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Information on the glow-worm's world distribution is still very patchy, but Figure xx gives a very rough impression of its range. This map has been produced by shading countries in which glow-worms have been seen, though of course they are unlikely to be found in every region of any country. Nevertheless it does show that 'our' glow-worm is actually an extremely widespread species, wit a range stretching from Portugal and Britain in the west right across Europe and Asia to China in the east. It also survives further north than any other firefly, almost reaching the Arctic Circle. Yet it has apparently never managed to colonise the New World, presumably because it evolved after Europe parted company with the Americas 50-100 million years ago.
No-one knows exactly when the glow-worm first arrived in Britain. In fact it has probably had to return to this country several times, having been forced out by the harsh conditions of successive Ice Ages. After each of these, as the climate began to warm up, sea level would have been much lower than it is today and a land-bridge would have connected most of the East coast of England to the rest of Europe. Along with many other animal and plant species the glow-worm would have been able to walk across what is now the bottom of the North Sea and then to work its way across Britain. It would have been a fairly slow march, perhaps only a hundred metres or so in each generation, and there would have been countless obstacles such as rivers and mountains which would have to be skirted around, but each warm period lasted for thousands of years, giving the glow-worm plenty of time to spread into suitable habitats throughout the country.
Most of the islands around the British coast lie on a sea bed no deeper than the North Sea, so for as long as the land-bridge to Europe persisted they would have been part of the mainland, allowing glow-worms to colonise them on foot. But later, as sea level rose, each glow-worm population would have been marooned on its own island. Ireland on the other hand is separated from the coasts of England and Wales by a much deeper channel, so that even in the depths of an Ice Age, when sea level was at its lowest, Ireland would have remained largely isolated from mainland Britain, though it would have been connected to the West coast of Scotland for a while. We do not know whether glow-worms have ever managed to use this bridge into Ireland after any of the earlier Ice Ages, but since the last one they have been unable to cross the Irish Sea (there certainly seem to be plenty of suitable habitats, and from time to time there have been reports of glow-worms in Ireland, but these may have been introduced).
Eventually though the next Ice Age would begin, forcing out all but the hardiest of species. During the past two or three million years this cycle has been repeated more than twenty times, but we cannot tell how many times glow-worms have returned to Britain.
Though glow-worms can be found in most parts of mainland Britain they are at their most abundant in southern England (Figure xx). This may be because in this country they are at the limit of their environmental tolerance and have difficulty surviving the cooler climate further north. Certainly in both Sweden and Norway they show a very similar distribution, present in the south but absent in the north. Although they can be found on a wide range of soil types, they seem to be particularly abundant on chalk.
Glow-worms can be seen in grassland of almost every description, including downland, pastures, meadows, roadside verges, railway embankments, churchyards, golf courses and even lawns. The one type of grassy habitat which they do seem to avoid is grassland which has been 'improved' by treating it with herbicides and fertilisers. They also occur in moorland, heathland, quarries and occasionally woodland, where they are normally confined to the more open and grassy areas such as rides and clearings.
Several authors have commented that glow-worms appear to have an affinity with water, and they can often be found in reedbeds and along the banks of ponds, lakes and canals. In a survey carried out in 1992 40% of the glow-worm sites were within a hundred metres of some form of open water (though of course this does not necessarily mean that they could not have survived if the water had not been there). Glow-worms may have a liking for wet sites but they are certainly not restricted to them, and many glow-worms carry on quite happily on sites without any open water at all. All their habitats do have at least one thing in common though: they generally support healthy populations of snails.
4. A THREATENED SPECIES?
With such a wide distribution the immediate future of the glow-worm globally seems quite secure, but within Britain it may be less safe. Accurate statistics are hard to come by for an insect which is nocturnal and is only easy to see for a few weeks of the year, but a large body of anecdotal evidence suggests that there has been a steady fall in the British glow-worm population, certainly since the 1950's and possibly before that. Indeed they have completely vanished from many haunts where they were once common. This decline appears to be a general one, not confined to any particular habitat or part of the country.
Although there has certainly been no shortage of suggestions as to why the glow-worm is disappearing from our countryside, there has been very little research to test any of the theories. Possible causes for the decline include:
Habitat destruction. There can be no doubt that many sites which once supported glow-worms have been lost through the complete destruction of their habitat. Thousands of acres of grassland have been built upon, ploughed up for arable farming, converted to conifer plantations or 'improved' using fertilisers and pesticides. So habitat destruction is certainly one factor working against glow-worms, but it cannot be the only one. There are many sites which have remained superficially unchanged for decades and yet have still lost their glow-worms.
Habitat fragmentation. As more and more glow-worm habitat is destroyed the surviving sites are becoming increasingly isolated from each other, separated by growing expanses of urban and arable land. Like most insects glow-worm numbers can fluctuate dramatically, often increasing or decreasing by a factor of ten or more from one year to the next, as a result of changes in food supply, predator populations, weather conditions and a host of other factors. Even without human interference these fluctuations will from time to time cause a local population to disappear altogether. A century ago, when glow-worm sites were less isolated than they are today, the gaps left by these local extinctions would soon have been filled by individuals wandering in from nearby sites. But as fragments of suitable habitat become more and more isolated this recolonisation takes longer and longer until eventually it virtually stops altogether. Then when a local population becomes extinct it stays extinct. Glow-worms are likely to be particularly sensitive to habitat fragmentation for at least two reasons. Firstly, being predators they tend to occur in smaller numbers than their prey, and so may be more susceptible to local extinctions. Secondly, as we have already seen, although the male is relatively mobile the wingless female is almost completely sedentary throughout her adult life. So the glow-worm's ability to colonise new sites, or to recolonise vacated ones, is limited by how far the female larva can travel on foot. A river, a ploughed field or even a busy road can pose quite a formidable barrier and reduce the chances of an empty site being repopulated. It is interesting to note here that had the roles been reversed, with the male being the flightless sex, then this situation would not have been quite so serious. To establish a new colony a winged female from a nearby population could simply have mated with a male on her own site, flown to the new site and laid eggs there. As it is, with the female wingless, a female larva must walk, rather than fly, to the new site and then somehow find either a male which has flown there or one which has also walked there as a larva and then pupated. The chances of this happening seem slim for all but the closest of sites.
Pollution. There is another reason why being a predator may put the glow-worm in a vulnerable position. Being at the top of a food chain it is likely to absorb any pollutants eaten by the snails on which it feeds. Pesticide and herbicide sprays drifting from nearby fields or industrial chemicals washed from the air by rain become concentrated as they pass up the food chain from plant to snail to glow-worm, where they may accumulate. Many pesticides are particularly soluble in fat and since the larva's main role in life is to stockpile the food reserves which it will need as an adult most of its body is packed with globules of fat. Compounds which were originally too dilute to kill either the plants or the snails may build up in these fat-bodies, to be released when the fat reserves are drawn upon by the pupa and the adult. This may kill the glow-worm outright or simply lower its fitness, for example by reducing the number or viability of the eggs it lays.
Distraction by artificial lights. Male glow-worms are normally very precise in distinguishing between females of their own species and other light sources, but they are sometimes drawn to house lights or street lamps. It may be that they treat an artificial light not as a female glow-worm but as the moon or stars and, in trying to navigate by it, spiral in towards it just as moths do. It is quite possible that on sites close to bright artificial lights males either become distracted by them or find it difficult to spot females in the glare, but we do not yet know how important a factor this is in the overall fall in glow-worm numbers. The glow-worm sites reported in the 1992 survey tended to be well away from any form of artificial lighting. On the other hand a few females were found displaying directly under streetlights, but we do not know whether this prevented them from attracting males.
Insufficient grazing. Most of the grassland in Britain was originally created by clearing forests to make way for farming, but left unmanaged trees soon begin to move in and the grassland reverts first to scrub and then to woodland. Prior to this century the invasion of grassland by trees was largely held in check by grazing animals, particularly sheep and rabbits. But within the last hundred years many sheep farmers have abandoned the unimproved and unmanaged downland slopes which once provided one of the richest habitats for glow-worms, preferring instead the flatter, 'improved' fields of the valleys. The impact of this decline in sheep grazing on downland habitats was compounded in 1953 by the introduction of the myxomatosis virus. Within the first few years it had killed an estimated 99% of the rabbits in Britain, and although with time the virus has become less virulent and the rabbit more resistant, the rabbit population is still just a fraction of that immediately before myxomatosis. Less grazing means longer grass and more scrub, conditions which have proved intolerable for a number of grassland plants and animals, including several orchids, the wheatear and the large blue butterfly (which disappeared from this country in 1979). The glow-worm may be another victim of scrub invasion, in which case it may be possible to slow its decline, at least locally, by reintroducing sheep grazing to selected grassland habitats.
Changes in climate. There has been growing concern recently over the possible consequences of the 'greenhouse effect', in which the burning of fossil fuels leads to a build-up of carbon dioxide in the atmosphere. This may reduce the amount of heat radiating back into space, leading to global warming, which in turn could have secondary effects such as changes in rainfall and other weather patterns. Detecting these changes against the background of an already variable climate is extremely difficult and open to a variety of interpretations, so as yet there is still very little agreement as to whether the burning of fossil fuels has actually affected our climate and, if so, to what extent and for how long. These questions will have to be answered before a link between global warming and the disappearance of glow-worms can be suggested, but it certainly a possibility.
A lot more research will be needed before the cause, or causes, of the glow-worm's gradual decline is known, and only then can we look for ways of halting it. In the next chapter we will look in more detail at some of the questions which will need to be answered.
5. WHAT DON'T WE KNOW?
As with most things, what we do know about glow-worms is dwarfed by what we have yet to find out about them. The list of questions awaiting an answer is endless, and each question is likely to involve answering several others, but the following sixteen would make a good starting point.
How widespread is the glow-worm's decline? Are its numbers dwindling throughout the world or just in particular regions or countries? Information is scarce at the moment but in Denmark for example glow-worms were very common at the beginning of this century but are now restricted to a handful of sites. Naturalists in Sweden and Switzerland have noticed a similar fall in numbers. How long has the decline been going on, and are all habitats equally affected? To answer these questions it will be necessary to monitor glow-worm populations on a large number of sites over a period of many years.
How important is habitat fragmentation in the disappearance of glow-worms? To tackle this question we would have to follow population changes on a large number of sites varying in size and the degree of isolation from other glow-worm populations. If habitat fragmentation is one of the reasons for glow-worms disappearing from some sites then we would expect small, isolated sites to be most at risk.
What effect is pollution having on glow-worms? To study this we need to measure the concentrations of substances such as pesticides, herbicides and other man-made compounds in the tissues of glow-worms living on a wide range of sites and to relate these concentrations to any long-term changes in the glow-worm population of each site.
To what extent are males distracted by, or attracted to, artificial lights? Do some types of light cause more disturbance than others and over what sort of distance will they draw males? This question could be studied either by watching the response of individual males (either captive or wild) to a range of different lights, or by looking for changes in glow-worm numbers on sites where lights have been installed nearby.
Are glow-worms affected by changes in grazing? Here we would need to look at the abundance of glow-worms in fields grazed by different species (such as rabbits, sheep or cattle) and at different intensities. For example one count, at Aston Rowant National Nature Reserve, recorded more than twice as many glowing females in an area of grassland grazed at two sheep per acre as in a corresponding area grazed at six sheep per acre. The total number of glow-worms counted was just 48, making it far too small a sample to draw any firm conclusions, but it does suggest that too much grazing (or grazing by the wrong species) may be as harmful as too little.
Has a change in the climate contributed to the fall in glow-worm numbers? This will be one of the most difficult questions to answer and will have to wait until we have a much better understanding of how our climate works and how it has changed.
It often appears that the female glow-worms emerge a few days earlier than the males, but is this really true? So far relatively little is known about males in the wild because it is normally very difficult to find them until they have arrived at a female, so any males emerging before the first females would go unnoticed. Recently however, Malcolm Jennings has developed an ingenious 'battery-operated female' which uses a light-emitting diode to imitate the real thing. So effective is this mimicry that in one instance two males which had begun to mate with a female immediately abandoned her and went over to the battery-operated version! Devices of this sort should make it easier to study males without having to rely on females to attract them.
What is the average lifespan of a male glow-worm in the wild?
Does the male mate just with the first female that he finds, or does he then go on to look for others? In a study at Sevenoaks Reserve 32 males were collected from females, marked with white ink or typist's correction fluid and released in the same place. Not one of them was caught again, even though there were often other females still glowing just a few inches away. It may be that not enough males were marked, or it may suggest that each male normally only mates with one female. A lot more studies will be needed to be sure.
How important are weather conditions to glowing females and flying males? Males seem to be put off flying by strong winds and low temperatures, and females tend to huddle low in the grass on wet nights, which might make them more difficult to spot from the air. This factor could be particularly important if our climate is indeed changing.
How far will a typical male fly in search of females? Again the battery-operated females should help in answering this question. By using several of them, spaced at different distances, and by marking any males that they attract, it should be possible to discover how far (and how fast) a male can fly. The shorter the male's range the more difficult it is likely to be to recolonise lost sites, and the more inbreeding there will be within each glow-worm population.
What is the range of a glow-worm larva? Can it survive in apparently inhospitable habitats such as arable fields? This too will determine how readily a new or lost site can be recolonised.
What are the larva's food preferences in the wild? In captivity they will tackle virtually any type of snail they are given, but might they be more selective in the wild? The only way to find out would be by patiently watching wild larvae.
How feasible would it be to reintroduce glow-worms to sites where they have become extinct? Restocking might be possible on sites where they have disappeared through natural fluctuations in their numbers, but it is less likely to work where the habitat has been damaged, for example by pollution. How many individuals are needed to stand a reasonable chance of becoming established, and at what stage in their life cycle should they be introduced? This will depend on which stage is the most vulnerable in the wild: if for example predators eating the eggs account for the most glow-worm deaths then it would be better to hatch them in captivity and release the young larvae. So far very few attempts have been made to introduce glow-worms to new areas, but at least one has been successful: 119 larvae, hatched from the eggs of a single female, were released onto an area of grass at Sevenoaks in 1988 and by 1993 they had established a small breeding colony.
What are the glow-worm's habitat requirements? Even on a site which supports a healthy glow-worm colony they are often restricted to one particular area. By measuring as many environmental conditions as possible, in areas both with and without glow-worms, it may be possible to find out how sensitive they are to factors such as temperature, frost and the type of vegetation. For example the 1992 survey mentioned earlier seemed to suggest that glow-worms may prefer south and west-facing sites to north and east-facing ones, but many more records will be needed to check this.
Is it possible to manage habitats so as to encourage glow-worms? A small-scale study at Sevenoaks for example suggests that it may be possible to create a better habitat for them by cutting the grass at least once during the summer. By looking at the numbers of glow-worms in habitats managed in different ways (for example by scrub clearance, grazing, mowing or even burning) we may be able to find the glow-worm's 'ideal' habitat.
Only by finding answers to at least some of these questions will it be possible to tackle what are perhaps the most urgent and yet most difficult questions: why are glow-worms disappearing, and how can this decline be halted?
Some of the questions will require a lot of detailed research involving a lot of technical apparatus, but many could be tackled by anyone with enough time, patience and enthusiasm. At the same time it is obviously essential that we do not cause any unnecessary disturbance to the glow-worms that we are studying, otherwise we will simply be adding to the problems which they already face.
6. GLOW-WORM WATCHING
No amount of words or photographs can replace the experience of seeing glow-worms for yourself. The earlier chapters of this book should give you some idea of when and where to find them, but to summarise:
When? The glow-worm season varies considerably from one site to another, and from one year to the next on the same site but in general the best time to look for them is between mid-June and mid-July. Even during the season there will be odd nights when there happen to be no females about (for example if they all succeeded in attracting a male the previous night), so do not give up if you fail to see any on the first visit. They can be seen most clearly on a moonless or overcast night. The females normally begin their display between about ten and eleven o'clock at night, and few carry on much beyond midnight. As a rough guide it is worth waiting until your eyes can no longer make out colours in your surroundings before beginning your search. Outside the breeding season glowing larvae can still be found occasionally, though their dim lights make them much harder to find than the females.
Where? Glow-worms can be found in most parts of the country, though they do appear to be more common in southern Britain than in the north. It is worth looking for them on almost any area of unimproved grassland, but the best sites of all are on downland which is unploughed and unsprayed. Other habitats which often support glow-worms include canal towpaths, grass verges, gardens, heathland, moorland, old quarries, woodland rides and clearings, river banks, lake margins, churchyards and clifftops. In urban and suburban areas one of the most promising places to search is on a disused railway line. These act as 'biological corridors', unbroken strands of habitat which stretch for miles and allow glow-worms (and many other animals and plants) from the surrounding countryside to penetrate right into the centre of a town. You can often save yourself a lot of fruitless searching simply by contacting your County Trust for Nature Conservation, who will probably be able to point you towards a nearby nature reserve where glow-worms have been seen. It is probably best to start with a known glow-worm site before looking for new ones.
Despite the brilliance of the glow-worm's light it can easily be swamped by the beam of a bright torch, especially if the female is glowing deep inside a bush or a tussock of grass, so it is often better to familiarise yourself with the site first during the day and then rely on natural light when searching. Females do not seem to be particularly disturbed by torchlight and once you have found one she will usually continue to glow while you take a closer look at her.
We are always interested to hear of new sites, or to receive more details of known sites (all records are treated in confidence to avoid any possible disturbance, and will not be made public without your permission). Please include the following details: Ordnance Survey grid reference (most important); name of nearest town; county; type of site (garden, nature reserve etc); habitats; dates and numbers of glow-worms seen. Please send any records to:
Photography The one essential piece of equipment for photographing a glowing female is a camera which has the facility to leave the shutter open for long periods. To avoid vibration the camera will need to be mounted on a tripod (with some models it is possible to reverse the central column of the tripod so that the camera is suspended between the legs, allowing you to get it much closer to the ground) and operated with a cable release. You may need to use a torch so that you have enough light to focus by. The time for which the shutter has to be left open will of course depend on which film and which lens you are using, but as a rough guide it is probably best to take a series of photographs, starting with an exposure of about ten seconds and roughly doubling the time with each picture, up to about two minutes. Using extension rings or a macro lens in addition to your standard lens will of course increase the exposure time.
Choosing the right female is quite important. A female glowing partway up a grass stem is likely to give a clearer, less cluttered picture than one on the ground, but may sway about in the slightest breeze, so it may be worth waiting for an absolutely still night or improvising some form of wind-break. Some females will remain perfectly motionless, but many have the unfortunate habit of slowly waving their abdomens from side to side. If this happens while taking a picture it is usually possible to avoid getting a smudged image simply by placing your hand in front of the lens until she has stopped moving.
If you are using a manual aperture setting it is probably best to have the lens fully open (i.e. the lowest F number). Although this gives a relatively shallow depth of field, making it fairly critical to get the glow-worm correctly focussed, it does keep the exposure time to a minimum, thus reducing the risk of the picture becoming blurred by movement. Finally, you might like to experiment with the use of a flash during the exposure to bring out the shape of the female and her surroundings.
Finding glow-worms may require a little time and patience, but the reward is certainly well worth the effort.
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