Art. III.—The Habits and Life-history of the New Zealand Glowworm.
[Read before the Wellington Philosophical Society, 8th October, 1890.]
My former paper on this insect (Trans. N.Z. Inst., vol. xix., p. 62) was written considerably over four years-ago, and is, I regret to say, very incomplete in its details, as well as being in some places absolutely misleading. I will therefore, with the permission of the Society, completely recast my account of the natural history of the glowworm, the present paper thus entirely superseding my previous one. I trust that this course may be allowed, as the insect is one of unusual interest, and also excessively difficult to observe. It is therefore desirable that a complete account of its habits and life-history should be carefully recorded.
My first attempt to discover the nature of the New Zealand glowworm was in January, 1885, when I captured several
specimens and recorded them as dipterous larvæ, but, through pressure of other work, did not figure or carefully examine them.
Exactly a year later I had the insect again brought under my notice during a conversation with Mr. E. Meyrick, who told me that he had written a short note on the animal, which is quoted in full in my previous paper (Trans., vol. xix., p. 62). He stated as his opinion that the larva was referable to one of the Staphylinidæ (Coleoptera), and carnivorous. The light he considered attracted, and the web entangled, minute insects on which he supposed the larva to feed. It will be seen that his supposition as to the nature of the insect is entirely contradicted by subsequent investigation, whilst the latter conjecture has not in any way been corroborated, as we are still entirely in the dark as to the use of either the web or the light to the larva.
During February and March, 1886, I instituted very careful observations on the larvæ, keeping several specimens in captivity. From these I ascertained that the light was not exhibited at all regularly, sometimes being brightest at night, and sometimes in the early morning hours. I have since noticed that, in the natural state, the larvæ shine most brilliantly on dark damp nights with a light north-west wind.
The web referred to above is suspended in a rocky or earthy niche in the banks of streams in the densest parts of the forest. It consists of a thick glutinous thread stretched across the niche, and supported by several smaller threads running right and left, and attached to the sides and end of the cavity. On this the larva invariably rests, but when disturbed immediately glides back along the main thread, and retreats into a hole which he has provided at the end of it. From the lower side of this central thread numerous smaller threads hang down, and are always covered with little globules of water, resembling a number of minute silver-beaded necklaces, constituting a conspicuous, though apparently unimportant, portion of the insect's web. I should mention that all these threads are constructed by the larva from a sticky mucus exuded from the mouth.
The organ which emits the light can easily be seen by a reference to Pl. VIII., fig. 1. It is situated at the posterior extremity of the larva, and is a gelatinous and semi-transparent structure, capable of a great diversity of form. It can be withdrawn or extended at the will of the larva, which, however, can immediately cease to shine without withdrawing it. This action is most likely effected by shutting off the air from the tracheæ ramifying through the “light-organ,” which, being, no doubt, largely composed of phosphorus, only becomes
luminous when in contact with the oxygen in the air-tubes. Larvæ cease to shine on very cold nights, in the day-time, and in a room which is artificially lighted.
During my observations in 1886 one of the larvæ disappeared, and I naturally assumed that it had buried itself in the earth, and was undergoing its transformation into the pupa state. This was apparently confirmed by the emergence, in about a month's time of a fly which was afterwards identified by Baron Osten-Sacken as Trimicra pilipes, whose larva is well known, and has nothing to do with the present insect. It is most unfortunate that a larva of this Trimicra should have got into the breeding-cage without my knowing it, and thus deceived me.
Further investigations were instituted at the end of 1886, when I discovered a luminous pupa suspended in one of the webs in the manner represented at fig. 2, which I have since several times reared from the glowworm, and which is consequently the real pupa. It is a curious animal, furnished with a large process on the back of the thorax, which is attached to the web, and holds the pupa suspended in the middle of the niche previously inhabited by the larva. The light is emitted from the posterior segment of the pupa, but is much fainter than in the larva, and a distinct organ is not apparent. It is frequently altogether suppressed for days together. This pupa died in a few days, and all the larvæ then-under observation also died.
Larvæ were again procured in August, 1888, but this time I did not succeed in getting any of them as far as the pupa stage. I should mention that the larvæ are only to be obtained by walking up the bed of the stream in the big gully of the Botanical Gardens at night, with a bull's-eye lantern. A piece of thin stick is rapidly introduced behind the larva as soon as it is detected, which always adheres to it, and is thus taken away, web and all, and carried home in a tin box with damp moss, &c.
On the 1st September, 1888, I obtained another supply of larvæ, placing them this time in a large bell-glass with stones and ferns, the bottom of the glass having about 1in. of water in it. This I conceived would closely resemble their natural habitat. During all my expeditions I always examined a great number of the webs, and could never find any remains of insects entangled. I also noticed that the largest larvæ were always concealed in the deepest niches in the bank, and frequently behind large cobwebs, where they would stand a poor chance of capturing insects. I also think that there must be a very great mortality among the larvæ, judging from the number of minute ones always observed, in natural conditions, compared with large ones. On the 21st December I
found that two of the larvæ had changed into pupæ resembling the one I found two years before. These unfortunately became mouldy and died, and it was not until the following April that I succeeded in rearing the true fly, which I discovered on the morning of the 4th standing beside its old pupa-skin. Two enlarged drawings were then made, and for-warded to Mr.Skuse, of Sydney, and Baron Osten-Sacken, of Heidelberg, Germany. Both these gentlemen had been previously furnished with specimens of the larvæ in alcohol. They then expressed their opinion that the fly of which I sent a drawing probably did not result from the luminous larva, and that it would be desirable to postpone publication until another specimen had been reared. Convinced though I was of the accuracy of the observation of the 4th April, I determined to verify it, and again procured larvæ, which all died. On the 10th July, 1890, I got about twenty large larvæ, spending upwards of three hours in the bed of the stream. Two of these changed during August into pupæ, one of which died, but the other gave rise to another fly exactly resembling the one reared on the 4th April, 1989. The circumstances connected with the emergence of this fly are so entirely conclusive that it may perhaps be well to relate them in detail. On examining the pupa at 8 a.m. on the 14th, I observed that it had become much paler in colour. At 2 p.m. I noticed the fly perched on it, with its head down towards the tail of the pupa, and the extremity of the abdomen of the fly still within the pupa-skin. In this position it remained until the following day at 5 p.m., when I transferred the fly into a large glass-topped pill-box, which I placed on the table in my sitting-room. On returning to the room at 7 p.m. without a lamp, I was astonished to see the inside of the box brilliantly lit up, the extremity of the fly's abdomen giving out a strong light about half as bright as that emitted by a full-grown larva. The whole of the phenomena relating to the emergence of this fly from the pupa, and its subsequent luminosity, were also observed by my brother, who was present at the time, and can fully corroborate these statements if necessary. As this was a female fly, I decided to take her up into the big gully in the Gardens, where the larvæ are abundant, and see if she would attract males. As soon as I arrived I put the box down in the bed of the stream, and the fly immediately lit up so as to again strongly illuminate the inside of the box. There were lots of larvæ all round, so that I considered it likely there were also flies. After thirty-five minutes I visited the box, but found nothing had arrived. I then left her for ten minutes more, and returned, with the same result. During this expedition I again carefully examined many webs of the larvæ, and took a quantity of the mucus from them home and
examined it with the microscope. I could, however, find no trace of insects entangled on their remains. I am quite at a loss to explain either the light, the web, or the food of the larva. I must leave these points for future investigation.
I should mention that the flies reared on the 4th April, 1889, and the 14th September, 1890, were both females, as it is conceivable that this may have an important bearing on future inquiry as to the use of the light.
I attach a seientific description of the fly, which has been kindly drawn up by Mr. Skuse, of Sydney, for the present paper.
Genus Bolitophila, Hoffm.
Bolitophila, Hoffm., Meigen, Syst. Beschr., i., p. 220, pl. 8, figs. 1–4, 1818; Macquart, S.à B. Dipt., i., p. 126, 1834: Messala, Curtis, Brit. Entom., xiii., p. 581, figs. 1–3, 1836: Bolitophila, Walker, Ins. Brit. Dipt., iii., p. 71, pl. xxiii. fig. 7, 1856; Winnertz, V. z.-b. G., Wien, xiii., p. 672, pl. xix., fig. 5, 1863.
Head small, roundish, fore part flattened. Eyes broadly oval, a little emarginate on the inner side above. Ocelli three, arranged in a somewhat bent line on the front. Palpi prominent, incurved, cylindrical, four-jointed; first joint very small, the following of almost equal length, the fourth the longest. Antennæ setaceous, pubescent, in the ♂ as long as, in the ♀ shorter than the body, 2 + 15 jointed; the joints of the scapus cyathiform; the flagellar joints cylindrical, the terminal one very small, almost gemmiform. Thorax small, oval, highly arched; scutellum small, roundish; metathorax acclivous. Halteres large. Abdomen very long and slender; in the ♂ linear, sub-cylindrical, 8-segmented, without the anal joint; in the ♀ laterally compressed, 9-segmented, the last segment small. Legs long and slender; tibiæ with very short, weak spurs; the fore tibiæ with a single range of spines on the inner side, and the hind pair with one range on the inner and two ranges of shorter and weaker spines on the outer side. Wings large, microscopically pubescent, as long as or somewhat larger, than the abdomen, with obtusely-cuneiformly narrowed base; incumbent in repose. Costal vein uniting with the tip of the third longitudinal vein at or somewhat beyond the apex of the wing; auxiliary vein complete, joining the costa, united to the first longitudinal vein by the sub-costal cross-vein; third longitudinal vein with an anterior branch (which is sometimes wanting), the branch short, almost vertical, ending in the tip of the first longitudinal vein or in the costa; small cross-vein
short, situated about midway between the origin of the third longitudinal vein and the inner end of the second posterior cell; fourth longitudinal vein starting from the base of the fifth longitudinal vein; fork of the fifth longitudinal vein united at its base to the fourth longitudinal vein by a short cross-vein; sixth longitudinal vein perfect.
Bolitophila luminosa, sp. n.
♀. Length of antennæ, 0.090in. (2.27mm.); expanse of wings, 0.250in. × 0.070in. (6.34mm. × 1.77mm.); size of body, 0.380in. × 0.040in. (9.64mm. × 1.01mm.).
Antennæ very slender, as long as the head and thorax combined; joints of the scapus yellow, tinged with brownish; flagellar joints elongate, progressively diminishing in thickness, brown. Hypostoma brown. Palpi yellow. Front and vertex black. Thorax black or very deep brown, levigate, with a median yellow line, the humeri and lateral borders pale-yellow or whitish; two convergent rows of short black hairs from humeri to scutellum; some black bristly hairs above the origin of the wings; pleuræ deep-brown, tinged with pale-yellow; scutellum black; metanotum brown, bordered laterally with yellow. Halteres pallid, the club black. Abdomen slender, sub-cylindrical, five times the length of the thorax, dusky-brown, the segments indistinctly, especially the hindermost ones, tinged with yellowish anteriorly; densely clothed with very short black or dark-brown hairs; extremity and lamellæ of the ovipositor yellow. Legs long and very slender. Coxæ pale-yellow or whitish, the fore and intermediate pairs with the extreme apex, and the hind pair with almost the apical half, dusky-brown; trochanters dusky-brown; femora pale-yellow or whitish, the hind pair black at the apex; tibiæ and tarsi black. Tibial spurs black. In the forelegs the tibiæ and metatarsi of about equal length; the tarsi twice the length of the tibiæ. Wings shorter than the abdomen, pellucid, with a delicate yellowish tint, and almost the apical half infuscated with grey. Costal vein uniting with the tip of the third longitudinal vein somewhat beyond the apex of the wing; auxiliary vein terminating in the costa opposite or somewhat beyond the inner end of the second posterior cell, the sub-costal cross-vein situated near its base; first longitudinal vein running straight into the costa opposite a point before the tip of the posterior branch of the fourth longitudinal vein; third longitudinal vein gently arcuated at its base, strongly arcuated towards its tip; posterior branch of the fifth longitudinal vein abruptly reaching the margin.— F. A. A. Skuse.
Hab. Wellington, New Zealand (G. V. Hudson). Type-specimen in the Australian Museum, Sydney, N.S.W.
Obs. In this species, as in the European Bolitophila tenella. Winn., the anterior branch of the third longitudinal vein is wanting.—F. A. A. A. S.
Explanation of Plate VIII.
New Zealand Glowworm. (Larva of Bolitophila luminosa.)
Pupa of safne.