None of the mycetophilid larvae described by Madwar (1937) closely resembles that of B. luminosa. Perhaps the nearest is Brachypeza radiata (Jenkinson), which however, has tracheal stigmata, and a body nearly cylindrical. Madwar describes the hooked areas (his locomotory pads) as lying across the segmental line, whereas in B. luminosa, the pad always appears to the present writer to lie in front of each segment. Madwar states that on that part of the pad in front of the segmental line the hooks point towards the head, and those on the pad behind the line point backwards. This is different from B. luminosa. According to Madwar the gastric coeca (mucus glands of the present author's papers) probably produce digestive enzymes. In Brachypeza these glands are poorly developed compared with B. luminosa. If Brachypeza could be regarded as more generalized, it would appear that the larva of B. luminosa has become specialized in the following: (1) The pad hooks point forwards. (2) The mucus glands are highly developed for the production of mucus, not apparently for secreting enzymes. (3) The shape of the body is cylindrical only in front, the hinder portion being more specialized. (4) There are no lateral chordotonal organs as in Brachypeza, so far as Ganguly and the present writer could ascertain. (5) Respiration is apneustic. (6) The anal papillae have elaborate chordotonal organs, which if present in other Mycetophilidae have not been found by authors previous to Ganguly. (7) The reflector and luminescent part of the milpighian tubes are absent in other mycetophilid larvae, though the
malpighian tubes of these larvae are carried down posteriorly. Dr. Gouri Ganguly kindly lent the writer her sections of the British Ceroplatus larva. In it the mucus (?) glands are poorly developed in comparison with the same organs in the N.Z. and N.S.W. larvae of Bolitophila; there are some doubtful cells in the anal papillae, which Dr. Ganguly believes may be sensory in nature. The section of the body is cylindrical, without dorsal folds as in B. luminosa (Pl. 39, fig. 1). The silk glands are very well developed in Ceroplatus. The malpighian tubes reach back as in other mycetophilids like Brachypeza. As is well known, this larva of Ceroplatus has luminescent fatty tissue cells, and leads the same sort of predaceous existence as B. luminosa.
Future study on the genus Bolitophila in the Australian region should include the breeding out or capture of the imagines of the Blue Mountain localities. A further investigation of the ecology of the Ida Bay and other Tasmanian localities, especially with reference to the supposed predators, Idacarabus and cave spiders, would be rewarding. Captive N.S.W. larvae could be fed on house flies in order to see whether food alters the bright colour of the larvae How far these bright colours and dark pigment are due to the Australian sunlight is not known, as cave specimens from N.S.W. have not been examined by the writer for comparison. The chordotonal organs of the N.S.W. form appear to have longer protruding setae than the N.Z. form, but this observation needs confirmation. The sensory setae had become broken off on all the mounted N.S.W. larvae brought back to Dublin.
The writer has had the privilege of discussing the relation between larval and adult forms in the Nematocera, with Mr. David Lee. He states that small larval differences will not necessarily appear in the adults. The modifications of the larva refer to that part of an insect's modus vivendi, and usually the imagines of different species, as would be the case with the glowworms, live similar lives and would not be prone to anatomical differences Mr. Lee showed the writer the large collection of Mycetophilidae at the N.S.W. School of Tropical Medicine. There are few adult mycetophilids from this region as large as the imago of glowworm. The Ceroplatidae seen were all smaller than B. luminosa.
It is quite certain that the Tasmanian and the N.Z. insects are different species as based on observations of the male. The antennae in both N.Z. and Tasmanian males are larger than those of the female of the N.Z. insect Since it now seems to be true that the N.Z. female usually does not luminesce, the male no doubt finds his mate by the use of sensillae in these comparatively very large antennae. It will be remembered from previous reports (Gatenby, 1959; Gatenby and Cotton, 1960) that three live female N.Z. adults were never seen to light up, two having mated, one unmated. The only report of luminescence in the N.Z. female comes from Hudson (1955), whose observation cannot be doubted. The scarcity of glowworms in N.S.W. as compared with N.Z. is due to the fact that the climate in Australia is not so suitable.
The drawing in Pl. 39, fig. 3, was made freehand from a living specimen, and was as accurate as the author could make it under these conditions. The last segment appears particularly swollen as compared with the previous drawing of the N.Z. specimen (Gatenby, 1959; Pl. 23, fig. 4), but in Fig. 7 in the same paper, something of the appearance of the present Pl. 39, fig. 3, is shown. The comparative size of this segment depends on the amount of haemocoelomic fluid directed into it, as the larva struggles under the coverslip. But this segment appears whiter and more translucent in the N.S.W. form because of the deeper pigmentation of the segments in front. The author got the impression that the mesenteron in the N.S.W. form was shorter than in the N.Z. form, but this will need more careful observation of freshly anaesthetised larvae of different sizes, but it is a point for future
Fig. 1—Tasmanian larva (about 23½ mm in length), from dorsal surface. Fig. 2.—Front end of N.Z. larva showing imaginal discs of legs, wings and halteres. Fig. 3.—Free hand sketch of N.S.W. larva, to show zones of colouration. Fig. 4.—Head of N.Z. larva, slightly from one side to show eyes and antenna base. Fig. 4A.-Part of section of head of larva showing the two types of eyes.
Fig. 5.—Part of snare of N.S.W. larva scraped from rock and fixed in 70% ethanol. Fig. 6.—Larval light organ and reflector in transverse section. Fig. 7.—Plan of externals of N.Z. larva, showing imaginal discs, testes, and hooked areas. Fig. 8.—Last three abdominal segments of Tasmanian larva from the side. Fig. 8A.-Position of short straight (Y) and long curved (X) chordotonal sensory setae (see Pl. 42, Fig. 16). Fig. 9.—Plan of hooked areas or pads of N.Z. larva. Fig. 10.—Transverse section of newly metamorphosed pupa showing hollow reflector and male ducts forming. Fig. 10A.-Row of hooks of two individuals from same N.Z. locality to show variation in size and pigmentation.
fig. 11.—The Tasmanian male. Fig. 11A.-First five segments of the antenna for comparison with those of the N.Z. male in Fig. 12A. Fig. 11B.-Coxa and femur of N.Z. male, hind leg Fig. 12-N.Z. female. Small specimen Fig. 12A.-Part of the antenna of N.Z. male.
Figs. 12B and 12C.-External genital segment of N.Z. female. (12B from ventral surface.)
Fig. 12D.-The contour and lengths of the Tasmanian and N.Z. adults antennae. fig. 13.—Halteres of the N.Z. and Tasmanian (A) forms.
fig. 14.—Course of nerve fibre (AN) towards the light origin (LO). Male Fig.-15.-Ditto in female. Note.-These photographs taken under phase-contrast, appear to show a background of congulum at (R) where the space is however, really quite empty under ordinary microscopic observation.
fig. 16.—The curved external sensory seta (S) of the chordotonal organs. The base is seen where the seta emerges from the cuticle. The muscle of the palp and one of the overlying chordotonal organs can be seen sweeping across from upper right to lower left. fig. 17.—Photomicrograph of the hooks on the eight segment. These point forwards.
observers. In the same way the arrangement of the sense organs in the anal papillae depends somewhat on the amount of tension in the papillae, but the same four types of sensillae were present in both N.S.W. and N.Z. forms. Good fixed stained preparations of the internal sensory organs in the anal papillae are hard to make, only a small percentage of whole mounts being useful.
Regarding the nervous system of the larva and imago of the N.Z. form, the following may be stated: Pyramidal prolongations of the nerve fibres (Pl. 42, fig. 15., NE) have been found abutting against the cells of the light organs, and for the time being these have been assumed to be the nerve endings. These, however, have a nucleus near where they adhere to the light organ cell. The actual nerve fibres have been traced to the ganglion in the seventh segment. It was not known what type of nerve ending to expect in connection with the light organ. There are least three branches emerging from the seventh segment ganglion, one to muscles, one to the light organ, and a large branch undoubtedly continuing down to the scolophores in the anal papilla on each side. In the adult as well, nerve ramifications from this ganglion, go to the hind gut and receptacula seminales. No muscles have been found in connection with the larval or adult light organ. It is significant, however, that the cell membrane of the light organ cells has been shown by electron microscopy, to be thrown into folds and tube-like indentations often of a succular nature (Text-fig. 2), and it is suggested that the light is dowsed by closure of the openings of these tubules, thus cutting off oxygen. In some of the preparations, it would appear that tracheoles penetrate the light organ cells, but the writer is very doubtful about this.
There is now much information, derived from electron microscopy, on the ultra-structure of the light organ of the American coleopterous firefly (Photinus pyralis) and a certain amount is now known about the same organ in B. luminosa. The work on Photinus was carried out by Beams and Anderson (1955), and recently W. Bertaud, of the Dominion Physical Laboratory, has made a preliminary study of B. luminosa. The light organs of the coleopterous insects has been much studied by optical microscopy. The important paper by Beams and Anderson still leaves us in the dark as to how this insect controls its light. In Text-fig. 3 the present writer has attempted to interpret Beams and Anderson's excellent electron micrographs. It will be noted that the tracheal end cell (TEC) has concentric bodies in it, shown at a higher power in fig. 4. These bodies (M) were at one time thought to be contractile elements, and in some way were believed to compress the tracheal trunk (T) which passed through both the tracheal end cell, and the end bulb (EB), and thus to cut off the light which in Photinus flashes brightly at intervals as the insect flies. Beams and Anderson have now shown that the supposed contractile elements are really elongated mitochondria, each possibly in a saccule (F), and that all tracheoles are provided with taenidia (the internal supporting spiral), which presumably causes them to be incompressible. Taenidia in such tracheoles cannot be resolved by light microscopy and were not seen by previous observers of Photinus.
In this insect, the reflector is a separate layer (R) containing urate bodies, and actually abuts against the columns of luminescent cells (LC), the space between the two in Text-fig. 3, being left for purposes of simplicity. Air enters at (TT) and presumably is used by the battery of mitochondria in (TEC) and (EB), but two tracheoles (T) continue past (EB) going on to lie between the various luminescent cells (LC, of which only one is shown). Thus it is evident from the work of Beams and Anderson, that not only do the tracheal end cells (fig. 4) partake of this air, but the luminescent cells also have their own supply (T) and there is no confirmation that this supply is cut off by controlled compression within either (TEC) or (EB). Beams and Anderson could not find nerves going to (TEC) or
(EB). We are forced to consider the bags (F) enclosing the mitochondria, certainly present in (TEC) and probably also in (EB), as a possible mechanism of control. But the relationship of the mouths of the bags, and the cell membrane abutting against the central tracheole is obscure. If there are infolds from the centre of the cell forming the bags, closure of their mouths might provide some sort of control. There are no infolds in the cell membranes of the photogenic cells of Photinus. Only in (TEC) and (EB) can such folds be identified as possible.
Turning now to B. luminosa, we find a much simpler arrangement. In the first place, the luminescent cells in this mycetophilid do not appear to contain those photogenic granules, which are so conspicuous in Photinus (PG in Text-fig. 3). In the parts of the luminescent cells examined by Bertaud with the electron microscope, no such granules were seen. In Text-fig. 5, the reflector of B. luminosa is the tracheal supply as well, but swarms of mitochondria lie near the cell wall as in Photinus (Text-fig. 5, M). The infoldings of the cell membrane (MF in Text-fig. 5) have been referred to above. It should be noted that in both Text-figs. 3 and 5, the proportions of tracheal end cells, and luminous cells, and the infolds in the latter figure are quite out of proportion. The arrows point to the direction of light output, (EC) being the external cuticle of the beetle. It is well to notice that in the American firefly two factors have to be considered. First the turning on of the light, and secondly the mechanism of intermittent flashing. The latter does not occurr in B. luminosa. Only the question of the method by which the light is turned on and doused needs an answer.
In Photinus the repeated bright flashing would need a considerable storage of material, which would become exhausted after a number of flashes had been emitted. But some workers might prefer to believe that the extraordinary richness
of mitochondria in the tracheal end cells may have some connection with this. Since the American authors have found no nerves going to the tracheal end cells, the mechanism of intermittent flashing is quite unknown, but the bag-like arrangement of the mitochondria might account for this if the tracheal end cell contracted rhythmically, allowing the escape of stored energy or some sort of stimulus sent across the cell membrane of the luminescent cells. Since the luminescent cells of Photinus are crammed with photogenic granules, the necessity for additional reinforcement from the tracheal end cells would seem unnecessary. It should be remembered that mitochondria observed in vivo are known to be able to elongate and contract quite quickly, but the hypothesis of compression of the tracheal leads would seem to be disproved definitely by Beams and Anderson. The same would apply to B. luminosa, where Bertaud's micrographs of the tracheal reflector of the larva show that the smallest tracheoles have taenidia, and the reflector contains no elements which could be contractile. In Bolitophila, the mechanism of control must be in the luminescent cell itself, and the cell membrane folds (MF in Text-fig. 5) are the only modification indicating a possible mechanism of control—a mechanism one may point out in this case, which is activated from the 7th abdominal ganglion. While the present writer hesitates to disagree with a worker of Beam's calibre, it seems surprising that no nerves have been found going to the tracheal end cells. It is still possible that the flashes in Photinus are triggered off in some way by nerves which have been overlooked by Beams and Anderson, and which presumably go to the tracheal end cells.
As has been remarked, G. V. Hudson alone has seen luminescence in an adult—a female. But recent re-examination of the sectioned material (e. g., that in Pl. 42, Fig. 14) provides no reason for believing that anatomically, luminescence would be impossible. On the contrary, the tracheal system of the adult appears more efficient than that of the larva (Pl. 42, fig. 14, TT, for the large air channel which eventually connects to the tracheal stigmata). The capacity to luminesce presumably does not apply to the male where collapse and degeneration of the light organelles is evident, in one case at least. Past observers, as well as the writer, have noted that light shining on the beads of mucus droplets increases the area of luminescence, and to get this result the larva must throw its light downwards. For other reasons also it has been concluded that the glowworm rests ventral surface upwards. Proof of this in wild larvae can only be advanced by previously setting up a microscope in the field, arranged so as to give sufficient magnification to examine the head, and slope of the anal papillae, and then turning on a light source suddenly so as to observe the larva before it could escape. The use of coloured screens for the light source would be worth investigation—the reaction of the larvae to different colours not being known.
The possiblilty that the size and arrangement of the segmental hooked areas might be useful taxonomically is at present doubtful owing to the fact that a number of larvae got from one situation in New Zealand showed considerable variation. A further examination of a larger number of larvae from N.S.W., N.Z., and Tasmania may, however, be helpful.
The following problems need further research:—
The position of wild larvae on the snare.
The proportion of forward or backward retreats into the hiding place.
The study of parts of the snare wiped on cover-slips, dried and stained.
The manner of origin of the suspensory thread of the pupa.
Repetition of previous lighting up experiments with larvae.
The study of possible predators, especially in caves.
The main reason for mortality in large communities of larvae.
The physical nature and attractiveness of the glowworm light.
The reaction of glowworms to lights of different colours.
A study of nerve endings with recent techniques.