by John Tyler
(Text version only: illustrations not yet prepared for online version)
1. WHAT IS A GLOW-WORM?
2. THE GLOW-WORM'S LIFE
The newly hatched larva
Finding a meal
Having a wash
A change of clothes
The larva's calendar
The adult female
The glow-worm's glow
The morning after
The adult male
Why can't female glow-worms fly?
4. A THREATENED SPECIES?
5. WHAT DON'T WE KNOW?
6. GLOW-WORM WATCHING
1. WHAT IS A GLOW-WORM?
Walk along an unlit country lane on a summer night and with luck you may come across small points of vivid green light shining amongst the grass. These are glow-worms. The name is misleading though, because they are not worms at all but beetles, which can be distinguished from other types of insect by the fact that most species have a pair of thick wing-cases that close over the insect's back like wardrobe doors to protect the wings. These wing-cases are unique to beetles and can be seen clearly for example on ladybirds.
The glow-worm belongs to a family of beetles known as the Lampyridae or fireflies (this is another confusing name, as they are certainly not flies either!). The fireflies are a huge group containing over two thousand species, with new ones being discovered all the time. The females of many species have more or less stunted wings and wing-cases, whereas the males are normally good flyers and still have the characteristic beetle wing-cases.
The feature which makes fireflies so appealing to even the most casual observer, and the one which earns them their name, is their ability to produce an often dazzling display of light. To stumble upon a cluster of a hundred or more glow-worms displaying is like looking down on the streetlights of a miniature town (though it is an increasingly rare sight nowadays), and in the tropics some species gather in their thousands to form walls of lights, all flashing on and off in synchrony. The light is used by the adult fireflies as a signal to attract a mate, and so each species must develop its own 'call-sign' to avoid being confused with other species glowing nearby. So within any one area each species will differ from its neighbours in some way, for example in the colour or pattern of its light, how long the pulses of light last, the interval between pulses and whether it displays in flight or from the ground.
However, at least one firefly has been able to crack the code and make the signals of other species work to its advantage. The female Photuris versicolor, an American species, can impersonate the female of another firefly called Photinus by mimicking the pattern of flashes that a genuine Photinus would use to attract a male. When a hopeful male Photinus arrives expecting to find a mate, the female Photuris turns on him and eats him. It is thought that the predatory female can distinguish between the signals of several different species, and send the appropriate reply to each one.
The fireflies are an extremely varied group. Some for example have larvae which can live (and glow) underwater, and one Jamaican species, Photinus synchronans, is even said to avoid predators by mimicking lizard droppings! Another species, Lamprophorus tenebrosus, has been known to take care of her eggs until they hatch, a rare thing amongst insects, which normally abandon them as soon as they are laid. She will stand guard over her clutch of eggs and gather them up if they become scattered.
The firefly's almost magical light has attracted human attention for generations. It is described in an ancient Chinese encyclopaedia written over two thousand years ago by a pupil of Confucius. They often featured in Japanese folk medicine, and in 13th Century Arabia fireflies were crushed, mixed with attar of roses and dripped into the ear as a treatment for suppuration. All over the world they have been the inspiration for countless poems, paintings and stories. Here in Britain for example there are plenty of anecdotes describing how glow-worms have been used to read by or as emergency bicycle lamps when a cyclist's batteries have failed without warning. Early travellers in the New World came back with similar stories, of how the native people of Central America would collect luminous cucujo beetles (a type of click beetle) and release them indoors to light up their huts. Girls would wear them in their hair for decoration, or a few of them could be tied on a thread around the feet to light up the forest paths at night.
Fireflies very similar to those we see today have been found fossilised in rocks which were formed about thirty million years ago, and their ancestors were probably glowing long before then. The species found today in North and South America are quite distinct from those which live in Europe and Asia, suggesting that the ancestors of the two groups parted company a very long time ago and have remained largely isolated from each other ever since, each evolving in their own direction. It is impossible to be sure exactly when and where the first firefly appeared. The highest concentrations of firefly species today are to be found in the tropics of South America, which may just mean that they prefer the conditions there, or that this is where they first evolved. Wherever it was that they first arose, fireflies have since spread to almost every part of the globe. They may have travelled by sea, on flotsam carried along by ocean currents, or on foot, at a time when South America was still connected to Africa, Antarctica and Australia. Today fireflies can be found almost anywhere outside the Arctic and Antarctic circles, from Tierra del Fuego in the south to Sweden in the north.
Britain is perched on the northern edge of the area in which fireflies can survive, and so we have just two species (compare this to Jamaica for example, where about fifty species live in a country about twenty times smaller that the British Isles). One of them, Phosphaenus hemipterus, is sometimes known as the Little Glow-worm, being the smaller of the two (the male is about 6 to 8 mm in length and the female about 10 mm). Very little is known about Phosphaenus, but it is said to be active during the day and the adults of both sexes are only weakly luminous. Perhaps its one claim to fame is that it seems to be the only known firefly species in which neither the male nor the female can fly. The female has no wings at all and the male's are too small to be of any use. They are covered by equally tiny wing-cases which make him look as if he is wearing a waistcoat. Phosphaenus can be found in Europe, though it seems to be relatively rare (or perhaps just overlooked) wherever it occurs. There are records of it from as far afield as Newfoundland, but it has probably been carried there by accident, perhaps in the ballast of ships. In Britain it seems to have been restricted to a small area of south-east England, and even there it has not been recorded for some years, so it may now be extinct in this country. The other species, and the only one which most of us are ever likely to see in Britain, is the common or European glow-worm, Lampyris noctiluca, which is widespread and relatively abundant. This is the species on which we will concentrate in this book.
2. THE GLOW-WORM'S LIFE
When people talk of seeing a glow-worm they are normally referring to the brightly glowing adult female. This is certainly the most conspicuous stage of the life cycle, but in fact it makes up no more than two percent of her lifespan. As with many insects the glow-worm's life is divided into four distinct stages: the egg, the larva (equivalent to the caterpillar of a butterfly), the pupa (or chrysalis) and the adult.
The glow-worm begins its life in the autumn, as a pale yellow egg, roughly spherical and about a millimetre across (Figure xx). The freshly laid egg is extremely fragile and would burst at the slightest touch, but within a day its surface has hardened into a shell. For the first few days it may sometimes glow with a very faint yellow light, which seems to come either from the yolk of the egg itself or from a thin coating of 'glue' which the mother used to stick the egg in position. The egg usually takes about 35 days to hatch, but the exact time varies according to the temperature, from about 27 days in hot weather to more than 45 days in cold weather. At first the egg appears to be a featureless milky mass, but by the time it is due to hatch the segments of the young glow-worm can be seen pressed tightly against the inside of the shell, with its body arched over and its head touching its tail (Figure xx). By now the larva's light organ is fully developed, its glowing signalling that the egg will soon hatch.
The newly hatched larva
Once it is ready to leave the egg the larva presses its body against the sides until the shell bursts open, and then climbs out. When it first emerges the larva is about five millimetres long and has soft, smoky grey skin, but within a few hours this has hardened and darkened until it is virtually black. Seen from above the larva is made up of a head followed by twelve body segments, each with two yellow spots at the hind corners (Figure xx). At this stage there is no obvious difference between males and females, but from studies of larvae reared in captivity it seems that the two sexes are born in roughly equal numbers. Although its size will increase enormously over the next year or so the larva's overall appearance will hardly change at all until it is ready to become an adult: a large larva simply looks like a scaled-up version of a small larva (Figure xx).
In many insects the larval and adult stages of the life cycle have become specialised for different tasks, and this is certainly true of the glow-worm. The larva devotes much of its life to feeding and building up it food reserves so that as an adult it will be free to concentrate all its efforts on the task of finding a mate and reproducing. The young glow-worm has quite a specialised diet. Like many other firefly species it feeds almost exclusively on snails and slugs, and over millions of years of evolution its body and its behaviour have become beautifully adapted to hunting, killing and eating them.
After a few hours' rest to allow its skin to harden, the young larva is ready to begin the search for its first snail. This may mean several days of marching on an empty stomach, but a typical larva can comfortably maintain a pace of about five metres an hour. This may not sound much, even compared to a snail, but kept up non-stop it would allow the larva to search 120 metres in a day.
Finding a meal
The larva improves its chances of finding snails by concentrating on their favourite habitats. One of the few people who has studied the sorts of conditions that the glow-worm larva looks for is Hans Schwalb, a German researcher, who found that its preferences are very similar to those of the snail. For example the larva is most active at night, when snails do most of their feeding. Captive larvae which were allowed to choose from a range of different light intensities usually opted for complete darkness. They appear to ignore red light though, so fitting a red filter to a torch is a useful way of studying them at night. Captive larvae also showed a preference for moist conditions, again like snails. Given a choice between wet and dry sand they usually chose the wet, and if they were allowed to choose between different degrees of humidity they usually went for 100% relative humidity. In fact glow-worm larvae are particularly sensitive to drying out, and at a relative humidity of about 45% most die through loss of water within a matter of hours. Glow-worm larvae also resemble snails in avoiding very hot conditions. Schwalb found that captive larvae which were allowed to wander along a gradient of temperatures, from the boiling point to the freezing point of water, never willingly went above 40 C.
Because it is largely nocturnal, sight is of very little use to a hunting glow-worm. Even when it is about during the day the larva usually keeps low in the vegetation, where lighting and visibility are often poor. Each of its eyes consists of just a single black facet which detects movement and changes in brightness rather than forming an image as our eyes do, so it certainly could not spot a snail at any great distance. But the larva makes up for its short-sightedness by using its stubby antennae, together with six extremely sensitive feelers (called palps) around its mouth, to explore the ground in front of it. Whenever a larva is walking its palps can be seen constantly waving about, stretching out to touch anything in its path. They seem to be the young glow-worm's main point of contact with the outside world, and it often appears to be totally oblivious of anything going on even a few centimetres away.
It is not clear whether the larva just stumbles upon its meals or whether it can actually follow the trail of mucus left by a snail. Larvae in captivity do sometimes seem to be attracted to surfaces over which snails have wandered, but at other times a larva will walk straight across the trail left by a snail just seconds earlier. But whether it finds its prey by luck or judgement, sooner or later the larva will come face to face with a snail, and then it must tackle a tricky problem: somehow it must overcome a creature which is often more than two hundred times its own weight - the equivalent of a child taking on a bull elephant. And if they meet on a grass stem and the glow-worm attacks too clumsily the snail will just pull itself into its shell, fall to the ground and escape. So, not surprisingly, the larva normally approaches the snail with great stealth and explores its skin very cautiously before delivering the first bite.
The larva's jaws (or mandibles) are sharp and sickle-shaped, curving inwards to meet in front of the mouth. Each has a narrow tube running down its length and opening near the tip (Figure xx). The larva gives the snail's foot a series of gentle nips, quickly drawing in its head after each one. The head is much smaller than the segment immediately behind it (which extends forward like a hood) and is mounted on a long flexible neck so that it can be quickly pulled into the hood at the first sign of danger. As it does so the larva's eye becomes covered by a fold of skin, which seems to act rather like an eyelid, wiping the eye clean and protecting it from knocks and scratches.
It takes less than a second to deliver each bite, but each time that the jaws pierce the skin a small amount of brown toxic fluid is pumped down the hollow mandibles and into the snail's body. The poison is produced in the larva's intestine and is able to digest proteins. The number of bites that the larva needs to overcome the snail depends on their relative sizes: a single bite from a well-grown larva may be enough to halt a snail about a centimetre across, but it may need to bite larger ones ten times or more.
While it is waiting for the poison to take effect the young glow-worm will often ride on the snail's shell (Figure xx), but it is always careful not to set foot directly onto the mucus-covered skin. From time to time the larva will clamber to the entrance of the shell and put its head round the corner, apparently checking whether the meal is ready. Slugs of course do not have this convenient platform and so they have to be worked on from ground level (Figure xx).
At first the snail may try to defend itself by covering its body with a thick lather of mucus froth and the larva has to be careful not to get its legs or antennae stuck. But no matter how cautious the glow-worm is, accidents will happen and larvae often become glued to their snail, or else find it difficult to release their grip after a bite and end up trying to break free by towing the snail along backwards. But as the poison begins to act on the snail's nerves and muscles the victim first becomes paralysed and is then slowly digested into a 'broth' which the larva can lap up. It has a sieve of hairs inside its mouth which it uses to strain off any lumps of flesh which are still too large to swallow, and a pair of pointed blades on the inside edges of its mandibles to break them into more manageable pieces. As it feeds, the larva pours more poison directly through its mouth to speed up the digestion.
Throughout most of the meal the snail is paralysed but still alive. Its heart rate, which rose rapidly after the first bite, now begins to fall as the poison takes effect, but is still going as much as sixteen hours later. In fact in some cases the partly eaten snail has then been known to recover and crawl away!
In captivity, larvae will often feed in a pack, with as many as fifteen of them gathered around the shell of a single paralysed snail, but in the wild larvae are likely to be spread out much more thinly, so sharing their meals in this way is probably a very rare event.
Having a wash
Feeding is a slow and messy business for a glow-worm and the larva often pauses to wander off a little way for a rest and a wash before returning to its meal. To clean off the mucus and partly digested flesh the glow-worm has evolved an extremely versatile little device at the tip of its abdomen. The tail organ is made up of a cluster of branching, cream-coloured tentacles (Figure xx). Each of these is armed with several hundred backward-pointing hooks, less than a hundredth of a millimetre in length and arranged in neat rows across the tentacle (Figure xx). When not in use the organ can be turned inside out and stored away inside the larva's tail. The glow-worm uses it as a scouring pad, painstakingly wiping down every joint and segment of its body and paying particular attention to its face.
As well as being extremely useful for personal hygiene the tail organ has two other uses. Firstly, as it walks along the larva repeatedly curls its tail under its body, uses the tail organ to anchor itself to the ground and then straightens its tail to push itself forward (it can also be used in reverse to pull the larva out of the slimy mess of a half-digested snail). Secondly, in its travels the larva sometimes wanders up plant stems or over stones, and if it loses its footing while climbing the grip of the tail organ is strong enough, at least in small larvae, to take the full weight of its body. The larva, hanging by its tail, can then swing from side to side until its claws are able to find a new foothold. The grip of the glow-worm's tail organ appears to work on the same two principles as a fly's foot. On soft or uneven surfaces, such as soil or plants, it uses the rows of hooks to cling on. But on much smoother surfaces like stone it relies on capillary action, in which a thin film of liquid on each tentacle binds it to the ground (tiny drops of this liquid can occasionally be seen if a larva is allowed to walk over a sheet of glass).
Although very little has been written about their food preferences in the wild, in captivity larvae do not seem particularly choosy about which snail species they will tackle. All of the following species have been eaten in captivity:
Scientific name English name
Aegopinella nitidula Smooth snail
Arianta arbustorum Copse snail
Arion ater Black slug
Arion circumscriptus Slug
Arion hortensis Slug
Arion subfuscus Slug
Cepaea hortensis White-lipped banded snail
Deroceras reticulatum Slug
Discus rotundatus Rounded snail
Ena obscura Lesser bulin
Helicella itala Heath snail
Helicella obvia Snail
Helicopsis striata Snail
Helix aspersa Garden snail
Helix pomatia Roman snail
Isognomostoma isognomostoma Snail
Monacha cantiana Kentish snail*
Oxychilus alliarius Garlic snail
Oxychilus cellarius Cellar snail
Trichia hispida Hairy snail*
Vitrea crystallina Crystal snail
Zebrina detrita Snail
* These species are also known to be eaten in the wild.
The larva's tastes may change as it gets older. For example, a fully grown banded snail makes a good meal for a large larva, but might be too big to be killed by a newly hatched one. On the other hand the large larva would find it almost impossible to get at a small species such as the rounded snail once it had retreated into its shell.
A change of clothes
Because each snail can be several times larger than the glow-worm itself, its body must be able to carry large quantities of food. This is processed into fat and then stored in hundreds of round fat-bodies, which in a well fed larva fill almost the whole body cavity, from head to tail. Each fat-body is about a fifth of a millimetre across and may be pale yellow or pink, depending upon where it is in the body. Each segment of the larva's body is made up of rigid plates on the upper, lower and side surfaces. These are brownish black in colour and act rather like plates of armour, strong enough to support the larva's body and protect it from injury, but largely unable to stretch or bend as the larva grows. On the other hand the skin between the plates, which is a pale cream colour, is far more flexible and elastic. In an unfed larva it is folded into deep creases, so that the rigid plates are almost touching in places, but as the larva feeds the skin both unfolds and stretches, allowing the body's volume to increase dramatically. The larva also has bands of this flexible skin between one segment and the next, and between the left and right halves of the upper plate, which spread apart so that it can expand still further. When this happens a pale yellow line of soft skin appears down the centre of the back: the mark of a well-fed larva (Figure xx).
But the flexibility of the glow-worm's skin eventually reaches a limit and then the larva must shed its old skin and replace it with a larger size. The new skin forms beneath the old one, which then becomes detached and splits open along a built-in line of weakness, allowing the larva to climb free. In many beetles this line of weakness runs down the centre of the back, but for some reason in the glow-worm it forms along the edges of the front segments. The larva, often lying on its back, emerges head-first and discards the old skin by a combination of wriggling, arching its body and alternately expanding and contracting each segment. Several of the larva's body segments have a pair of large backward-pointing bristles on the underside and these may act as spikes to push the old skin away. Moulting can be a slow process, often taking several hours or even a day to complete. And, when the newly clothed larva does finally emerge, it is extremely fragile and would be easy prey for predators such as ground beetles, but after a few hours in the air the skin has hardened and assumed its characteristic black colour. In the course if its life the young glow-worm will moult several times (no-one is yet sure of the exact number but it may be about five for males, and possibly more for females). Moulting is often brought on by a large meal, which stretches the skin to its limit, and the larva sometimes protects itself at this vulnerable stage by moulting inside the shell which it has just emptied. But even if it avoids being eaten by predators, moulting is a hazardous operation and larvae often die or become disfigured if they are unable to shed their old skin successfully. In some cases part of the old skin may become lodged around the new one, preventing it from expanding as the larva tries to grow. In other cases a patch of the old skin may actually fuse with the new one, so that although the larva can moult, it has to keep the moulted skin attached to its body. In one adult female this problem seems to have repeated itself at the next moult as well, forcing her to carry around the skins of both the larva and the pupa.
The glow-worm larva is able to produce a yellow-green light from a pair of organs on the underside of its body, near the tip of its abdomen, and although this light is very much fainter than the female's, on a dark night it can be seen more than five metres away. The larva appears to use its light in at least three different ways, two of which seem to have straightforward explanations, whilst the third is more puzzling:
Firstly, if the larva is disturbed, for example by handling or a vibration, it will sometimes switch on its lights for a few seconds and then turn them off again. It seems quite likely that this is just a defensive reaction, intended to scare off a would-be predator.
Secondly, some larvae have been known to glow continuously for hours at a time without any apparent provocation. These are often fully grown larvae which will soon be pupating, so it might be that this glow, which is very much like the adult female's, is just part of the preparation for adulthood, at a time when the larva's body is undergoing all sorts of internal changes.
These two types of glowing can also be seen in adult glow-worms, but the third type seems to be unique to the larva. As it walks along it sometimes produces definite pulses of light, each one normally lasting a few seconds and consisting of a period of a second or so while the brightness builds up, followed by a period of steady brightness and then a final period during which the light fades and goes out altogether. Each of these pulses is separated from the next by a longer interval of darkness lasting several seconds, creating an effect rather like a miniature lighthouse. This is the larval display which is most often seen in the wild and sometimes leads to the larva being mistaken for an adult female, but it can be distinguished in at least three ways: (1) the spot of light appears much smaller and fainter than the female's, (2) the light is often produced while the larva is on the move, whereas females are nearly always stationary, and (3) the light comes in pulses rather than the steady glow of the female. This pulsing of the light often gives people the impression that the glow-worm has switched off its light because they have disturbed it, whereas in fact the larva is probably completely unaware of them and will light up again a few seconds later.
Unlike the other two types of glowing, which the larva seems to be able to do at almost any time (though of course it is most noticeable in the dark), the larva normally only starts this 'lighthouse' display once the sky around it has reached a certain darkness. This light threshold may be the reason why larvae often fail to glow on bright moonlit nights, and it may also explain why glowing larvae are less often seen in midsummer, when evenings stay relatively light, whereas after about the end of August sightings become much more common.
Several authors have exercised their imaginations by trying to suggest reasons for the larva's curious display, but so far none of them can really be said to have been proven. Here are a few of the most commonly offered explanations:
Firstly, it is possible that the larva's glow has no purpose at all and is just a by-product of the light organs which are developing inside it, ready to be used in the adult female. So far no-one knows for sure whether both male and female larvae glow, but if they do then this explanation would be seriously weakened because adult males do very little glowing. It also fails to explain why the larval light is pulsed, rather than constant like the female's. The other problem is that, as we will see later, the two larval light spots make up just a small portion of the light organ in the adult female. The rest of it does not appear until she pupates, so why must these two spots be switched on several months earlier? It also seems a waste of energy to keep producing light which you don't need.
It has also been suggested that the larva's light is used as a torch to light its way at night, but if this is so then why does it keep switching it on and off, and why does it carry it at the rear, where it lights up where it has come from rather than where it is going? In any case we have already seen that the larva has very poor eyesight and relies much more on touch to find its way around.
Another suggestion is that the larva uses its light as a lure to attract its prey. Apart from the fact that there is little or no evidence to suggest that snails are actually attracted to light, it may seem odd that the larva should make it more difficult for the snail by continuing to keep moving along as it glows.
A fourth possibility is that the light is a signal to other larvae, either calling them in to share a meal or warning them off to avoid overcrowding. The larvae of other firefly species have been found sharing snails in the wild, but our larva gives its performance whether it has found a snail or not. And the glow-worm's poor eyesight, together with the low visibility of its habitat, would probably mean that it would have to come within half a metre or less before it could see the signal, which would not be of much help in preventing overcrowding.
Perhaps more plausible is the idea that by sending out pulses of light every so often the larva is deterring potential predators. This is after all what it does when it is actually attacked, and by using pulses rather than a constant glow it would make it more difficult for the predator to locate (it certainly works for humans trying to find larvae in the dark!). This tactic may not work for invertebrate predators such as spiders and centipedes, whose eyesight is as bad as the glow-worm's, but it may help to scare and confuse creatures like shrews and woodmice.
A similar possibility is that the larva is actually protected by unpleasant-tasting, or even poisonous, chemicals in its body, and that its light is meant not just to scare predators off but to warn them that if they do attack they might regret it. Many insects which are active during the day, such as ladybirds and some moths and caterpillars, use bright colours and striking patterns to advertise the fact that they taste revolting. Once a predator has tried a few and learnt to associate the markings with the taste it is unlikely to make the same mistake again. Bright colours would obviously be of very little use to a nocturnal insect like the glow-worm larva, so it may have chosen light instead. There are certainly reports of birds, lizards and even ants refusing to eat fireflies, but as yet no-one seems to have looked (or tasted) to see whether they contain any unpleasant compounds.
At the moment it is impossible to tell which, if any, of these explanations is the correct one. Of course, it is just possible that they all are!
The larva's calendar
Almost nothing is known about how quickly the glow-worm larva grows in the wild, but if captive ones are anything to go by, its life seems to consist of a short spurt of growth sandwiched between two long and relatively static spells during which it feeds very little and does not change very much in size.
A typical pattern might be as follows. During the autumn of its first year, if there are enough snails about, the larva will probably be able to put on enough weight to moult once or twice. But then as winter sets in it becomes more and more lethargic and eventually goes into a sort of hibernation. This makes good sense, as snails become increasingly hard to find in winter and those that do survive often seal themselves into their shells with a strong sheet of dried mucus, which makes them virtually glow-worm-proof. Larvae often spend the winter under logs, stones or leaf litter, their bodies drawn in like concertinas, with the rear half of each segment overlapping the front half of the one behind it.
I have even found larvae hibernating in the hollow cores of tree stumps in the middle of a reedbed. Nearby, another larva was passing the winter in the centre of a reedmace stem, where it had adopted a tunnel bored out and abandoned by a bulrush moth. The tree stumps and the reeds were surrounded on all sides by water, so the larvae must have been feeding in the reedbed during the previous summer, when it had largely dried out, and then retreated upwards as the water level began to rise in late autumn. They would be marooned there until the water receded again the following spring.
In the spring of the second year, when the larvae wake from their 'sleep', some are not much larger than they were when they first hatched, but then they get down to the business of eating in earnest. Although a larva may live for fifteen months or more, most of its growth will be crammed into the next five months or so, and some larvae will put on so much weight during this time that they will need to shed their skins twice in a single month. The female larvae grow much more rapidly than the males and by the end of August they can be recognised by their larger size, even though it will be another nine months or more before they become adults. By about the end of September the growth spurt will be coming to an end and the larva will stop moulting, lose its appetite and become less active, preparing to sleep through its second winter. The larva begins to feed again in the spring of its third year, but by now it is already nearly full-grown and may not need to moult again before it pupates.
A typical larva's growth is summarised in Figure xx, which shows the average size of 24 larvae reared in captivity and illustrates clearly both the growth spurt and the difference in the rates of growth of males and females. It is important to remember though that in the wild, where conditions and the availability of snails can be quite different, the larva's growth rate and the precise timing of its moults may also vary. The length of a glow-worm's body is a very poor indicator of its overall size because the larva expands and contracts as it moves, so the measurement used in Figure 9 is the width of the pronotum, the large 'hood' at the front of its body. This is relatively rigid and so gives a much more consistent measure of size. The width of the pronotum can also provide a useful guide to the sex of the larva: if it measures less than about 4mm then the larva could be either a male or a young female, but if it is 4mm or more then the larva is likely to be a female.
Only a small proportion of larvae will have survived the full fifteen months or so. Many will have been eaten by predators, or succumbed to fungal infections or dehydration, so that on average only about two larvae from the original brood of fifty to a hundred will live long enough to reproduce two years later. It also seems that some larvae may need an extra year before they are ready to pupate, but it is not yet clear how commonly this happens.
In early summer the larvae which are ready to pupate that year seem to shake off their nocturnal habits and can often be seen striding purposefully along in broad daylight. As we shall see, the adult female rarely moves far before she dies, so it may be that this final larval stage is the one in which glow-worms are able to spread out in search of new habitats.
When it has finished its walkabout and is ready to begin the transformation into an adult the larva usually searches for some form of cover such as a log. Larvae preparing to pupate often gather together in small groups, and it is fairly common to find six or more side by side under one log. It is possible that they gang together in this way by producing a scent to attract other larvae, or each larva may simply wander from log to log until it stumbles upon another one and then stays with it. However they do it, this 'ganging' makes good sense as it saves a lot of time and travelling when the adults emerge and start looking for a mate.
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