The Eyes of the Flies

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THE EYES OF THE FLIES WILFRID

S.

GEORGE

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


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The Eyes of the Flies by Suffolk Naturalists' Society - Issuu