Extremely abundant in all damp situations from September till May, being one of the last insects to disappear in the autumn. The larva is found under stones, &c., in every stream, feeding on various aquatic insects and crustaceans. When very young the wing-cases are scarcely discernible, but gradually become more distinct at each moult, until the larva assumes the form shown in the illustration (Fig. 3a), which is taken from a specimen about a week before the emergence of the perfect insect. In all these insects it would be much more convenient to regard the metamorphosis as consisting of only two stages, viz., larva and imago, as there is really no condition analogous to the quiescent pupa of other orders. The female is rather stouter than the male, which is the sex figured, and her abdomen is of a dull bronze colour, instead of metallic blue. The only other dragonfly found in my neighbourhood (Wellington) is the pretty little Telebasis zealandica (Fig. 4), which occurs in similar situations to the last, but is not quite so common. The male is of a brilliant red colour, the female being bronzy green, but she may be readily distinguished from the same sex in Lestes colensonis by her smaller size. The larva of this species is rather more attenuated than that of the previous insect, and is of course considerably smaller.
Family Ephemeridæ.
Ephemera, n.s., near Coloburus[[20]] (Plate [XVI]., fig. 4, 4a larva).
The well-known mayflies are very extensively represented in New Zealand, hovering in swarms over running water during the summer evenings.
The larva of the present species (Fig. 4a) occurs abundantly under stones in rapid streams. It may be immediately distinguished from its numerous congeners by its large head and conspicuous black eyes. It is carnivorous, feeding on various small insects, chiefly those belonging to the present family, but in lack of these it will even devour individuals of its own species. It is consequently a most difficult insect to rear, and it was a long time before I succeeded in obtaining a single imago in captivity. When mature the insect leaves the water, and an apparently perfect imago escapes through a rent in the thorax in the usual way. In a few hours, however, a second moult occurs, the wings gaining additional size and beauty, and the anal setæ becoming very much more elongated than before (Fig. 4). This second change, which has so perplexed some entomologists, is merely an apparent departure from the general rule, a careful examination of the exuviæ of the dragonflies, and pupa shells of many other insects, revealing a delicate membrane within, which invests the imago, and is cast off at the same time as the harder external envelope. In the case of the mayflies, the retention of this internal membrane some two or three hours longer than usual, will fully explain its apparently unique metamorphosis.
Family Perlidæ.
Stenoperla prasina (Plate [XVI]., fig. 3, 3a larva).
This is the green gauzy-winged insect which we see flying feebly over running water, during the twilight, throughout the summer.
Its larva (Fig. 3a) is aquatic, hiding itself under stones, and devouring the unfortunate Ephemeræ found in similar situations. Towards the end of its career the rudimentary wings become very conspicuous, at which time it is a most interesting object. The curious appendages on each side of the abdomen are gills, which the larva is constantly vibrating, in order to obtain a fresh supply of aërated water. When mature, it ascends the stem of some aquatic plant, the skin becomes dry and brittle, and finally bursting, allows the perfect insect to escape, and in a few hours its wings are sufficiently hardened for flight. Several other species occur in New Zealand, one of the commonest being Perla cyrene, a black insect much resembling S. prasina, but considerably smaller; its larva may be occasionally found, and is at once known by its dark colour.
Terrestrial Group.