The Larva.—Omitting, for the moment, the very aberrant Vermiformia (see p. [464]), it is the almost universal rule for the egg to hatch out as a hexapod larva. The larvae of the genus Pteroptus are said to be eight-legged. Winkler has stated that the early embryo of Gamasus possesses eight legs, of which the last pair subsequently atrophy, but this observation requires confirmation.
The Nymph.—The nymph-stage commences on the acquisition of eight legs, and lasts until the final ecdysis which produces the imago. This is the most important period of Acarine life, and is divided into a prolonged active period, during which the animal feeds and grows, and an inert period, sometimes prolonged, but at others very short, and differing little from the quiescence observable at an ordinary moult, during which the imago is elaborated. In many species the nymph is strikingly different from the imago; in others there is a close resemblance between them. It would appear, from the cases which have been most thoroughly investigated, that the imago is not developed, part for part, from the nymph, but that there is an “histolysis” and “histogenesis” similar to that which occurs among certain insects (see vol. v. p. 165). There may be more than one nymphal stage.
The hypopial stage occurs in the Tyroglyphinae, the “Cheese-mite” sub-family. Here some of the young nymphs assume an entirely different form, so different that it was for a long time considered to constitute a separate genus, and was named Hypopus. The animal acquires a hard dorsal covering. The mouth-parts are in the form of a flat blade with two terminal bristles, but with no discernible orifice. The legs are single-clawed, and all more or less directed forward, and they are articulated near the middle line of the ventral surface. Suckers are always present under the hind part of the abdomen.
It appears that these remarkably modified nymphs are entrusted with the wider distribution of the species, and that they are analogous to the winged individuals which occur in the parthenogenetic generations of the Aphidae. The ordinary Tyroglyphus is soft-bodied, and requires a moist environment, and exposure to the sun or prolonged passage through the air would be fatal to it. The hypopial form is much more independent of external conditions, and its habit is to attach itself by its suckers to various insects, and by this means to seek a new locality, when it resumes the ordinary nymph-form and proceeds with its development.
Classification.—There is no generally accepted classification of the Acarina, though several eminent Arachnologists have attempted of late years to reduce the group to order. Widely different views are held concerning the affinities of certain groups, and there is no agreement as to the value to be accorded to the characters which all recognise. Thus Canestrini[[366]] allows thirty-four families, while according to Trouessart[[367]] there are only ten.
Trouessart’s scheme of classification is in the main followed in the present chapter.
Sub-Order 1. Vermiformia.
This Sub-order includes the lowest and most aberrant forms of the Mites. They are entirely parasitic, and of very small size. The abdomen is much elongated, and is transversely striated. There are two families, Eriophyidae[[368]] (Phytoptidae) and Demodicidae.
Fam. 1. Eriophyidae (Phytoptidae).—These are the so-called Gall-mites. The curious excrescences and abnormal growths which occur on the leaves and buds of plants are familiar to every one. Various creatures are responsible for these deformities, many being the work of insects, especially the Cynipidae among the Hymenoptera, and the Cecidomyiidae among the Diptera. Others, again, are due to Eriophyid Mites.
Though the galls originated by Mites are often outwardly extremely similar to those of insect origin, they can be at once distinguished on close examination. Mite-galls contain a single chamber, communicating with the exterior by a pore, usually guarded with hairs, and the Mites live gregariously within it, apparently feeding upon the hairs which grow abundantly on its inner surface. In Insect-galls each insect larva lives in a separate closed chamber.