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Art. XLI.—Notes on the Life History of Charagia virescens.
[Read before the Otago Institute, 8th October, 1878.]
The larva lives chiefly on the extremely hard wood of the Black Maire (Olea apetala), but I have also found it in a tree known to the settlers about Masterton as the wine-berry or New Zealand currant tree (Aristotelia racemosa). The wood of this last differs widely from the Maire, being very soft and white. I believe I have once or twice found the larva in other trees. C. virescens passes certainly three years in the larval state, probably four. In this stage of its existence it is extremely plentiful wherever the Black Maire abounds. It is not easy to find a single tree of this species that is not more or less honeycombed by its ravages; the imago, however, is far more rare. Of the larva I have obtained twenty specimens in about an hour, notwithstanding the loss of time in getting them out, whereas I have only come across seven specimens of the perfect insect in four seasons. I believe I have once seen it on the wing, but I am not positive about it. I have generally come across it half dead, partially stiff and much faded and frayed. I have also found the wings, the insect having evidently fallen a prey to some bird.
When newly emerged the perfect insect is very beautifully marked with blackish markings, but these soon fade, or get rubbed off, and the insect then presents a pretty uniform green with a few whitish markings.
Although the larva is so plentiful, it requires a little practice to detect its burrow readily. If the limbs or trunk of a Black Maire be carefully examined, a more or less diamond-shaped mark, two or three inches in the side, may often be noticed, which varies slightly in tint from the surrounding bark; and if this patch is pressed with the finger it gives way; if the patch is torn off it is seen to be composed of yellowish or greyish silk, covered on the outside with scraps of bark, lichens, excreta of the larva, etc., the whole
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forming a wonderfully close imitation of the natural bark of the tree. Beneath this covering is seen a cavity, the depth of which varies from a quarter to half an inch, and rather above the centre of the cavity is the entrance to the burrow. The cavity around the entrance of the burrow is used by the larva to turn itself in. The burrow at first takes a course inwards and upwards for one, two, or more inches; this upward inclination preventing the entrance of water. Then the burrow turns downward in a nearly vertical direction. This vertical portion of the burrow varies in length according to the age of the larva. That of a full-grown larva is about four or five inches long. I have seen them eight inches and proportionally wide. I have ascertained positively that the larva frequently inhabits the same burrow for more than two years, and I am of opinion that they generally keep to the same burrow during the whole larval stage. But I have occasionally found larvæ in terminal shoots which would not admit of their attaining full growth, and consequently they must in these cases change their ground.
When the larva has attained its full size, it spins, at the top of the vertical portion of its burrow, a contrivance very much resembling that of the Trap-door Spider, as an additional security against its foes during the pupa state.
The imago emerges in October and November. The best time to obtain the pupa is in September and the early part of October. It is easy to ascertain if the insect in a burrow is in the larval or pupa stage, for, if the exterior web is torn off, the larva, if inside, will replace it by the next day.
The larva is flesh-coloured, tinted with purple; head dark brown, with a few strong bristles; spiracles black; segment next the head darker than the rest, horny, with a large black mark on each side, just above the spiracle.
The pupa is flesh-coloured, inclining to brick red; head and thorax deep chestnut brown. The semi-transparent wing-cases show the markings of the future imago.
I have examined these larvæ and pupæ in their different stages up to the time when by stripping off the pupa case, just before the insect was ready to emerge, the easily recognised Charagia virescens was disclosed.
I do not think that the larva of C. virescens is the larva which is attacked by the fungus Cordiceps robertsii. Not only do the two larvæ differ in the size of the head and shape of the body—the larva of C. virescens being more cylindrical and with proportionately a larger head—but I do not see how C. virescens could get into the ground, which is the position in which the larva, which is attacked by the fungus, always is found. I think that the fungus-attacked larva is probably a Porina.