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Measurements of Adult Insects, Etc.
These were made with a ruler and magnifying glass, and can only be regarded as approximate.
Waitomo specimens: Female, entire length, 17 mm; individual wing, 9 mm; wing span, 20 mm; antenna, 4 mm. Male, entire length, 9 mm; single wing, 8.5 mm. The Tanypus (Chironomidae) from Waipu which formed the main food were 8 mm body length.
Hudson gives the wing span of the female (from Wellington) as 17 mm; length of larva, 25 mm; pupa, 7 mm. The present author's Waipu specimens of pupal exuviae measured 14 mm, suspensory cord 16 mm. On January 1, the larvae in the Wellington Botanical Gardens measured 10–12 mm. Large specimens appeared to be absent, but in some cases imaginal discs were present in larvae examined.
The Fate of the Reflector During Pupation.
In Pl. 26, Fig. 9, R, is the reflector and luminescent organs (LO) of the larva in dorso-ventrally oblique cross section. The reflector is supplied with two large tracheal tubes shown at (T) in Pl. 25, Fig. 7, and is itself formed by a dense tangle of tracheoles. It will be seen that each light organ tube lies in a reception groove in the reflector. Probably the sharp points at the anterior and posterior ends of the reflector (shown in black in Pl. 25, Fig. 7) are forward and backward continuations of the edges of such grooves. The reflector is partly divided in front.
Now in the adult, both in the male and especially in the female, there is an organ (r) shown in Pl. 27, Fig. 10, which is believed to be the remains of the reflector. It is so intimately connected to the luminescent tubes that it is almost certainly a reflector, as no other organ exists near this in the larva. But the organ marked (R) in Fig. 10 is a hollow structure, which may or may not have contained air. So far as can be ascertained, this sacculated structure is related to tubules, which are tracheal.
The Food.
As was stated by Wheeler and Williams, the food within the mesenteron consists of insects. In Pl. 24, Fig. 6B, obtained from a larval gut, are chopped portions of the legs of a chironomid or similar insect. The larva may, as has been claimed by Norris, suck out the body contents of its prey. But afterwards it carefully chops
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up the whole insect into suitable sizes for swallowing. Perhaps this is why it is so rare to see entangled insects in the snares. Apart from insects, Norris mentions that the larvae will attack crustaceans, by which he presumably meant wood-lice (Isopoda). It is difficult to believe that an adult wood-louse would not be able to plough its way through the short fishing lines seen in holes on banks where such lice abound. It is not known whether the mucus droplets of Bolitophila contain some substance which immobilizes snared arthropods, but this mucus is non-toxic to freshwater Protozoa. Experiments with weak oxalic acid made at Dublin by Ganguly did not appear to support this. The droplets from glow-worms' mucus glands have not yet been analysed. It seems likely that the sticky mucus, covering the body of struggling snared insects, gradually smothers them. Small larvae from Arapuni not much larger than 5 mm all had food in their mesenteron.
The main food of cave dwelling Bolitophila is midges (chironomids, said to be a Tanypus). A requisite for the existence of glow-worms inside the darkness of caves is a river or pools in which the various aquatic larvae are carried. Larvae of Bolitophila may be seen in dry caves, but only near the entrance. In larvae examined after capture, food can be seen in the mesenteron, during the months when these observations were made (September to January). Compared with the light of the North American firefly, the intensity of luminescence in the New Zealand glow-worm is very much less, so far as the human eye and memory can be relied on. In the case of the North American insect, the flashes can be seen clearly from within a brightly lighted room, as the insects fly past outside. There seems no doubt that under the same circumstances the light of the New Zealand insect could not be noticed.
It is therefore a matter for surprise that the modest light of Bolitophila is sufficient to attract various flying Diptera. The light of the New Zealand glow-worm is distinctly green, and how far some sort of relation between the positive phototropism of the midges and the nature of the light has been built up to the advantage of the predatory glow-worm larva, is a matter of speculation; the present writer did not have the time or facilities for exploring this point in a scientific manner.
It has been stated by J. H. Richards that the light of B. luminosa contains no ultra-violet. In the case of the beetles Photinus, Pyrophorus and Lampyris, there is a continuous spectrum over a very narrow range of wave lengths (about 486μμ to 720μμ in Pyrophorus, the most luminescent of insects). It is known (Wiggles-worth) that within this range, the human eye is most sensitive. So far as the present writer knows, the light of B. luminosa has not been analysed spectroscopically, and its exact range is unknown at present. This may be different from the light produced by Coleoptera.