The flies have it

by Richard Jones

To merit a common name, insects have to be relatively common, and they have to be pretty or sinister, pest or helper, biter or worth eating. In other words, they have to get noticed.

Richard JonesSunday has been warm enough to sit outside, in a tee-shirt, so it's not surprising that there are still insects flying about. Today it was flies that caught my eye.

Episyrphus balteatus is perhaps the commonest and most distinctive hoverfly in Britain. Although there are about 250 species of hoverfly in the UK, and roughly 100 of them are black and yellow wasp mimics, this one is immediately recognizable by its narrow parallel-sided body shape and the fact that some abdominal segments have two black bars across the yellow rather than one. The overall effect is of a hoverfly that has rather more than its fair share of narrow markings.

For some reason its English name is the marmalade fly. This is a fairly recent coining (within the last 10 years), but I don't think I've ever worked out how on Earth it came to be so called. The strange thing is, when I saw one today, I didn't immediately recognize it and I leapt forward to have a closer look. It had the body almost entirely dark, with the normally strong background yellow reduced to only the faintest whisper. This species is well known to vary tremendously in size and colour. It's nice to know that I can still be challenged by even the relatively 'easy' hoverflies.

There was no mistaking the next fly, a huge bristly orange and black critter, Tachina fera. No English name for this one, despite being the size of a bumblebee. The trouble is that to merit a common name, insects have to satisfy two important criteria. First they have to be relatively common. And second they have to have some impact on humans: they have to be pretty or sinister, pest or helper, biter or worth eating. In other words, they have to get noticed. Tachina fera is large, but it has the secretive life style of searching in the long grass and herbage (away from flowers) for moth caterpillars in which to lay its eggs. This is the first time I've seen it in the garden. It sunned itself for a few seconds, then it was off.

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Gardeners' World Web User 09/11/2007 at 16:17

Reply to LouLou: Without seeing the flies I can only guess, but they sound as though they may be cluster flies, Pollenia species. These mottled grey/brown flies are larger than houseflies, but smaller than blowflies. They get their name from the fact that they cluster together to overwinter, usually in lofts or attic rooms. They are known to cluster on the leeward side of houses, gusted over the roof by the wind, then dropping down and flying back into the shelter. Thousands of them can gather together. You're doing all the right things to get rid of them. You have to stop them coming in, and this may be via the loft then down through the light fitting. Although they may annoy you, don't worry, they do not spread disease.

Gardeners' World Web User 15/11/2007 at 20:16

Thanks Richard, at least I know what name to suffix to the expletives now! I thought they were just houseflies but I'm sure you are right, that bathroom is exactly at the leeward corner of the house, away from the prevailing south westerlies. As it's the guest bathroom, I think I'll print off your explanation and hang it up in there - along with the dust-buster vac!

Gardeners' World Web User 31/08/2008 at 03:12

I'm experencing this pesty fly as I type. I have plug any little hole I can find. My kitchen sits above the basement which is all dirt. The weather is very warm and I hope no wild critter died under there. There is no smell and this is a little freaky.

Gardeners' World Web User 28/11/2011 at 18:30

Why do thousands of flies invade my bathroom every Autumn and then proceed to die? I can understand they may be looking for somewhere to spend the winter but why do they die when they get in and if they are going to die why can't they do it outside or at least in the loft! I can't find the point of entry, though I have patiently stood sentry duty to try and find out - they just seem to materialise out of thin air. It is north facing. I've tried fly killer around the windows, blocking the air vents and hanging up bunches of lavender but still the window sills are covered in dead and dying flies. Help!