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Many fireflies prepare for pupation by digging into the ground to create a small chamber, but the glow-worm does not bother with this and instead it just curls up and lies motionless, normally on its back or its side, for a few days until it is ready to shed the last of its larval skins. As with the other larval moults a split forms across the front edge of the thorax and along the sides of the first three segments. Sometimes the pupa frees itself from the old skin by turning onto its front and wriggling away from it, but more often it just lies on its back and expands, contracts and wriggles its abdomen until the crumpled skin has been pushed up to the tip of the tail and can be shaken off (Figure xx). As it is shedding the larval skin the pupa excretes a drop of clear liquid, which often remains balanced on the tip of its tail for several hours or even days.
The pupa's skin is extremely thin and translucent, with none of the armouring of the larva, and through it can be seen the beginnings of the adult structures. At first the areas which are to become the hard parts, such as the legs and toughened plates, are a pale yellow colour, in contrast to the soft skin between them which is pink, but after a few hours the whole pupa becomes a more uniform olive green.
During the pupal stage most of the organs of the larva's body are completely broken down, to be replaced by the adult versions. Among the few organs which escape this fate are the twin spots of the larval light organ, which remain visible through the pupa's skin and will be passed on to the adult. The pupa will often glow in response to handling or a vibration, but will also light up for no apparent reason. It will also wriggle about quite violently if exposed to bright sunlight. The male pupa can easily be distinguished from the female by his smaller size and by his narrow strap-like wings, which are lacking in the female (Figure xx).
Compared to the months spent as a larva, the pupal stage is quite brief, lasting about 8-12 days for a female and 11-15 days for a male. As we shall see later, the transformation from larva to adult seems to involve rather more drastic structural changes for the male than it does for the female, and this may be the reason for him taking a few days longer than her to complete the change.
The adult female
Female glow-worms often appear a few days before the first male. In most years this happens in June or July, though the precise date varies from year to year and from site to site. In some years it is possible to find the odd one as early as May or as late as October.
The adult female bears a striking resemblance to the larva. She too has a segmented body and no wings or wing cases, but she can be recognised by the lack of pale spots at the hind corners of each segment. A pale line of unpigmented skin runs down the centre of her back, through which her long tubular heart can be seen beating once every couple of seconds or so, the pulse travelling up the body from tail to head. She has no proper jaws because she does not need to eat: she will live entirely on the food reserves which she built up as a larva. This makes her life something of a race against time, in which she must attract a male, mate with him and lay her eggs before her energy supply runs out.
The glow-worm's glow
It is the female's light which has earnt the species its name, and the structure of the light organ is probably the most thoroughly studied aspect of the glow-worm's biology. It is set on the underside of the body, towards the tip of the abdomen, and consists of large luminous bands on both the sixth and seventh segments and a luminous spot on either side of the eighth segment (Figure xx). (There are occasional oddities, such as females with an extra spot or two on the fifth segment, but these are extremely rare.) The spots on the eighth segment are those which first developed in the larva and were glowing even before it hatched from the egg, but the two main luminous patches on the sixth and seventh segments were formed much later, once the larva had reached maturity. Starting from the undersurface and working towards the interior of the body, each light organ is made up of three layers (Figure xx), corresponding roughly to the lens, light bulb and reflector of a car headlamp. First there is a transparent window of toughened skin, through which the light shines; then the glowing layer itself, which is made up of large light-emitting cells; and finally a layer of cells packed with crystals of uric acid which act as mirrors, reflecting light back through the window (it is the crystals in the reflector layer which give the underside of the female's tail its pale colour when seen in daylight).
The glow-worm's light, like that of all fireflies, is produced by a string of chemical reactions. The central players in these are two compounds with very similar names but very different structures: luciferin and luciferase. Luciferin is a fairly small molecule by biological standards, consisting of just three rings of atoms (Figure xx). Not all of the details of the light-producing reaction have been worked out, but it appears to involve building up the luciferin molecule's energy and then allowing it to fall apart again, releasing that energy as light. First, oxygen and another molecule called adenosine triphosphate (ATP) are attached to the luciferin molecule, making it more energetic and less stable. This causes the luciferin to throw off some of its atoms, releasing energy in the form of light.
The luciferin molecule is completely dwarfed by its partner, luciferase. Whereas luciferin contains just a couple of dozen atoms, luciferase is made up of over ten thousand. It is an enzyme, a protein consisting of a single chain of carbon and nitrogen atoms with other groups of atoms branching off to either side (Figure xx). The chain is coiled up on itself, not randomly like tangling up a piece of string, but in a very precise and controlled way, giving each luciferase molecule the same three-dimensional shape.
The role of luciferase in the light-producing reaction is as a catalyst, bringing the various molecules together and holding them in the correct positions while they react with each other. Unlike the other molecules, luciferase is not used up in the process, and once the reaction is complete it can let go of the spent molecules and move on to gather a fresh set.
The instructions for making luciferase are carried by the glow-worm's genes, which specify such things as the number of atoms in the main chain and the type of side branches which they carry. Millions of years of evolution mean that each firefly species now carries its own form of the luciferase gene. It is the structure of the luciferase, rather than the luciferin, which determines the colour of a firefly's light, and a change to just a few percent of the gene is enough to change the colour of the light. Different individuals of the same species may carry slightly different versions of the luciferase gene and so produce slightly different colours, and some species may even use different luciferases in different parts of their bodies (for example the luminous click-beetle Pyrophorus lagiophthalmus often has green headlamps and orange tail-lights).
The luciferin-luciferase reaction is used to produce light in a wide range of distantly related groups such as flies, jellyfish, bacteria and fungi. Many luminous groups contain non-luminous species, and vice versa. It may be that the genes controlling the reaction have evolved independently in each group, or that they first appeared over a thousand million years ago in the common ancestor of all these groups and have subsequently been switched off in species and groups which no longer need them. If this is the case then the light-producing genes are probably lying dormant in many non-luminous species, including ourselves.
Incidentally, firefly luciferase has also proved very useful in medical and biological research, where the luciferase gene can be attached to other genes, 'lighting up' to indicate when those genes are active in a plant or animal. Luciferase can also be used to search for life on other planets, because it is able to detect minute amounts of ATP, which is found in every living species (at least on Earth).
The reaction of luciferin and luciferase produces a yellow-green light which covers the region of the spectrum to which the human eye is most sensitive, making it visible many metres away. The reaction is extremely efficient, wasting less than 2% of its energy as heat, compared to about 96% in a typical electric light bulb, so a brilliantly glowing female feels completely cold to the touch. The oxygen needed for the reaction is carried to the light organ by a network of branching air tubes running from a pore on either side of each segment. In fact provided that enough oxygen is reaching the light organ it can continue to glow even after death: one female which was accidentally trodden on and crushed was still glowing several hours later. The female appears to be able to control the bands and spots of her light organ independently of each other, often flashing her sidelights when disturbed and only switching to full-beam when trying to attract a mate.
The female usually begins to glow soon after dusk, which during the season is generally between about ten and eleven o'clock. The start of her display seems to be triggered when the light intensity around her falls below a certain level, which may explain why females in the darkness of a wood will often start glowing considerably sooner after sunset than others on nearby grassland. As well as being able to use darkness as a guide, each female is equipped with an internal 'clock'. So, for example, if a female is kept in constantly dim light with no sunset to act as a cue she will glow approximately once a day, just as she would in the wild. Strangely though, under these conditions the interval between displays is usually slightly less than the 24 hours that one might expect (in some cases her day may be as short as 17 or 18 hours), so that she starts glowing earlier and earlier each night. No one knows why this should be so, though the same effect has been noticed in other animals. During the display she may stay close to the ground, often at the base of a grass tussock, or may climb half a metre or more up a grass stem to make herself more conspicuous to searching males. Because the light organ is set on the underside of her body she has to twist her abdomen over so that the light can be seen from above (Figure xx).
Glowing females can occasionally turn up in very unexpected places. During a survey in 1992 one was reported on pebbles on a beach, another had to be removed from a Post Office and two were found on a farm: one on a sheep's back and the other behind a pig's ear! One female had somehow managed to get onto an island in a pond (perhaps it had fallen in and drifted across, possibly on a bit of flotsam, or perhaps the pond had dried out at some time, allowing it to walk across).
Females sometimes seem to seek out very open areas for their displays. In old quarries for example they can often be found on piles of sand or gravel, several metres from the nearest vegetation, and in gardens they may choose rockeries, paving slabs or driveways, positions which would certainly make them easily spotted by passing males. At the other extreme are females which seem to hide their lights deep within bushes or among dense grass stems, where it is hard to imagine a male ever finding them.
Many species of firefly are able to switch their light on and off quite rapidly, each species producing a characteristic pattern of flashes, but our glow-worm maintains a more or less steady light. However she will often swing her tail slowly and rhythmically from side to side, which when seen through a screen of grass can give the impression of a slow brightening and dimming. Rain does not usually put her off her glowing but on wet nights she will often display close to the ground rather than climbing a grass stem. She usually keeps up her display for two or three hours and if after that time she has not been successful in attracting a mate she will stop glowing, retreat into the grass and return for another performance the following night.
The female is extremely sedentary and can often be found night after night in precisely the same spot. Moving would waste energy, which is at a premium if you are unable to eat. A few females are a bit more adventurous though and may wander a couple of metres between displays (this can make it rather tricky for someone who is trying to count glow-worms as it is hard to know whether the same female has reappeared or whether a new one has started to glow). It is normally only virgin females which glow: once she has mated she is unlikely to repeat her display, though if she becomes separated from her partner before they have finished mating she will often light up again immediately. Clusters of between two and six glowing females can sometimes be found within a few centimetres of each other. These clusters may well be the result of the 'ganging' of larvae mentioned earlier.
The morning after
No-one really knows where a typical female glow-worm spends her days. Even when her position is carefully marked while she is glowing at night, it is not often possible to find her again the following day. It is extremely rare to come across females above ground during the day, though one possible exception was a female which displayed from the middle of a dense patch of brambles, about half a metre above the ground. Every night she was seen on exactly the same twig, within an inch or so of the same spot, and it seems very unlikely that she could have found her way back to it night after night, through the tangle of branches, so she may have been staying there during the daytime.
Where possible the female may try to escape from the light by going underground. In one of the very few cases where this has actually been seen, a female was watched one evening as she crawled down a vertical burrow about 5 mm in diameter and about 8 cm deep. It is unlikely that the female could have done the tunnelling herself, and it seems more probable that this was the abandoned burrow of a solitary bee. The female glow-worm, acompanied by a male, was found at the bottom of the tunnel the following morning. A few evenings later another female was spotted disappearing down a similar burrow. This one had a scrum of five males trying to mate with her, but as she crawled down the narrow tunnel they were all pushed off and left wandering around trying to find her. On another site a female which had been glowing on a path was found the next day in the gap between two paving stones.
The adult male
At first glance the male glow-worm is so different in appearance from the female that it is almost hard to believe that they belong to the same species (Figures xx and xx). Most of the external differences between the two sexes stem from the fact that the male is very much the active partner. He is the one who must do the travelling, searching for displaying females. For this reason he has well-developed wings, protected when not in use by leathery, dark brown wing cases. Unlike us, a male glow-worm's eyesight actually improves with age: as a larva each of his eyes would have had just one facet, but now it has more than two thousand (compared to a female's mere three hundred), allowing him to receive a much more detailed picture of his surroundings. Like the female he does not feed once he has become an adult and so has no jaws, and like her he has the two small luminous spots, inherited from the larva, at the tip of his abdomen, but he lacks the main light bands which are so conspicuous in the female. The male glow-worm does not generally become airborne until half an hour or more after the females begin their display, and usually finishes his search flight before many of the females have stopped glowing. If it is too windy, or if it has been raining, he may even abandon flying altogether that night. When searching for a mate the male flies within a metre or so of the ground, scanning the grass below him for the glow of a female. The upper surface of his thorax extends over the top of his head (Figure xx) and may act as a visor to protect his eyes against knocks. Although he has never seen a female before, he instinctively knows what to look for: the light must be of the right colour, the right brightness, the right size and the right pattern, with two bands and two dots. In one experiment males were able to distinguish between the patterns shown in Figure xx and go for the one most resembling a female (the one on the left).
Once he has spotted a female, the male stops flying and falls to the ground. His aim is remarkable. In one experiment females were placed in glass tubes just 25mm in diameter and yet almost two thirds of the males were able to drop straight down the neck of the tube, while the rest landed within 20cm of the female. The male then covers the remaining short distance on foot. His visor has two clear windows, one above each eye (Figure xx), and these may be to allow him to see any females perching overhead. Having reached the female he uses his sensitive antennae to feel his way onto her back and begins to mate with her. If they are left undisturbed mating may carry on for several hours, but it is quite common to find a female with anything from two to five males (and on very rare occasions as many as eight) jostling for position on her back (Figure xx). A challenger may attempt to prise the mating male away from the female by forcing the front edge of his thorax under that of his rival, lifting him off his feet and pushing him backwards. If this tactic fails he may crawl round to the tip of the female's abdomen and, again using the front edge of his thorax, try to lever the pair apart. By repeatedly dislodging each other in this way two or more males may take it in turns to mate with the female, who often seems oblivious to the struggle taking place on her back. Her main concern, once she has attracted a mate (or several) is normally to make her way back to ground level and her daytime shelter. She may continue to glow for some time after she has started to mate, but her tail is no longer twisted over to display her light organ, so from a distance she often gives the impression that she has 'gone out'. In fact unlike many fireflies female glow-worms seem unable to switch off immediately, even if they are picked up and handled, and the reason may be to do with the structure of their light-organs. In the light organs of most fireflies which can flash on and off very rapidly there is usually a specialised cell, called an end-cell, at the point where each air tube meets a light-producing cell, and it has been suggested that these end-cells somehow act as taps to control the supply of oxygen to the cells. In the glow-worm's light organ these end-cells are missing, so that the air supply is connected directly to the light cells, and this may be the reason why the glow-worm takes longer to switch off. (It would be interesting to know whether the glow-worm larva, which can switch on and off several times a minute, has end-cells, but so far no-one seems to have looked for them.)
Not all glow-worms live long enough to find a mate however. Both males and females can end up being caught by web-spinning spiders, which normally have very poor eyesight and do not seem at all put off by a meal which glows. Some females are unable to attract a male before their energy supplies run out, so their lights get weaker each night and finally go out altogether (few females can survive more than about ten nights of glowing). Ironically, celibacy appears to prolong active life, at least in glow-worms and particularly for females. Studying captive adults, Hans Schwalb found that males which had not mated lived up to three days longer than those which had. This may not sound much, but it is quite significant for an insect whose adult lifespan may only be a week. The effect was even more marked in females, in which unmated individuals lived up to ten days longer than mated ones (in fact this difference probably has more to do with the rigours of egg-laying than it does with the actual business of mating, which seems to involve very little effort, at least for the female).
The male is often the first to go. As he approaches the end of his life he becomes more and more reluctant to fly. He frequently falls onto his back, and each time he does it seems to take him longer to right himself. Finally, after an adult life of just a week or so, he dies.
What the female does next depends largely upon whether she has been able to find a mate. A female which has not mated will continue to glow for as long as she can, holding on to her eggs until the last possible moment in case a male arrives, and then lays them immediately before she dies. She does not bother to arrange them, or even to stick them to a surface. These eggs do not harden as fertilised ones would, but shrivel and decay within a few weeks.
Meanwhile the female which has been able to mate behaves very differently, seeking out a suitable place in which to lay her eggs. If she chooses the wrong spot the eggs may either dry out or become waterlogged, so she uses a pair of sensitive feelers at the tip of her tail to test the surface carefully before laying.
Studies of captive glow-worms suggest that the eggs are not fully mature when the female first emerges from the pupa, so a female which attracts a male and mates on her first night as an adult must wait for a few days before the eggs are ready to be laid, whereas an older female can lay eggs almost immediately after mating.
Egg-laying normally takes just a few days. It is very rare to find an egg-laying female in the wild, but what few reports there are suggest that she lays her eggs close to the spot where she had been displaying. They appear to prefer fairly moist positions, for example under logs and stones, at the base of grass stems or in moss. The eggs may be laid singly or in clusters and each one is stuck to the surface with a film of quick-drying glue. The number of eggs which a female can produce is roughly proportional to her size. A small female may be no more than about 12 mm in length, whereas a large one may be over 25 mm, so a clutch may vary from a couple of dozen eggs to well over a hundred and fifty, but a typical number is probably somewhere between fifty and a hundred. By the time she has finished laying her abdomen, which was once swollen with eggs, has collapsed completely and is little more than a flattened bag (Figure xx). She will be dead within a few days and will not see the new generation hatch to begin the next round of their extraordinary life cycle.
Why can't female glow-worms fly?
It may seem strange that female glow-worms have no wings. After all, in the ancestral firefly, from which all others evolved, both sexes were probably able to fly and at first sight it seems odd to have thrown away that ability during evolution. And yet the glow-worm is not the only species to have done so: a number of other fireflies also have flightless females (though, as far as we know, only Phosphaenus hemipterus has flightless males), so there must be some advantage to it. The answer may be that having evolved a light organ the female does not need to fly in order to find a male. Instead she can just flag one down with her glow. Wings cost a lot of energy to produce, and to use, and there is also a limit to how big a body they can carry, so by doing away with them the female is free to grow larger and put more of her energy into producing eggs. This theory is supported by the fact that in firefly species in which both sexes can fly they are usually about the same size, whereas in species where the female is flightless she is often several times heavier than the male.
An interesting parallel occurs in moths. Here too there are species in which one sex has become flightless; again it always seems to be the female; again she has first had to develop an alternative way of calling up a mate (in this case a scent capable of drawing in males from several miles away); and again by sacrificing her flight she has been able to grow much larger than the male.
But why is it always the female that loses her wings and never the male? Firstly and most importantly, there is no real advantage in him being any larger than he already is, as he only has to produce lightweight sperm rather than heavy and energy-expensive eggs. And secondly, in virtually every insect species studied so far it is the male who is programmed to take the initiative in mating: it is he who must hunt for females, and so his wings are more important to him than the female's are to her.
Although being flightless has made good evolutionary sense to the female glow-worm for millions of years it does limit her ability to travel from site to site, and this could now have serious consequences, as we shall see later.
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