Habitation of the Glow-Worms—Waitomo Caves

(By E. J. Barrett.)

“….A radiance became manifest which absorbed the whole faculty of observation—the radiance of such a massed body of glow-worms as cannot be found anywhere else in the world, utterly incalculable as to numbers and merging their individual lights in a nirvana of pure sheen…… To bow the head in adoration of Beauty was but to meet its whole shimmer reflected, unwrinkled, in the quiet river below.” In such eloquent language does a distinguished visitor to New Zealand describe those glittering tabernacles of the underworld, the Waitomo Caves,

The Glow-worm Fly (Arachnocampa Luminosa) and Larva.
(The cross and line indicate the actual sizes of fly and larva respectively.)

which lie in the heart of bush-clad hills seven miles from the Hangatiki Railway Station on the Main Trunk Line. These caves rank high amongst the great natural wonders of the world and every year their popularity as the Mecca of the tourist becomes more firmly established. To how many of the thousands of visitors who have made the boat journey upon the silent underground river which flows through the famous Glow-worm Grotto, has not the question of the “how” of the wondrous radiance of this grotto suggested itself? Stalactites, gorgeous in their mantle of tiny lights stretch down from the roof of this grotto, from which again myriads of similar lights shine down on the enchanted tourist. This wonderful radiance is produced by living organisms. Believing that the subject will be of sufficient interest to the many readers of our Magazine who have planned a visit to those Caves during the present summer season, I shall set down as briefly as possible what science is able to tell us about the fascinating glow-worms which have made Waitomo famous throughout the world.

The phenomenon of light production in the animal kingdom is one of very great interest and has occupied the attention of investigators for many years. A number of insects are self-luminous because they have special light-producing organs; others again are luminous because of the presence of light-producing bacteria in their blood—the luminous sand-hoppers belong to this group. Still other insects owe their luminosity to the injestion of luminous food. The glowworms belong to the group of insects which have special light-producing “photogenic” organs. These organs are situated near the apex of the abdomen, or along the sides of the body of the insects in which they exist. The structure of the light emitting organs is, in all cases, essentially the same, and consists of an outer (light emitting) layer, and an inner (reflective) layer. Delicate trachae (breathing tubes) and nerves connect one layer with the other. The external covering or skin of the photogenic organs is translucent. A microscopic examination of the reflective layer of these interesting organs reveals the cells of which they are composed as containing urate crystals which act as a barrier to the internal dispersion of the light. It is astonishing to observe that the luminious efficiency of the light of these insects—it is especially true of the fireflies—is nearly 100 per cent. It is 97 per cent. greater than the ordinary gas flame (all the rays of which, above three per cent., are heat or chemical rays), 90 per cent. greater than the electric are, and 65 per cent. greater than sunlight itself.

“Queens of the insect world…… What lamp so fit, so pure as thine?”

Though called glow-worms, they are not, in reality, worms at all. The glow-worms of the Waitomo Caves are the larvae of flies of the fungus gnat family, as the glow-worms of Europe are the wingless females of a Lampyrid beetle. The fly and the larva figured in the illustrations page 23 (which have been sketched from Mr. G. V. Hudson's fine reproductions) are individual specimens of the wonderful creatures which are the agents —the Alpha and Omega as it were—of the living radiance of Waitomo. The fly is very rare in entomological collections, being difficult to capture on account of its preference for a dark habitation. It is about twice the size of a mosquito, has dark wings and a long and banded abdomen. The larva of this fly (the glow-worm) is a very delicately constructed little creature, which, when full grown, is anything up to 1 ½ inches in length. Like most larva of flies its body is segmented and semi-transparent. Those who possess a microscope know how fascinating and instructive is the diversion of placing such creatures under the lens and watching the internal organs at work. From such seemingly unimportant activities on the part of investigators, have major discoveries been made for the benefit of mankind. But to come back to the theme of our study. The glow-worms attach themselves by a fine horizontal thread to the roof of the caves in which they are found. This thread is tubular in structure, and being adequately lubricated by a substance produced and exuded by the glow-worms, the little creatures glide backwards and forwards within it as the conditions of safety and danger dictate. From the main thread described, the glow-worms will send out as many as fifty vertical Threads measuring up to two feet in length, a gossamer-like structure of great beauty it is, streaming down from the roof. But what an elaborate, cunning

A Ghmpse of the world-ramed Glow-Worm Grotto at Waitomo. (Note the hanging threads of the glow-worms.)

and formidable death trap it is! Says Mr. Tonnior, the able entomologist of Canterbury Museum who is making researches into the life history of the glow-worms:—

“When a small insect like a midge, attracted by the glowing light of the ‘worms,’ comes in contact with the thread (which is covered with a sticky substance) it is lost without hope, as it sticks there in spite of its efforts to escape. The vibrations generated by its efforts to escape attract the attention of the ‘worm’ which speedily winds up the hanging thread at the end of which hangs its victim, just as an angler would do with his catch. The poor midge is chewed up and completely absorbed by the ‘worm.’”

As with every form of plant and animal life, the life of the glow-worms depends upon the food supply. In Waitomo they live for several months because the food conditions are ideal. The river flowing through the Caves is rich in organic matter upon which the larvae of midges feed in millions. These larvae develop into flies which are attracted to the roof of the cave by the light of the glow-worms, and captured and consumed in the way described. When the glow-worm grows to maturity it re-absorbs its streaming threads, hangs down from the main horizontal thread and commences that most wonderful of changes which culminates in its complete transformation from larva to fly.

Thus do living organisms produce the dazzling beauty of the Waitomo Caves.