Do you remember? Windscreens were covered once, at the end of a car trip in high summer, with an insect massacre: splattered moths and squashed flies and wasps and gnats and God knows what. But in recent years more and more drivers seem to be finding their windscreens clear.

Is it just a vague perception? Or might it correspond to something real and serious, the widespread disappearance of insects in general? Conservationists are starting to think the latter proposition is true, and scientists at the Royal Society for the Protection of Birds have devised a simple but original device to test it.

The splatometer, a piece of PVC film that attaches to the front of your car, will start to give a proper statistical basis to the increasing feeling that insects are vanishing, which is shared by many of Britain's senior entomologists.

"Anecdotal evidence pointing to the decline of British insects abounds," said Dr George McGavin, acting curator of entomology at the Oxford University Museum of Natural History. "Most people over the age of 50 talk of seeing many more species of moths, butterflies and other insects when they were children."

With a colleague, Dr McGavin in 2000 examined insect records in Warwickshire from 100 years ago and the present day, and found that about 20 per cent of the species surveyed (including beetles, bees, dragonflies and butterflies) had disappeared or were in marked decline. A closer examination showed that 394 beetle species alone had been lost, a decline of 24 per cent.

Britain's best insect records are kept by Rothamsted Research, the former government agricultural research station at Harpenden in Hertfordshire. Rothamsted supervises a network of 16 insect suction traps around Britain, which have been emptied daily, with the insects kept, for more than 30 years. This year a pilot project for English Nature assessed how much change there had been in the total weight of insects caught in four traps since 1970.

There had been no change in the trap at Rothamsted itself, slight declines in traps in Devon and Kent, but a 60 per cent decline in the trap in Herefordshire. "That was a very dramatic drop, but it is not yet possible to speculate on the reasons," said the scientist running the project, Dr Richard Harrington.

When specific records for six moth species at Rothamsted were examined last year, five were found to have suffered substantial declines; many more are now being examined. It is a situation paralleled with butterflies; with bumblebees; and with mayflies, the upwing flies of rivers on which trout feed. The trend seems to be especially serious with Britain's 4,000 beetle species: many, such as ladybirds, are tumbling in numbers.

But although these individual declines are becoming well known, there is a lack of data that might indicate general declines across Britain as a whole, which is where the splatometer comes in. It consists of a simple, postcard- sized piece of transparent PVC film, which sticks to the front of the car - on the bumper, number plate or body - and traps the insects that collide with it.

At the end of a given journey, an identical piece of film is slipped on top of the original to protect its spattered haul, and the whole is peeled off (without leaving a sticky residue) and sent for analysis.

The beauty of the system, and the aspect that makes it possible to use on a large scale, is that each plate can be analysed quickly and automatically by a computer picture scanner, which can give an accurate reading of the marked area. The mindboggling task of trying to count the insects by hand never arises.

For the past few weeks the splatometer has been tested on the cars of RSPB staff based at the society's headquarters in Sandy, Bedfordshire, supervised by two young scientists, Dr Richard Bradbury, an expert in the decline of farmland birds, and Dr Mark Telfer, an entomologist.

They gave The Independent an exclusive demonstration - Dr Bradbury's 20-mile car journey to work produced a boldly splattered plate - and they are convinced now that the splatometer will work, will be easy to use, very popular, and can be rolled out for mass use, very probably next summer.

Once operating, it will produce a statistical baseline for insect abundance against which declines in future years can be accurately measured. And straight away it will yield some really significant results, such as regional variations, and date variations.

The widespread disappearance of such once-common farmland birds as yellowhammers and grey partridges is what has set the ornithologists of the RSPB on the trail of insect decline; the one is thought to be caused by the other. But what is causing such widespread insect decline remains to be established. It is probably a combination of factors, including habitat loss, changes in land management practice, and climate change.

The phenomenon has hitherto received virtually no publicity. This may be because many people see insects as "creepie-crawlies", and feel the fewer the better. But it may also be because a conservation community, which long ago alerted the world to threats to the giant panda, the tiger and other "charismatic megafauna", is only now waking up to the fact that things appear to be going badly wrong with insects and other invertebrates, or, as the great Harvard zoologist Edward O Wilson famously called them, the "little things that rule the world".

SPECIES IN DECLINE

BEETLES

Large numbers of Britain's 4,000 beetle species are thought to be declining in abundance and range.

This applies especially to the larger ones and those associated with rotten wood, such as the stag beetle, the subject of a biodiversity action plan. The loss of large beetle species may be behind the extinction of one of Britain's most attractive birds, the red-backed shrike, which fed on them.

BUTTERFLIES

About three quarters of Britain's 55 butterfly species have declined in recent decades, according to Britain's leading authority, Dr Jeremy Thomas. Two have become extinct - the large tortoiseshell and the large blue (although the large blue has been successfully reintroduced). Several more species, including the high brown fritillary, the pearl-bordered fritillary, the wood white and the Duke of Burgundy, have virtually gone.

MAYFLIES

The numbers of mayflies and the other 50 aquatic upwing fly species on which trout feed may have declined by as much as about 60 per cent since the Second World War, according to a study organised three years ago. The Millennium Chalk Streams Fly Trends Study was based on the records and recollections of 365 experienced anglers on the chalk streams of southern England. The anglers said that they thought the numbers of flies were plunging.

MOTHS

Many of Britain's 900 or so larger moths are thought to be rapidly declining. When records for six species, caught in the moth trap network run by Rothamsted Research over 30 years, were examined, five, including once-common species such as the garden tiger and the magpie moth, were found to be plummeting in number. A large number of other moth records are now being scrutinised.

Additional research by Thomas Chappell

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