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The Problem of the Supposed Dousing of the Light
In the North American firefly, which is a coleopterous imago, the light flashes at intervals. This is to suggest that the light is either under the control of the insect, which is supported by experiments, or that the energy is built up and becomes released in waves. In the case of the cave dwelling dipterous Bolitophila, the light is continuous so far as the present writer has been able to ascertain. In those glow-worms living outside in the open air, the light seems to be shut off during the day. Certainly when artificial light is put on or near glow-worms at night, they all gradually shut off their lights. Hudson has stated that the glow-worm's light is brightest before dawn, when presumably most prey would be snared.
It has been widely believed that noises of various kinds cause the New Zealand glow-worm to douse its light. This appears to be believed by the guides at Waitomo; when there, the writer shouted in ordinary and falsetto voice, and clapped hands,
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but no noticeable effect was apparent in the great number of specks of light seen on the roof of the cave.
On the alleged evidence of dousing, and of the presence of screens of pigmented cells and fat, just in front of the light source in Bolitophila, Gatenby and Ganguly (1958) suggested a possible mechanism for the supposed sudden dousing. This hypothesis is now abandoned by the present writer. No sudden dousing of the light occurs when various noises are made; nevertheless the withdrawal forwards of the segment containing reflector and light organs has been noted in living larvae observed under a coverslip. Larvae dislodged with a knife fall down the bank but still light up. Some then put in the box did douse. Radio music and clapping together pieces of iron (tire levers) had no effect; and (supersonic) dog whistle was also tried without effect on glow-worms at Arapuni—that is on individuals living outside on banks, where the ability to douse rapidly could be expected to be most useful. But lighted larvae on a table in a darkroom soon douse if the table is accidentally shaken during experiments. In the case of larvae living on or under banks in the forest where retreat tunnels are available, the supposed sudden dousing is caused by the alarmed larvae moving quickly into their hiding place. They do this in about three seconds, especially when near their tunnel, so their light has the appearance of being turned off. On the other hand, it is true that among a number of larvae carried home in a tin box, some will be found luminescent on arrival, some not. The reason for this is not known, but it is certain that the larvae can fade out their light more slowly (in about a minute); but this is quite different from what has been written about sudden dousing, on alarm.
It is not known whether the larva itself excavates its hiding place—but this is likely. Since the larva usually backs into this tunnel, its position brings about the quicker covering of its light, and presents its hard head and jaws towards any enemy which might try to enter the larva's hiding place. In hard-walled caves, few larvae possess a hiding tunnel, and when disturbed they move along to the end of their horizontal web and stop, uncertain what to do.