Country diary: mesmerised by the courting ritual of the semaphore flies

Country diary: mesmerised by the courting ritual of the semaphore flies

Country diary: mesmerised by the courting ritual of the semaphore flies

Crook, Country Durham: in an aquarium at the end of my garden, the female fly reacts with splendid indifference to a frantic whirling dance from her suitor

The semaphore fly

Wed 18 Aug 2021 00.30 EDT

It might be the glint of sunlight on water that attracts semaphore flies to the little aquarium at the end of the garden, the arena where they perform their entertaining courtship ritual.

Poecilobothrus nobilitatus is small but exquisite, 7mm long, with emerald eyes that burn with inner crimson fire in the sun, a bronze body shot through with green and purple iridescence and, in males, white spots on the wing tips. It can comfortably walk on water, long legs supported by surface tension, where its feet leave tiny dimples.

Their performance might have gone unnoticed, but for the fact that the aquarium is at eye level, arranged so that our grandchildren can watch unfolding aquatic dramas through its transparent sides.

The male semaphore fly’s courtship dance follows a predictable pattern. He spots a female feeding on the water surface and a brief aerial dogfight ensues, performed with such speed and agility that it is impossible to follow with the naked eye. She settles again on the water and he lands in front of her, white wing tips whirling in a frantic effort to semaphore his intentions, often met with total indifference. Usually, she continues to feed, rarely turning to face him. I have watched these rituals over the floating duckweed many times, and have yet to witness a successful mating.

As this afternoon wears on, swarms of water fleas perform their own jerky dance just under the flies’ feet, sometimes falling prey to the female semaphore flies fishing for them through the meniscus. Silver, sunlit bubbles of oxygen from pondweeds stream to the surface. Diving beetles scoot down into the depths, limbs paddling frantically to counteract the buoyancy of the scuba-tank air bubble trapped under their wing cases. Water slaters, refuse recyclers of the pond, trundle through silt on the bottom. Daily routines of pond life in summer, revealed through the simple magic of glass.

And all is suddenly visited by chaos, in the shape of the blackbird who baths in the tank on hot afternoons. Spray flies in all directions, but when he has gone, even before the ripples subside, the never-give-up semaphore flies have returned to their wobbly dancefloor.

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