view of the ground as well as 

 of objects above it. 



The reason for duplication, however, is not always 

 so apparent. In the majority of composite eyes the 

 convex surface is covered with lenses of uniform size, 

 but in those to which the present note relates, namely, 

 dragon-flies, White (or Cabbage) fly, and Aphides, this 

 is not the case. 



Among the dragon-flies — a very highly developed 

 type — each eye presents a continuous convex surface, 

 but the lenses of the upper part are much larger than 

 those below. The transition from large to small is 

 quite abrupt, but as the curvature of the surface is 

 continuous the line of demarcation is not noticeable 

 without the use of a magnifying glass. 



In the White-fly (Alevrodes proletella, etc.), where 

 the eyes are well divided, the relative position of 

 the large and small lenses is reversed, the large lenses 

 being below. 



The eyes of Aphides present for the greater 

 part of their area a convex surface carrying 

 lenses of equal diameter, but not far from the 

 posterior margin there is a small prominence 

 bearing a few lenses on its summit and sides. 



The appearance of the eyes of dragon-flies is 

 so well known that it is scarcely necessary to give 

 figures, but it may be remarked that the areas 

 covered respectively by the large and small lenses 

 differ considerably both in form and extent in 

 different genera. 



White-fly is chiefly known as a pest in green- 

 houses, and until its appearance in unusual 

 numbers in the autumn of 192 1, I had never 

 given it any attention. Any one, however, 

 who examined the perfect insect with a magni- 

 fying glass might well be excused for taking 

 it (as did Linnaeus) for a small moth, but if the 

 course of its development is followed up from the 

 egg to the imago it is seen to be more nearly allied to 

 the Aphides. 



Far the best account of it is given by Reaumur 

 (" Memoires," Tome II.) in 1736, and having myself 

 repeated his observations of its transformations, I 

 can confirm the accuracy of his description. All 

 Reaumur's specimens were apparently taken from 

 tin' leaves of Chelidonium >najus, but this plant is not 

 abundant in the neighbourhood of Exeter. 



NO. 2771, VOL. I IO] 



White-fly, however, feedson a great variety of leaves, 

 and I have taken it from cabbage, cucumber, tomato, 

 campanula, veronica, and from many composites. 

 There is a considerable difference in these cases both 

 in the size of the perfect insects and in the density of 

 the cottony down with which they are coated, which 

 gives them their white appearance, but whether this 

 implies real specific differences or is only a result of 



* 



Fig. i. — Alevrodes. 



a, Camera-lucida sketch of A. proletellax 25. 



b, Head, side view. 



c, Head, from below. 



d, Headxi25 (60 in the reproduction), front view to show the difference 



in the size of the lenses in the two divisions of the eye. The contents 

 of the head and the exterior down have been removed. The specimen 

 was taken from cucumber. Diameter of upper lenses 0-0003 in., of lower 

 lenses 0-0005 hi. 



different food, is, I believe, considered uncertain. 

 Among my own specimens, those taken from cucumber 

 were the smallest in size and had the thinnest coating 

 of down. The divided eyes were closely similar in all, 

 and the general appearance of the insect is shown in 

 the camera-lucida sketches, Fig. 1. When the head 

 is viewed from underneath, especially when it is so 

 turned that only the lower pair of eyes are visible, 

 the face is curiously owl-like, the proboscis standing 

 for the beak. 



Fig. 2. — Black Aphis taken from Laburnum. 



a, Head, side view. 



b, Head, front view x 50. 



[ c, Eye and part of headx 125 (60 in the reproduction), seen from above. 



The eyes are shown in more detail in the photo- 

 graph, Fig. 1. 



There is always some difficulty in photographing 

 such objects as require large magnification, but the 

 structure of which does not permit of these being 

 flattened ; and though much less detail is shown in 

 Fig. 1 c than can be made out by focussing each part 

 independently, the difference in the size of the upper 

 and lower groups of lenses is very apparent. 



Several species of Aphis taken from various plants 

 were examined, and in all of them the eves had the 



