Family Hepialidæ.
Hepialus virescens (Plate [IX]., fig. 1 ♂, 1a ♀, 1c larva, 1b pupa).
This gigantic insect is seen occasionally in the forest during the early summer. The larva (1c) tunnels the stems of living trees, feeding entirely on wood which it bites off with its strong mandibles. The plant most usually selected by the caterpillar is Aristotelia racemosa, called by the settlers "New Zealand currant," from its large clusters of rich-looking black berries, which appear in autumn. Other food-plants are numerous, the black maire (Olea apetala) and manuka (Leptospermum) being among those more frequently chosen.
This larva, for the most part, inhabits the main stem of the tree, its gallery always having an outlet to the air, which is covered with a curtain of dull brown silk, spun exactly level with the surrounding bark, and consequently very inconspicuous. These burrows usually run down towards the ground, and are mostly two or three inches from the surface of the trunk. In some instances the larvæ inhabit branches, in which case, if the branch is of small dimensions, the tunnel is made near the centre. These remarks only refer to galleries constructed by young larvæ, as the tunnel made by the insect prior to becoming a pupa is of a very complicated character and merits a somewhat detailed description. It consists of a spacious, irregular, but shallow cavity, just under the bark, having a large opening to the air, which is entirely covered with a thin silken covering, almost exactly the same shape and size as the numerous scars which occur at intervals on the trunks of nearly all the trees. Three large tunnels open into this shallow cavity: one in the centre, which runs right into the middle of the stem, and one on each side, which run right and left just under the bark. These are usually very short, but sometimes extend half-way round the tree, and occasionally even join one another on the opposite side. The central tunnel has a slightly upward direction for a short distance inwards, which effectually prevents it from becoming flooded with water; afterwards it pursues an almost horizontal course until it reaches the centre of the tree when it appears to suddenly terminate. This, however, is not the case, for, if the gallery floor is carefully examined a short distance before its apparent termination, a round trap-door will be found, compactly constructed of very hard, smooth silk, and corresponding so closely with the surrounding portion of the tunnel that it almost escapes detection. When this lid is lifted a long perpendicular shaft is disclosed which runs down the middle of the tree to a depth of 14 or 16 inches, and is about six lines in diameter. At the bottom of this the elongated pupa (Fig. 1b) sleeps quietly and securely in an upright position, the old larval skin forming a soft support for the terminal segment of the pupa to rest on. The upper end of this vertical shaft is lined with silk, which forms a framework on which the trap-door rests when closed. The lid itself is of a larger size than the orifice which it covers, and this makes it extremely difficult, if not impossible, to force it from the outside, whilst it fits down so closely to the aperture as not to be readily lifted. The object of this most ingenious contrivance is, in all probability, to prevent the ingress of insects, large numbers of spiders, slugs, and various Orthoptera being frequently found in both central and lateral tunnels, but they are quite unable to pass the trap-door. The galleries of different individual larvæ are all wonderfully alike, the only differences observable being in the length of the perpendicular shaft and the direction of the horizontal burrow, which is sometimes curved. These variations are usually caused by the presence of other tunnels in the tree, which the larva invariably avoids, although how it can ascertain that it is approaching another tunnel before actually reaching it, is hard to understand. As development progresses in the pupa, it becomes darker in colour, especially on the wing-cases, which in some individuals show the future black markings of the moth, as early as two months before emergence. Others remain quite white and soft, the green wings suddenly appearing through their cases a fortnight or three weeks prior to the bursting forth of the imago. Previous to this change the pupa works its way up the vertical tunnel, lifts the trap-door, which yields to the slightest pressure from within, and wriggles along the horizontal burrow until it reaches the air, the last three or four segments only remaining in the tree. The thoracic shield then ruptures, and the moth crawls out and expands its wings in the ordinary way, resting on the trunk of the tree until they are of sufficient strength and hardness for flight.
The perfect insect, although it must be common, is very rarely seen. It is best reared from the pupæ, which can be often successfully cut out of their burrows and kept amongst damp moss until they emerge. It appears to be much persecuted by birds, as we often observe its large green wings lying about on the ground.[[16]]
The curious "vegetable caterpillar," which is usually referred to this species, probably belongs to one of the larger subterranean larvæ of the family.
Family Bombycidæ.
Nyctemera annulata (Plate [IX]., fig. 3 ♂, 3a larva, 3b pupa).
This abundant species is usually mistaken for a butterfly by the uninitiated owing to its diurnal habits and conspicuous colouring. Its larva feeds on various plants, the most usual being a light green kind of ivy with yellow flowers, but its original food no doubt consisted of the "New Zealand groundsel" (Senecio bellidioides), on which it may now be occasionally taken in wild situations. Its general colour is black, with interrupted dorsal and lateral lines, the ventral surface and connecting membrane between the segments being slate-coloured. In younger larvæ there are also several slate-coloured lines extending the whole length of the insect, and thus dividing the black into squares. Round the middle of each segment, at its greatest circumference, a variable number of brilliant blue warts are situated, and out of these dense tufts of long black hair take their rise. There are, however, no warts along the ventral surface. This description applies very well as a rule, but the larva is subject to many slight variations. It remains in this state for nearly three months, or more, according to the season, and is very common, numbers being found on the different plants which constitute its food. The pupa (Fig. 3b) is of a shining black colour, with many longitudinal rows of small yellow blotches on the abdominal segments; there is also a stripe of the same colour at the tip of the wing-case. It is enclosed in a slight cocoon, formed of a mixture of silk and hair, and is attached near the ground to any firm object. The moth emerges in the course of a month or six weeks. It is very common, being found profusely in the neighbourhood of its food-plants, and appears in the greatest numbers during the early morning hours in the middle of summer.
For an account of a Dipterous insect, parasitic in the present species, I refer to page [59].