The male Glow-worm carries only the smaller of these lamps, the two spots on the end segment, which are possessed by the entire tribe. These luminous spots appear upon the young grub, and continue throughout life unchanged. And they are always visible both on the upper and lower surface, whereas the two large belts peculiar to the female shine only below the body.
I have examined the shining belt under the microscope. On the skin a sort of whitewash is spread, formed of some very fine grain-like substance, which is the source of the light. Close beside it is a curious air-tube, with a short wide stem leading to a kind of bushy tuft of delicate branches. These branches spread over the sheet of shining matter, and sometimes dip into it.
It is plain to me that the brightness is produced by the breathing-organs of the Glow-worm. There are certain substances which, when mixed with air, become luminous or even burst into flame. Such substances are called combustible, and the act of their producing light or flame by mingling with the air is called oxidisation. The lamp of the Glow-worm is the result of oxidisation. The substance that looks like whitewash is the matter [[66]]that is oxidised, and the air is supplied by the tube connected with the Glow-worm’s breathing-organs. But as to the nature of the shining substance, no one as yet knows anything.
We are better informed as regards another question. We know that the Glow-worm has complete control of the light he carries. He can turn it up or down, or out, as he pleases.
If the flow of air through the tube be increased, the light becomes more intense: if the same air-tube, influenced by the will of the animal, stops the passage of air, the light grows fainter or even goes out.
Excitement produces an effect upon the air-tube. I am speaking now of the modest fairy-lamp, the spots on the last segment of the Glow-worm’s body. These are suddenly and almost completely put out by any kind of flurry. When I am hunting for young Glow-worms I can plainly see them glimmering on the blades of grass; but should the least false step disturb a neighbouring twig, the light goes out at once and the insect becomes invisible.
The gorgeous belts of the females, however, are very little, if at all, affected by even the most violent surprise. I fire a gun, for instance, beside a wire-gauze cage in which I am rearing a menagerie of female Glow-worms in the open air. The explosion produces no [[67]]result: the illumination continues, as bright and placid as before. I take a spray, and rain down a slight shower of cold water upon the flock. Not one of my animals puts out its light; at the very most there is a brief pause in the radiance, and then only in some cases. I send a puff of smoke from my pipe into the cage. This time the pause is more marked. There are even some lamps put out, but they are soon relit. Calm returns, and the light is as bright as ever. I take some of the captives in my fingers and tease them a little. Yet the illumination is not much dimmed, if I do not press too hard with my thumb. Nothing short of very serious reasons would make the insect put out its signals altogether.
All things considered, there is not a doubt but that the Glow-worm himself manages his lighting-apparatus, extinguishing and rekindling it at will; but there is one circumstance over which the insect has no control. If I cut off a strip of the skin, showing one of the luminous belts, and place it in a glass tube, it will shine away merrily, though not quite as brilliantly as on the living body. The presence of life is unnecessary, because the luminous skin is in direct contact with the air, and the flow of oxygen through the air-tube is therefore not required. In aerated water the skin shines as brightly as in the free air, but the light is extinguished in water that has been deprived of its air by boiling. There could be [[68]]no better proof that the Glow-worm’s light is the effect of oxidisation.
The light is white, calm, and soft to the eyes, and suggests a spark dropped by the full moon. In spite of its splendour it is very feeble. If we move a Glow-worm along a line of print, in perfect darkness, we can easily make out the letters one by one, and even words when they are not too long; but nothing is visible beyond this very narrow zone. A lantern of this kind soon tires the reader’s patience.
These brilliant creatures know nothing at all of family affection. They lay their eggs anywhere, or rather strew them at random, either on the earth or on a blade of grass. Then they pay no further attention to them.