Art. XXII.—On Charagia virescens, Dbld.
[Read before the Wellington Philosophical Society, 18th November, 1902.]
The species which forms the subject of this paper is the largest New Zealand Hepialid, a very handsome species, expanding in the imago stage from 3 ½ in. to nearly 6 in. It has from time to time been described under the generic title of either Charagia or Hepialus, the latter being adopted by Mr. E. Meyrick and by Mr. G. V. Hudson. I, however, would urge the acceptance of Walker's name Charagia for virescens and its Australasian allies, not in the generic sense, but as a subgenus of Hepialus.
The larvæ of true Hepialus, the type of which is humuli, Linne, are subterranean root - feeders, perhaps sometimes phytophagous, never lignivorous. The species are found in Europe and Asia—excepting one, so far as I know un-described and therefore doubtful, from Guatemala—and nowhere else.
Three subgenera of Hepialus occur elsewhere—Phymatopus, Wallgr. (Europe, North America); Sthenopis, Pack. (North America); Cibyra, Walk. (Europe, Asia, South America).
The species which may be referred to Charagia are wholly Australasian, the larvæ lignivorous, feeding in vertical burrows, no doubt an important item of their pabulum being the sap of the tree. Geographical distribution, supported by the habits of the larvæ, warrants the title Charagia, despite the fact that no definite structural difference can be found between Charagia species and Hepialus species. It appears to be a more natural indication of affinity to associate
them thus than to arrange humuli, followed by wood-borers, and again by subterranean feeders, as is done, I believe, in the British Museum collection.
Hepialidœ appear to be remarkably uniform in the ovum stage; the size of the ovum varies between species, being smaller or larger by comparison. C. virescens ova are rather large; but all are smooth and spherical, and when extruded are yellowish or whitish, and afterwards turn black in colour. All the species of various genera of Hepialidœ of which I have knowledge agree in this respect.
No descriptions have as yet been published of newly hatched Charagia larvæ. I very much desire to obtain fertile ova for this purpose, having always failed hitherto, and the newly hatched stage is calculated to furnish evidence of affinity of greater value than the adult larva.
Mr. R. Illidge and myself have examined larvæ at a more advanced stage of several species, and there is remarkable uniformity—I may say almost identity—of structure between Australasian Charagia and virescens of New Zealand, which have abundant specific distinction in the imago stage. It is not necessary to detail the larval structure of Charagia, this having been, done in our paper on Australasian wood-boring Hepialidœ,* but for the purpose of this paper a note may be made that the prothorax has an anterior dorsal series of equidistant setæ, a mid-lateral scutellar concavity enclosing in virescens three setæ when young, one only in adult larvæ, with always a single seta below the concavity; spiracle enlarged and posterior; a lower anterior tubercle bears two setæ. The abdominal segments have normal single-seta trapezoidal tubercles; supraspiracular tubercle has two setæ; spiracle anterior; two subspiracular single-seta tubercles a little below posterior almost in line with each other; an anterior single-seta tubercle; and four setæ on base of the abdominal feet. Of these the prothoracic scutellar concavity appears to be peculiar to Charagia, the other features being more or less typical of all Hepialidœ.
Mr. Hudson mentions† that young larvæ (how young?) are olive-green in colour. I have examined many larvæ from about ¾ fin. in length without seeing this form, all being normal in colour and practically so in structure with more adult larvæ—i.e., yellowish or reddish. Amongst others, I once took a larva about 1⅛ in. in length which differed remarkably from the normal — an undoubted Hepialid from tubercle structure, a Charagia from prothoracic scutellar concavity, and almost certainly virescens, since this is the
[Footnote] * Proc. Roy. Soc. Queensland, 1900.
[Footnote] † “New Zealand Macro-lepidoptera.”
only species in New Zealand. The larva appeared to be marked by alternate brown and yellow transverse dorsal lines, with lateral and ventral brown spots, closer examination showing the elevated portions of segments, subsegments, and tubercle areas to be brown, hard, and polished (chitinous?), the incisions and ground-colour being yellow. The thoracic segments did not differ so from the normal. This larva, maybe, had retained its juvenile characteristics until a later stage than usual, or it is, perhaps, an exceptional form. In either case it is of considerable interest that a form of Charagia should occur having dorsal and lateral shields on the abdominal segments. Typical Charagia have only the prothoracic scutellum hard, the remaining segments being fleshy on the dorsal area.
In an account of this species it would be unwise not to mention the curious fungus, known commonly as the “vegetable caterpillar” (Cordyceps hugelii, Corda; Torruba robertsii, Hook.), which has been so commonly associated with the larva of C. virescens. I have examined specimens which retain the tubercles fairly well, and find the arrangement to be that of the Hepialidæ so far as it can be made out. I agree, however, with Mr. Meyrick—and believe Mr. Hudson has somewhere given the same opinion—that the larva is not C. virescens, which does not leave the larval burrows until emerging as an imago. As far as I know, the host has never been identified; being subterranean, it is probably of the Porinœ.
The larva of C. virescens, when full fed, retires to a vertical burrow, which it seals up transversely with a silken operculum at the entrance. It is not correct, I think, to term this operculum a “trap-door,” as I and others have done. The latter name suggests a definite hinged cover, whereas I do not think this to be the case. When the imago has emerged the operculum frequently remains attached at some point to the rim of the burrow. This is not invariably so, but when so attached it is always on that side farthest from the passage used by the pupa, otherwise the operculum might prevent egress. However, this is not an argument in favour of the belief in the trap-door, as the larva might easily provide for contingencies by making the one side weaker than the other at the suture of the operculum and the rim. As a matter of observation and examination, I believe the operculum of this species and C. daphnandrœ, which makes a similar prepupal operculum transverse to the burrow, is not hinged. C. eximia, another Australian species, makes its prepupal cover inner to the external cover, therefore vertical, not horizontal. Incidentally, which is the more ancient method?
The passage from the larval burrow to the outer chamber is effected by the pupa, which removes the operculum, and, I believe, the external cover also, but this I have not been able to verify by actual observation. An imago had failed to emerge from one pupa which I took from an outer chamber, the external cover being already removed. Here is an interesting problem: the external cover is usually demolished with considerable completeness, but the pupa, remaining as it does with the anal segments within the larval burrow, could not in many cases reach the external cover to remove it and permit free egress of the imago.
Examination of the pupa may here help us. The head, prothorax, and mesothorax dorsally are hard, longitudinally but finely corrugated on the head and mesothorax, irregularly so on the post-thorax, which is smooth posteriorly. Undoubtedly this is a protection to that portion of the contained imago, and would resist tremendous pressure. One would think the pupa requires less, not more, protection than the larva, which has done all the boring, and in its later days, having no more to do, it loses the dorsal hard area (scutellum), becoming more fleshy.
It seems that the removal of the operculum, and likewise the external cover, is due to aerial pressure from within, initiated by movement of the pupa—in fact, literally blown off. It must be remembered that the pupa is located at the lower end of the vertical burrow until the time of emergence. The pupa in bulk approximates very closely to the circumference of the burrow, and the segmental spines give it great power in forcing its way upward. The anterior portion of the pupa would necessarily need to resist great pressure, which, as we have seen, is possible by the corresponding greater strength of that portion. The pupa does not appear to expand when making its escape from the burrows.
I have for several years taken annually a fair number of imagines, both males and females, from the trunks of Hoheria populnea, Cunn., where they may be found drying their wings an hour or so before dark during the month of September in this district. The females are very variable, males more constant, the species being sexually dimorphic as regards wing shape and markings.
The males vary as regards the size of the typical white spots of the fore wings, and in having occasional additional ones, but always have faint pale-green ring-like markings also. Unlike the majority of its sex, one male which I have taken has all those usually pale-green markings of the fore wings distinctly and decidedly white in colour. I propose for this aberration the name “albo-extremus.” It must be very rare, as I never saw but the one specimen—which, indeed, I am now
sending to my valued correspondent Mr. Roland Illidge, of Brisbane, in recognition of the very great assistance he has from time to time rendered me in my studies of the Hepialid group of Lepidoptera.
The females vary as regards the dark markings. On the green ground-colour there are outlined a series of spots which in the male are white, also an outer series, and there are constantly dark costal markings. Numerous pale-green elongate markings are on the wing-surface generally, these likewise in some specimens being outlined with dark-fuscous. A not un-common form has the green ground-colour entirely replaced with a dark colour which may be rather rusty in hue, with the fine lines and markings green in colour but blurred in outline, or, as in the figure, very distinctly approaching black, with numerous transverse lines and spots clearly defined in green. The latter form appears to be that described by Butler* as Charagia hectori. If it is permissible to use as a varietal nomen a name which was originally applied in the specific sense, I should like to see this distinct and handsome form of the female recognised as ab hectori, Butl.
It does seem important that varietal forms of Lepidoptera should be distinguished by names, the comparative rarity or otherwise of which may then be placed on record with who knows what valuable result to science in after-years in a country like New Zealand, when the local fauna becomes, as it undoubtedly must, more or less modified by altered environment brought about by the agency of man, the effects of which may be better observed in a comparatively small area than in large continental areas.
May we speculate on what was the primitive stock from which C. virescens was derived—i.e., Charagia or Hepialus? There is little doubt the colour was green, the white of the male and dark female markings being more recently acquired. This is found in the greater permanence of the green coloration as compared with the other less constant colours. Everywhere in the Hepialidœ we find white is a varietal colour; indeed, in Hepialus humuli the male is uniformly white on the upper surface of both wings. The original form, resembling the female coloration and markings, still exists in the Shetland Islands, being known as “var. hethlandica.”
The Queensland species, C. daphnandrœ, is just such a species as might have been ancestral to C. virescens; the coloration of both sexes is the same, being green, with scarcely any wing-markings. Several Australian Charagia have silvery spots on the fore wings, amongst these being C. eximia. We can safely place the white-spotted C. virescens
[Footnote] * Proc. Zool. Soc. Lond., 1877.
as intermediate between. C. daphnandrœ and these latter species; the further similarity in larval habits indicates very close relationship, if not actual descent, of C. virescens from the Queensland species C. daphnandrœ.
Hitherto my friend Mr. Blackmore Lane has accompanied me at the proper season to search for C. virescens on the Hokowhitu property of Mr. James Ritchie, near the banks of the Manawatu River, where Hoheria populnea (thousand-jacket tree) grows in fair quantity; it was possible, however, to get there only at intervals of sometimes a week. This season, in addition to our joint visits, Mr. Ritchie made constant and almost daily search at dusk, and when successful brought his captures to me to prepare and set, and a larger number than ever before have passed through my hands.
The first specimen, a female, was taken at light on the 6th September, it having already deposited all excepting one of its ova, and that proved infertile. The weather being cold, it was not until the 19th September that others were taken, although repeated search was made. Subsequently the equinoctial gales were accompanied by frost, and none were seen until the 29th September; from this date they were taken with more and more frequency, sometimes one or two specimens only, sometimes many, until early in November.
At first, and during the prevalence of a rigorous atmosphere, a considerable number of crippled specimens were found, whereas in previous seasons, with more genial temperature, it has been exceptional to find any such; and to the same cause we have no hesitation in referring buff-coloured specimens (mentioned by Mr. Hudson in his work on Macro-lepidoptera), of which three males were taken on the 29th September, one being crippled. Two females of this form were taken at a later date.
We have not met with this aberration until the present season, but that it is due to atmospheric conditions acting on the developing imago in the pupa stage is strongly suggested by a curious specimen having its superior wings symmetrically streaked with golden buff on the usual green ground-colour. If atmospheric influence acted directly on the imago at the time of emergence one would expect the wings to be asymmetrically streaked. Is this coloration due to retarded emergence, and consequent alteration in the strength or nature of the pigment in the wing-scales?
Other notable specimens were one albo-extremus taken on the 29th September, and three other albo-extremus on the 15th October. Male specimens were the most plentiful.
Mr. Ritchie, however, believes this is due to the females frequently emerging later—i.e., after dark—and this opinion was supported by finding quite a number of freshly emerged females after dark on one occasion, there being no males observed at the same time. Two females have a white blotch agreeing with the white blotch frequently observed at lower part of the band on male specimens. More than one female specimen have dark “chocolate” spots about one-third from tips of superior wings, these being remarkably reminiscent of C. daphnandrœ.
The colour of Charagia virescens suggests protective resemblance; that it is really so I have little doubt. Mr. Ritchie one evening drew my attention to what he thought was a male specimen—which, in my opinion, was probably a female specimen—perched some 15 ft. up on a tree-trunk. It was getting dark, and objects at that distance in the shade amongst branches could not be clearly distinguished. With a long stick it was reached down, and proved to be a leaf split in the middle. Doubtless a sharp-eyed morepork often passes a specimen under the impression that it is a leaf.
One curious fact may be mentioned: ova of C. virescens are normally pale-yellow when first laid, becoming black after several hours. On several occasions this season black ova were extruded among the pale-coloured ones by freshly emerged females; and, moreover, within the abdomen of several specimens black ova were mixed amongst the others.
To observe if possible the manner of emergence from the vertical bore and through the operculum we obtained some pupæ. Having cut the wood so as to divide the bore between the operculum and the pupa, on every occasion that the top half was removed the pupa would slide up to investigate the cause of the disturbance in the atmosphere above. On touching a finger held above it the pupa would immediately descend again, apparently satisfied that all was right. This was repeated with different pupæ, proving that C. virescens can and does travel up and down its vertical burrow at will during the pupa stage. I now believe the operculum is cut or removed by pressure of the pupal headpiece, and not by aerial pressure, as I was previously inclined to think. One thing is certain, the cover is not a trap-door, in the sense of being hinged, until after emergence of the pupa.
Explanation of Plate XXVIII.
- Fig. 1.
Prothorax of unusual form, larva of Charagia virescens; magnified.
- Fig. 2.
Abdominal segment of ditto, dorsal aspect; magnified.
- Fig. 3.
" lateral aspect; magnified.
- Fig. 4.
Anterior segments of pupa, C. virescens; natural size.
- Fig. 5.
C. virescens, male, nov. ab albo-extremus; natural size.
- Fig. 6.
“female, ab hectori, Butl.; natural size.