35. Lauxaniidae – bristles

4th June 2019

Even though it was rainy, the cat and I went for a walk in the woods, both being tired of indoors. The cat sniffed and listened, tail and whiskers alert for interest and danger, yet indifferent to the nearby bark of a deer, frighteningly close if it had been a dog. Because of the rain there were a lot of refugees using leaves as parasols and I moved along at cat speed, looking upwards through the canopy for tell-tale shadows, turning over soft green new leaves where aphids hunched, spiders crouched and flies took their ease. Some inept tube-work lost me a few interesting-looking ones, and eventually, our paws and beard becoming damp, we headed home. I resorted to sweeping the undergrowth, careful for the snag of bramble tendrils, and this one caught my eye in the net because of its buttery colour, nothing quite like anything I have caught so far.

Bristles are often important in fly identification, and on a dark fly they can be a bit of a pain, the lighting having to be just right and the fly at the right angle before you can see them. No such trouble with one. There are no bristles (vibrissae) at the mouth edge beneath the eye, but there is a bristle at the end of the tibia of the middle leg on the top side (and underneath as it happens). The anal vein, which is the one nearest the bottom of the wing, is short and doesn’t reach the wing margin – the vein above it also doesn’t quite reach the margin, but peters out just after the cross vein. Finally, the P bristles are converging, forming an X shape behind the ocelli which are three primitive eyes that sit in a triangle in the centre of the forehead between the large compound eyes. These characteristics place this mellow fly in the family Lauxaniidae, while a key to the genera, also depending heavily on bristles, places it in the genus Meiosimyza.

Dipterists have an almost comical interest in the position, size and orientation of the bristles on their subjects. But apart from taxonomic utility, the precision with which hairs, bristles and spurs are placed upon the body provides one of the delights of looking at flies under a microscope. There are graded girths and lengths of hairs in rows down the thorax, single bristles in particular places, a tuft somewhere else, fancy combs of stubby hairs on the legs, well-maintained mustaches or proto-punk anarchy of hairs that arch inwards or outwards, upwards or downwards, forwards or backwards, pairs that cross or are parallel or divergent. Antennae can have a stubble or exuberant whorls of delicate verticils. The same fly can have no-nonsense jet black bristles in one place and a profusion of soft blonde fuzz elsewhere. The coiffure of flies is a wonder to behold – which is more than could be said for either the cat or I after our walk. But at least we got rid of the cobwebs.

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