ŒCETICUS OMNIVORUS, Fereday.
(Liothula omnivora, Fereday, Trans. N. Z. Inst. x., 260, pl. ix. Œceticus omnivorus, Meyr., Trans. N. Z. Inst. xxii. 212.)
(Plate [XIII]., fig. 6 ♂; Plate [III]., fig. 26, larva in its case; fig. 25 ditto withdrawn from case.)
This interesting species is seldom seen as an imago in the natural state, although the cases constructed by its larva are of common occurrence. Specimens of these cases have been noticed at several localities between Palmerston, in the North Island, and Invercargill, in the South Island, so that apparently the insect is common, and generally distributed throughout New Zealand.
The expansion of the wings of the male is from 1¼ to 1½ inches. The fore-wings are very elongate and narrow. All the wings are blackish-brown, and sparsely covered with scales, the hind pair being semi-transparent. The body is very hairy, and deep black. The antennæ are broadly bi-pectinate at the base, becoming almost filiform towards the apex. The female insect is apterous, having a close superficial resemblance to a large maggot. The head and thorax are very small, and the legs and antennæ rudimentary. The extremity of the body is furnished with a two-jointed ovipositor, and there are a few scattered yellowish scales on various parts of the insect. Its length is about 1 inch.
The eggs of this species are deposited inside the old case, which the female insect never leaves during the whole of her life. The young larva when first hatched is about ⅛ inch in length. Its head and three anterior segments are corneous and much larger than the others, which are rather soft with the exception of the last one. These little larvæ are extremely active, and immediately after hatching leave the old case, and roam in all directions over the tree, letting themselves down from branch to branch by silken threads. They carry the posterior portion of their body elevated in the air, walking whilst doing so by means of their strong thoracic legs.
The food-plants of this species are numerous. The following are a few of them: Manuka (Leptospermum scoparium and ericoides, Cupressus macrocarpa, Pinus insignis), and various species of willow, &c. These, it will be observed, include several introduced trees. In fact, the insect is a very general feeder. About three days after leaving the egg, the little caterpillar constructs a minute, conical-shaped, silken case, which it carries almost in an upright position on its posterior segments. Later on in life this case becomes too heavy to be held vertically, and is afterwards dragged along by the larva, and often allowed to hang downwards. The case has two apertures—a large one in front, through which the head of the larva is projected, and a smaller one at the posterior extremity, which allows the pellets of excrement to fall out of the case, as soon as they are evacuated.
Owing to the apterous and completely helpless condition of the female imago, it is evident that the dispersal of this insect must take place in the larval state. Distribution is of course quite impossible without a female being transported in some way, and from observations made on a good many larvæ of various ages, I am disposed to think that the migration of this insect to new localities takes place at an early age, possibly soon after its emergence from the egg. On this account I think that the occurrence of the moth in both North and South Islands is of great interest, as it would seem to indicate the existence of some connection between the two islands, at a period not sufficiently remote to have allowed any appreciable modification to take place in the insect's structure and habits. At the same time, it should be borne in mind, that the protection afforded the larva by its case, and its ability to feed on so many different plants, may have rendered any modification unnecessary for the preservation of the species during recent times. The length of the full-grown caterpillar is about 1 inch. The head is dull yellow speckled with black. The first three segments are very hard, dark brown, with numerous white markings. The remaining segments are considerably thickened near the middle of the insect, rudimentary prolegs being present on the seventh, eighth, ninth, and tenth segments of the larva. The anal prolegs are very strong, and are furnished with numerous sharp hooklets, which retain the larva very firmly in its case. As the caterpillar grows, it increases the length of its domicile from the anterior, causing it gradually to assume a more tubular form, tapering towards the posterior aperture, which is enlarged from time to time. The outside is covered with numerous fragmentary leaves and twigs of various sizes, placed longitudinally on the case, and, frequently, near the anterior aperture the materials, owing to their recent selection, are fresh and green. The interior is lined with soft, smooth silk of a light brown colour, the thickness of the whole fabric being about the same as that of an ordinary kid glove, and so strong that it is impossible to tear it, or indeed to cut it, except with sharp instruments. The size of the case, when the caterpillar is mature, varies considerably, ranging from 2¼ to 3 inches or more in length, and about ¼ inch in diameter, the widest portion being a little behind the anterior aperture.
During the day the larva closes the entrance, and spins a loop of very strong silk over a twig, the ends being joined to the upper edges of the case on each side; in this way it hangs suspended, the caterpillar lying snugly within. I have often known a larva to remain thus for over three weeks without moving, and afterwards resume feeding as before; this probably occurs whilst the inmate is engaged in changing its skin. At night the larvæ may be seen busily engaged: they project the head and first four segments of the body beyond the case, and walk about with considerable rapidity, often lowering themselves by means of silken threads; the only locomotive organs are, of course, their strong thoracic legs, which appear to easily fulfil their double function of moving both larva and case. If disturbed, these insects at once retreat into their cases, closing the anterior aperture with a silken cord, which is kept in readiness for the purpose, and pulled from the inside by the retreating larva. This operation is most rapidly performed, as the upper edges of the case are flexible, and thus fold closely together, completely obstructing the entrance. When full grown, this caterpillar fastens its case to a branch with a loop of strong silk, which is drawn very tight, preventing the case from swinging when the plant is moved by the wind, and also rendering the insect's habitation more inconspicuous, by causing it to resemble a broken twig. The anterior aperture is completely closed, the loose edges being drawn together and fastened like a bag. The posterior end of the case is twisted up for some little distance above the extremity, thus completely closing the opening there situated. It is lined inside with a layer of very soft silk spun loosely over the sides, and partly filling up each end. In the centre of this the pupa lies with its head towards the lower portion of the case, the old larval skin being thrust backwards amongst the loose silk above the insect.
The male and female pupæ may very easily be distinguished. The male pupa is rather attenuated, and has all the organs of the future moth plainly indicated on the integument, as is usual with lepidopterous pupæ. The female pupa, on the contrary, is merely a chain of segments, with a few faint indications of rudimentary organs on the anterior extremity. It is, moreover, much larger than the male pupa.