A fungus gnat , Sciarida (possibly Bradysia)

I am an amateur naturalist trying to learn something about everything living in my garden.

Buoyed up by my successful (?!) identification of a fly to species level in my previous posting, today I'm taking on a related, though tougher challenge: the little gnat in photo 1 (click on photo's to enlarge). This one was around in my garden mid-May last.

As I've discovered through writing this blog, to stand any real chance of identifying the smaller insects it's pretty much essential to have a microscope. This needn't cost the earth. Photo 1 was taken by holding a 'point and click' digital camera up to the eyepiece of a sub-£100 'DM2' stereo microscope. With 10x and 20x eyepieces this would probably suffice for a fair range of the needs of many amatuer naturalists though if you want to study the more minute structures such as mushroom spores, or the smaller pondlife, a microscope capable of 1000x magnification is needed. I have a (sub-£200) Westbury SP2 microscope which I've found to be thoroughly adequate for all my needs (the only minor drawback, for those in-the-know about such things, is I'm not certain this particular scope has the option to be equipped for 'dark field' operation, though this is a 'luxury' rather than a 'staple'). For anyone considering making a purchase, I have always been very satisfied with the service from Brunel Microscopes Ltd (I am unconnected with the company, and have received no payments for plugging them here!).

To set about identifying my fly I turned first to "A Key to the families of British Diptera" by D.M.. Unwin, available free here. This is designed specifically with the amateur in mind, being copiously illustrated to explain any technical terms. There are more than 80 families of British fly. The fact that my fly has long, thread-like, multi-segmented antennae immediately rules out more than 5o of these however, and places my fly in the nematocera, a sub-order of around 30 families. To distinguish between these its necessary, in part, to carefully examine your fly's wing. In my last posting on a crane fly I discussed the prehistoric origins of fly wings and the so-called 'Comstock-Needham' code for labelling up their veins. I'll not repeat this here and simply point out that I've labelled up the wing veins in photo 1.

With the wings of my fly in view, the key above pretty quickly bought me to a choice of my gnat being in one of two families: the Sciaridae or the Mycetophilidae. The 'decider' was the eyes. Photo 2 (taken from above looking directly down onto the antennae) shows my fly's 'left' and 'right' eyes, though this is a rather arbitrary choice of words since in fact the eyes are joined together to form one continuous band above the antennae. If ever you wanted an example of how 'alien' is the world of insects' senses, surely having eyes that join on top of you're head is one! Anyway, this 'eye bridge' decides against my fly being in the Mycetophildae and makes it a member of the Sciaridae. In searching for information on sciarid flies I came across two useful websites, the first dedicated to Sciariod flies, and the second, a list of free, online keys to different diptera.

The Sciarid flies are sometimes called 'fungus gnats', mushrooms being the larval food for some species. Mushrooms are not the only food however, and species have been reported emerging from a wide variety of substances from dead animals to birds' nests. Perhaps the most amazing thing I learnt about Sciarids in a short time searching is that the larvae of some occasionally undergo mass movements, thousands of them marching in columns several centimetres wide and metres long. I found an online paper reporting one such movement here [1]. It seems no one knows why they do this.

So far so good! Unfortunately, whilst identifying a fly to family level (Sciaridae in this case) is generally tractable, getting much further can be decidedly tricky. The first problem is that there are a lot of families of fly and finding a text-book or key that deals with yours can be difficult or indeed impossible. As luck would have it however, having become interested in flies and having something of a passion for natural history books, last summer I treated myself to some of the Handbooks for the Identification of British Insects from the Royal Entomological Society, amongst them Sciarid Flies by P. Freeman.

The book starts a little ominously:

[Sciard] taxonomy has always presented problems [...] the student has always been faced with a mass of similar looking species which he has been unable to group adequately.

Fortunately Freeman's book provides a detailed guide to identification, covering about half (according to this paper [2], as of 2005 there were 263 species of British Sciaridae in total) the British species across 18 genera.

So how did I get on sifting through the 18 genera of Sciaridae in Dr. Freeman's key? Well, On the basis of wing-vein shape and length, I was able to rule out 4 of the 18 genera; There was no sign of largish hairs ('macrotrichia') on my fly's wing veins, though there were tiny, downy hairs ( 'microtrichia'). This rules out another 5 genera; My fly has tiny spurs on its leg tibia (see photo 1). These are not 'distrinctly longer than the width of the tibia', ruling out the genus Corynoptera. After a little further work I was down to a choice between the genus Bradysia and the genus Lycoriella...and...I dropped my fly on the floor and lost it!!! On the basis of a couple of half-examined features, and the fact that Bradysia is the larger, more common genus, I'm going for Bradysia. But it all goes to show - you can't win 'em all!

Reference
[1] C. T. Brues, A Migrating Army of Sciarid Larvae in the Philippines, Psyche 58:73-76, 1951.

[2] Zoological Journal of the Linnean Society, 2006,146, 1–147. The sciarid fauna of the British Isles (Diptera: Sciaridae), including descriptions of six new species Frank Menzel, Jane E. Smith and Peter J. Chandler