In the Tetranychinae (see p. [472]), glands apparently homologous with the salivary glands of the Ticks have taken on the function of spinning organs. According to Donnadieu,[[355]] these glands, which resemble bunches of grapes, and are possessed by both sexes, open into the buccal cavity at the base of the chelicerae. The gummy fluid exudes from the mouth, and is combed into threads by the pedipalps. The legs of these mites are furnished terminally with curious hairs ending in a round knob, which are supposed to have some relation to their spinning habits.
The males are the busiest spinners, the time of the females being largely occupied in laying eggs among the excessively fine threads of silk with which the Mites cover the under surface of leaves. In the Eriophyidae (see p. [464]) corresponding glands are thought to furnish an irritant fluid which causes abnormal growths or galls upon vegetable tissues.
External Structure.—It is often stated, but erroneously, that there is no distinction between cephalothorax and abdomen in the Mites. Certainly no such division can be made out in the Hydrachnidae (see p. [472]) or in some other forms, but in the majority of Acari the cephalothorax is clearly marked off by a transverse groove or suture. In some cases the anterior portion of the cephalothorax is movably articulated with the rest, and forms a sort of false head called a “capitulum.” In most Mites the chitinous integument is soft and non-resistant, but it is otherwise with the Oribatidae or “Beetle-mites” (see p. 467), which are nearly all covered by an extremely hard and coriaceous armature.
Eyes are sometimes absent, sometimes present in varying numbers. They seem here to be of remarkably little systematic importance, as otherwise closely allied species may be either eyed or eyeless.
Normally Mites possess the usual Arachnid appendages, chelicerae, pedipalpi, and four pairs of ambulatory legs. The anterior appendages are, however, subject to a very great degree of modification, while in one Family, the Eriophyidae (Phytoptidae), the legs are apparently reduced to two pairs.
The chelicerae are sometimes chelate, in which case they are two-jointed, the distal joint or movable finger being always articulated below the immovable finger. Sometimes they terminate in a single claw or blade, the movable joint being obsolete. In the Ticks they exist as two long styles or piercing weapons, serrate on the outer edge.
The pedipalpi vary very much in structure, according to the habits of the particular form to which they belong. In the Sarcoptidae (see p. [466]) they are hardly recognisable owing to the extent to which they have coalesced with the maxillary plate. In many of the free-living forms they are leg-like feeling organs, but in others they are raptorial, being not precisely chelate, but terminating in a “finger-and-thumb” arrangement which is of use in holding prey. The extreme development of the raptorial palp is found in Cheyletus (see p. [473]), in which the whole appendage is remarkably thick and strong, and the “finger” is a powerful chitinous claw, while the “thumb” is replaced by movable pectinated spines of chitin. The Water-mites have a palpus adapted for anchoring themselves to water-weeds, the last joint being articulated terminally with the penultimate joint, and bending down upon it. Finally, in the “Snouted mites” (Bdellidae, see p. [471]) the palpi are tactile or antenniform, often strongly recalling the antennae of weevils.
The maxillary plates which arise from the basal joints of the pedipalps are always more or less fused, in the Mites, to form a single median transverse plate, constituting the lower lip or “labium” of some authors. In some of the Oribatidae the fusion of the maxillae is only complete at the base, and the free points are still of some use as masticating organs. In those free living Mites which have undergone no great modification of the mouth-parts two other portions can be distinguished, the upper lip or “epipharynx,” and the “lingua,” which forms the floor of the mouth, and is for the most part concealed by the maxillary plate.
The legs are usually six- or seven-jointed, and are subject to great variation, especially as regards the tarsus or terminal joint. This may bear claws (1–3) or sucking disks, or a combination of the two, or may simply take the form of a long bristle or hair.
The Cheese-mite has a claw surrounded by a sucker—like Captain Cuttle’s hook within his sleeve. The claws of those species which are parasitic on the hairs of animals are sometimes most remarkably modified.