Tuatara: Volume 8, Issue 2, May 1960
The New Zealand Glow-Worm*
The New Zealand Glow-Worm*
The scientific name of this insect is Bolitophila luminosa. Some years ago the name was changed. I think on unsatisfactory grounds, to Arachnocampa luminosa— so let us call it B. luminosa. This insect belongs to the group of fungus flies, Mycetophilidae, sometimes called shade-flies because they like damp shaded situations. Most of the larvae of these fungus flies live on damp rotten vegetation, or they bore in fungi such as mushrooms. In the British Isles it is difficult to find an old mushroom which is not being tunnelled by small white mycetophilid worms. From such lowly ancestors our glow-worm is descended. The first European scientist who noticed the glow-worm was Meyrick, about 1880, who saw it on the banks of a stream at Auckland. Meyrick thought the light came from the head end, whereas we know it comes from the tail end. Meyrick had noted the peculiar snare that this insect makes, and he rightly thought that the glow-worm might catch insects and eat them, as do spiders. Most other well-known fire-flies, or luminescent insects, are beetles, belonging to the family of which the click beetle is a member.
The distinguished New Zealand entomologist George Vernon Hudson, who died not many years ago, was very interested in glow-worms; he encouraged a young friend, Albert Norris, to help him in his observations. Norris was an exact observer, and all his descriptions of the behaviour of the glow-worm have proved correct. Unfortunately Norris died as a young man. He was one of many young people that G. V. Hudson had encouraged in studies of New Zealand entomology. No one knew what organ in B. luminosa produced the light, but in 1915 two Americans, Wheeler and Williams, had, after a visit to this country, taken some glow-worms home in a bottle of methylated spirit, and they correctly stated that the light organs were the swollen ends of the malpighian tubules (Fig. 1, L). Malpighi was a famous Italian anatomist of the Middle Ages, and his name is also linked with a part of the vertebrate kidney. In almost all insects waste matter is stored up in from four to eighty or a hundred malpighian tubules which connect to the insect gut where the mesenteron (stomach) and intestine join. Before an insect like a butterfly pupates, these stored-up waste urates are cleared out via the intestine.
The discovery of Wheeler and Williams was all the more remarkable as there seems to be no reason why part of the malpighian tubules should become so modified as to be able to produce light. In the beetle fire-flies, page 87 light is produced from modified fatty cells. This also is the case in a European mycetophilid. which also builds a web, not so elaborate as that of B. luminosa
About 1892, Albert Norris showed that the glow-worm was predaceous and carnivorous, and the snare was used to capture insects which had been attracted by the light. That this is true was finally proven by Wheeler and Williams in 1915, when they found pieces of chopped-up insects in the mesenteron of the glow-worms (Fig. 1, M).
Those who examine the snares of glow-worms living outside on river and creek banks, will notice that there is a curtain of hanging or vertical silken lines which are fastened beside a more or less horizontal silken runway on which the worm lies (Fig. 3). If you touch the snare the worm rushes immediately into a tunnel or crack (HP) into which the runway is continued. If you now carefully cut away the side of the tunnel you will find the worm still glowing, but it soon douses its light. Thus it can retreat into its hiding place in about three seconds, and this covers its light. People have thought that the worm can douse its light in a matter of seconds, but this apparent almost instantaneous dousing is due to its retreat into a crack or tunnel. We will return to this matter later.
The vertical hanging snare lines (Fig. 3, SL) are usually about an inch long; but in miners’ abandoned shafts, and in caves, these lines may be twenty-four inches long. On banks, the worms usually live in sheltered places, and they hang lines longer than an inch, if they have found that wind does not tangle up their net. The vertical lines have regularly placed droplets of mucus on them, so as to give the appearance of a string of beads. This mucus helps to entangle insects which fly against the curtain. One snare photographed years ago by Dr. Salmon had more than forty vertical lines. The larger the snare, the better the chance that it will catch food. Albert Norris said that the horizontal line in which the worm reposed was really a tube. It is true that the worm in this position is itself covered with mucus and silk, which resembles a tube. In the case of the wheel-like web of the spider Epeira. the spokes of tension lines are of plain silk, but the spiral lines of this snare have little sticky droplets. In this case the spider spins a silk line, and as it hardens, a gland covers the silk with a thin sticky substance which by surface tension becomes resolved into a single chain of droplets. It was thought that the droplets of the snare of B. luminosa might be so produced, but recent observations seemed to have shown that the worm emits a single thread, waits, and then spews out of its mouth a droplet of mucus the correct size. Then the silk line is continued, then another mucus droplet is put in place, and so on. When a chain of the right length is produced, it is held by the larva, and then stuck in the right place, at the right distance from other already spun vertical lines. Thus this insect is able to judge distance. If you sweep away the vertical curtain with a stick, the worm will that evening begin to make new lines, and will finish the job in a night or so. The very long lines seen in curves each probably represent the capacity at one time for secretion of the page 88 mucus and silk glands of the worm. This may seem unlikely, but it should be remembered that the spider Epeira can make a whole new web in about twelve hours, and the glands of B. luminosa are comparatively very large.
Now as regards the anatomy of the larva, Gouri Ganguly, an Indian woman zoologist who studied these larvae at Dublin, found that the two tiny thumb-like papillae at the end of its body contained chordotonal organs. These are peculiar sense organs found in various parts of the anatomy (usually on the legs) of insects. They are sense organs designed to register vibrations, and they consist of an arrangement like an elastic thread on which are wrapped sense cells (Fig. 2, SC). The struggles of insects which have been caught by the vertical fishing lines are thus noted by the glow-worm, which at once climbs down these lines and kills the insect with its powerful jaws. The struggles of the captured insect, and the descent of the glow-worm, make a muddle of the snare, but this is later straightened out by the larva. The latter just sucks out the blood of its prey, and if food has been scarce, as it must be in caves, the larva with its mouth-parts carefully saws the body and legs of the prey into the right size for swallowing whole. If there has been a good supply of food, the larva chops up only the juicy parts, and discards the harder regions of the prey.
When the larva is about a little more than an inch in length, it is full grown, and it now makes preparations for pupation. These have rarely been seen, and never properly described. We believe that the larva clears away all its vertical sticky lines and releases one end of its horizontal runway, so that it hangs in a bare space under a bank, or from the roof of a cave. It now sloughs off its larval skin, and becomes a hanging pupa.page break
Figs. 1-4. All figures semi-diagrammatic.
Fig. 1: Larva showing main organs. Fig. 2: Chordotonal sense organs in anal palp (P), in Fig. 1. Fig. 3: Plan of typical snare. The luminescent organ is on the left, head on right. The runway (RN) leads into the hiding place (HP) on the left. Fig. 4: Plan of recommended arrangement for keeping larvae for observation. Rotten branches are braced together, bark left on, or holes made for hiding places. Glass cover is essential. Rotten branches can be kept damp more easily than new ones, or than stones. Old cement slabs are also good.
B, brain; D, culture of fruit flies (Drosophila) in banana; H, head; HO, curved sensillary seta; HP, hiding place; J, jaws; M, mesenteron or stomach; MG, mucus gland; MP, 1, 2, 3, 4 parts of the malpighian tubes, No. 4 (L) being the light organ; N. nerve from chordotonal sense organs; O, oesophagus: OV, oesophageal valve; P, anal papilla; R, rectum; RN, horizontal runway; S, silk gland; SC, cells of chordotonal sense organ; SL, vertical fishing lines with mucus droplets; X, position of nerve ganglion enervating the segment with the light organ.
It is known that this pupa is occasionally luminescent. After a time, said to be three weeks, the adult insect emerges. The imago is something like a dark mosquito, but is much bigger. If you find a midge-like insect, body 17 mm. long, wing span 20 mm., antenna 4 mm., inside a cave, mining shaft, or under a wet bank where glow-worms live, it is likely to be the adult B. luminosa. Likewise if you find a hanging pupa 7-10 mm. in length in the same places, it is a glow-worm pupa. The Tanypus or chironomid midge which is often found in such localities is only 8 mm. body length, and is brownish. The glow-worm adult's abdomen has dark and light intersegmental parts and is unmistakable. Its wings are sooty at their outer ends.
It was stated by G. V. Hudson that the female adult is luminous, the male not. Recently anatomical work has been carried out on the adult insect and it is now known that both sexes have light organs, but in the male they appear degenerate. This supports the observations of Hudson and Norris. It is known that the females of some beetle fire-flies attract their mates by flashing the lights, and something like this may apply to the New Zealand glow-worm, but we are not at all sure of this. The glow-worm female lays up to eighty eggs.
The larva or glow-worm has a large nervous system consisting of a brain, a ventral nerve chain, and nerve ganglia in each segment except the last. We know that the chordotonal organs and muscles of the last segment have nerves going to and coming from the last abdominal ganglion in the seventh segment, but we are not yet certain about the innervation of the light organ. Recently some experiments have been made on the control of light by the glow-worm. If the head of a luminescent worm is cut off, the light goes off. The same takes place if the body is cut away above the ganglion in the seventh segment. If, however, the light organ is isolated from the seventh ganglion by a cut, the light comes on.
So far as we know, what actually happens during the life of the glowworm is this. When night falls this is noted by the larva's eyes; the latter are small, but capable of registering light waves. A stimulus from the brain is sent down via the central nerve cord to the ganglion in the seventh segment, and some sort of reaction here raises the block which has prevented the light from turning on. It is generally believed that the deprivation of O2 keeps the light organs from functioning, but how this is brought about in B. luminosa is not under stood. Further careful anatomical examination, which is being undertaken, may help to explain this.
There are many readers of this article who can help in solving some of our problems about the New Zealand glow-worm. If you live near glowworm caves, shafts, or banks, here is what you should try to do. Get some waterproof tags such as gardeners use, go out at night with a flash lamp, and put a tag and date near the biggest glow-worms you can find. Visit these places at least once a week to find the pupae when the worms pupate. These can be brought home and kept hanging in a damp, wide-mouthed, page 91 covered bottle in a cool place, until the adults emerge. These should be sent to Dr. Salmon at the Zoology Department, Victoria University of Wellington, who will see that they are properly studied. This applies especially to glow-worms found in the south of the South Island of New Zealand, and the north of the North Island. We need to know whether there is more than one species in New Zealand.
G. V. Hudson and Albert Norris bred glow-worms, and this is not too difficult. In Fig. 4 is a suitable arrangement. The secret is to keep the air damp— the glass cover should fog over with water vapour, or the worms will be uncomfortable. Rotting fruit in tubes (banana for fruit flies) is suitable to breed food for the glow-worms, but they can be fed on small house flies. The wings of the latter should be clipped off, or the flies will get off the snare.
Finally there is the question which everyone is bound to ask— how did it come about that the descendant of humble fungus larvae changed from a fungivorous life to a predaceous one? How did the physiological changes in the ends of the malpighian tubes arise, and finally, how did the larva learn to make a clever snare?
But these questions are similar to many which can be asked of other animals and plants. How, for example, did various molluses depart from their sedentary life and swim with the best of them in the ocean? It is true that other fungus flies have developed in a smaller way the peculiarities of B. luminosa. When I was a student we were taught to believe the Darwinian Theory of Natural Selection. Nowadays there are many biologists who find this theory unsatisfactory, but truth to tell the other theories seem equally difficult to believe. Another fungus fly (Ceroplatus) has the power to produce light, but not from its malpighian tubes; the light in this case comes from the fat body. This denotes a major physiological change in the metabolism of the fat cells; but how did these insects Ceroplatus and Bolitophila find that this capacity to produce light could be used to attract food, and how did this apparently chance physiological change come to be transmitted to, and used by its descendants? The Mutation Theory postulates that these major changes took place by comprehensive steps suddenly, which is not easy to believe. These major changes in Bolitophila are so many that it is not possible to believe that they all came at the right degree of development, and at the right time to produce the glow-worm as we know it to be today.
The Lamarckian Theory still has its adherents. It seems possible that, the behaviour and bodily changes of the animal during its life could be impressed on its germ cells and so transmitted to its descendants.
Here are some other problems which need solution:
|(1)||What is the effect of weather and electric storms on glow-worms? Why do they not light up on some nights?|
|(2)||Is the thread by which the pupa is suspended a part of the body. or is it the horizontal runway?page 92|
|(3)||What is the wave length of the light?|
|(4)||Do glow-worms survive in winter, and if so, at this time have they food in their gut|
|(5)||Does the mucus contain a poison? The mucus has been found to be non-toxic to cultures of protozoa and aquatic larval nematodes.|
|(6)||Does the insect mix mucus and silk, or do these keep separate at all times?|
|(7)||What is the duration of the stages in the life cycle?|
|(8)||Has the glow-worm or its pupae any predators? Keep any small parasitic insects which emerge from the pupae.|
|(9)||Does the light shine more brightly in an atmosphere of oxygen, and the reverse in carbon dioxide?|
|(10)||How do the newly hatched larvae manage to survive? Is this period the time when most glow-worms die?|
|(11)||Does the female mate just after emergence? Does she attract the male by lighting up?|
|(12)||Do the glow-worms shift the position of their snares if their food supply is unsatisfactory? Do they wander away before pupation? It has been noted that over a period of two months, at the Wellington Botanical Gardens, glow-worms in an isolated hole did not wander from the site, but if they are disturbed they may wander away from the marked places.|
|(13)||A flash lamp causes the glow-worm gradually to douse its light. Do different coloured screens all affect the worm in the same way? This is important from the point of view of watching the worm at night.|
For fixing pupae for anatomical examination, Carnoy's fluid is suitable. It is equal parts of chloroform, glacial acetic acid and ethyl alcohol. Leave pupae in this for two hours, then store in 90% alcohol. To send adult insects, put a little cotton wool in the bottom of a tube, then put in the insect, then a wad of cotton wool so as to leave the insect in a good space, then cork.
HUDSON, G. V., 1940— Fragments of New Zealand Entomology. Contains all papers up to 1940.
GATENBY, J. Bronte, 1959— Notes on Bolitophila luminosa. Trans. Roy. Soc. N.Z. 87 (3 and 4): 291-314.
* From the Zoology Department, Victoria University of Wellington.