THE EYES OF THE FLIES WILFRID
ON 28th May, 1962, while collecting hover-flies (Fam. Syrphidae Order Diptera) at Haiesworth, I watched an acrobatic Performance by three males of the species Eristalis nemorum, Linn. These bee-like " drone flies ", (slightly smaller than the very common drone fly, E. tenax, Linn.) were evidently competing for the love of a female of their species, who was imbibing nectar from hawthorn flowers. First, one male suitor hovered motionless about two centimetres immediately above (and in line with) the female, but he was disturbed by a second male who arrived and hovered above him. The hovering males rose and feil a few centimetres, but not in unison, then a third male arrived who " piled in on top ". There was now a Stack of four flies, a female continuing to feed, apparently unconcerned, with three males rising and falling in a vertical Stack, yet not touching each other, all facing one way, all within about ten centimetres. Unfortunately, I saw no happy ending. Possibly I disturbed them, perhaps the males disturbed each other, or it could be that the female had already mated. But it left me with the question " why such a precise vertical formation ? ". 1 am wondering if the answer lies in the design of the insect's eyes. They are the large, dome-shaped Compound eyes so typical oi insects, which in hover-flies occupy by far the greater part of the head. Such eyes, with a convex honeycomb of lenses, give what a photographer knows as a " wide angle " field of view. The actual field must extend to any surrounding area that is perpendicular to the eye-surface. It is quite usual in those insects where the male seeks the female by visual means for the males to have the larger eyes, and in many flies (such as the House Fly, Musca domestica, Linn.) it can easily be seen from a dorsal view that of a mating pair the female has her eyes spaced farther apart than are those of her mate. But in the great majority of hover-flies (E. nemorum included) the male eyes actually touch for a short distance on top of the head. The female has her eyes separated by a well-defined band, quite different from the male of her own species. I have tried, by diagrams, to suggest the probable fields of view oi male and of female drone flies. It seems likely that males can see directly above them, but that females have a narrow blind spot instead. It could be that by hovering in this blind spot the male is able to court her without frightening her away prematurely. But a male, not having this blind area, would be well aware of any
EYES OF THE FLIES
insect poised immediately above it, and no doubt resent it.
Situation w o u l d a p p l y to m o s t hover-flies, in f a c t t h e r e c o u l d b e a
direct connection between the hovering habit and the touching male eyes, for in the genus Helophilus the male hover-flies have separated eyes, and are less inclined to hover than are Eristalis flies. Could the simple eyes, or ocelli, spot the suitor ? (In both sexes these are seen as three little beads, spaced triangularly on a dorne at the rear top of the head.) I think not. These are probably more comparable to the photographer's light-meter, three being the smallest number of receivers capable of telling the brain which way the light is falling. These may well be the organs which will send any rare fly I have caught straight out of the window if I fumble my killing-bottle. And what part do the hairs bristling between the microscopic lenses on the Compound eyes play in all this ? Some hover-flies have them, others do not, and they vary from white to brown, or black. In E. tenax they are strikingly dark brown in two vertical lines on each eye, while the remainder are pale brown,, but the only use I know of is to enable fly-collectors to identify E. tenax easily. They may in fact aid the flies themselves to identify each other when courting (the vertical pattern shows well from above). These hairs, perpendicular to the eye-surface, can hardly have any optical effect, unless they take over the function of a photographer's lens hood, or counter the Wetting action of raindrops or of dew. Yet other species manage perfectly well without them 1 MALE Anterior views of heads of Eristalis
FEMALE nemorum Linn.
(magnified approx. 8 times)