Picture-wing flies

Posted: Wednesday 14 March 2012
by Richard Jones

[...] many people find small flies in their back gardens all the time, and most people ignore them. But this one is a jewel.

The picture-wing fly, Chetostoma curvinerve

On March 1st I found a small fly in the garden. Ordinarily, this might not be top news; I’m sure many people find small flies in their back gardens all the time, and most people ignore them. But this one is a jewel.

Luxuriating in the wonderful scientific name of Chetostoma curvinerve, it is one of the picture-wing flies, so called for their prettily marked wing patterns. Unfortunately the photo of my pinned specimen does not do the fly justice, but Brian Valentine has allowed me to put up his stunning image from Flickr. Note the distinctive brush of stout bristles jutting out from the face — most peculiar.

Some picture-wings, like the celery fly (Euleia heraclei) and the Mediterranean fruit fly (Ceratitis capitata) have achieved agricultural notoriety enough to warrant common names. Not Chetostoma. This is a rare and secretive beast, hardly known outside arcane entomological circles, and whose life cycle is still a bit of a mystery.

But things may be changing for Chetostoma. Although small (maximum length only about 7mm) the picture-winged flies are distinctively enough marked to make them a good choice for anyone wanting to graduate from butterflies, dragonflies and hoverflies to some of the other more obscure and interesting insect groups. Many can be identified from their wing patterns using a hand lens, or even down the barrel of a camera. Photographs of them put up on the Open University iSpot site get named quickly. Many popular insect field guides show pictures of the common and widespread species. And all this has spawned an active biological recording scheme to collate records.

For many scarce insects, a distribution map of records is often little more than a distribution map of the specialist entomologists out looking. But for groups like the picture-wings, it is the citizen recording by interested, but not necessarily expert, observers that can fill in the gaps and generate real scientific information about an organism.

This is exactly what has happened with Chetostoma. It used to be an incredibly rare fly, known only from a handful of sites along the south coast of England. During the last 15 years, though, Chetostoma has spread — first through the rich wooded landscapes of Hampshire, Sussex and Surrey, then through London into the other Home Counties, and now there are outlying localities in Wales, East Anglia and Yorkshire.

Quite why it is spreading is still, perhaps, open to question. Anyone finding the lily beetle shredding their Turk’s caps or stargazers may be astonished to learn that for 50 years, between 1890 and 1940, it was regarded as a rare oddity, known from gardens in London and Swansea (major ports of the time), but nowhere else…until it turned up near Guildford. There it obviously found the right niche, perfect climate, and perhaps mutated a slightly hardier race, because it was off and has spread relentlessly ever since.

No-one knows exactly what has happened to Chetostoma to aid its recent spread. I have one idea, though. According to all the textbooks, the maggots develop in the fruits of honeysuckle. My specimen, and several of those reported on sites like Flickr and other internet forums, were found on ivy. Honeysuckle and ivy are only very distantly related plants, so a switch from eating the leaves of one to eating the leaves of the other seems highly unlikely. Each has its own distinctive biochemistry and any switch is likely to play havoc with the digestive juices of even the toughest caterpillar. But they both produce small succulent fruits in autumn. The ‘point’ of fruits is to be eaten by birds or animals, thus aiding seed dispersal, and even widely differing plants produce fruiting bodies surprisingly similar in size, texture and chemistry. Switching fruits is a much more likely scenario. So look out for Chetostoma, appearing in a garden near you, any time soon.

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Chris9 22/03/2012 at 03:44

Hi We moved into our house 3 years ago and have been modernising it. We are now in the process of completely landscaping our semi-detached corner plot garden. The house was build in 1935 and so the garden is very matured. Unfortunately the garden is (we think) below the water table and is very wet in areas and doesn't drain off and the lawn never drys off in areas even on hot days. We have decided to completely strip the garden and lay drains and remodel the garden and lawn. One problem we have it Horsetail weed, at the moment we have a digger and it is moving and levelling off the lawn to relay and lay a patio. My question is while the earth is up and being turned over what chemical can we use to help to get rid of this pesky weed. Please help regards Chris

Skimpymonkey 28/04/2012 at 16:34

I saw one yesterday, it was sitting on top tip of my very young honeysuckle. I didn't know what it was!

Alina W 28/04/2012 at 16:43

I hate to tell you this, but there is no easy way of getting rid of horsetail, and your digger is probably spreading it.

Crush the growth underfoot and then spray with a glyphosate-based weedkiller. You won't get rid of this in one go, but will have to repeat the treatment several times.