Of all the Acari, the best known and most troublesome are those belonging to the family Ixodidæ; these infest the whole animal creation. They are furnished with a long cylindrical beak, armed with recurved hooks, formed of the two mandibles above and the long slender labium below. They have no eyes, nor apparently any dermaploptic sense, but there are various seemingly sensitive setæ distributed over the body and on the appendages. The whole of the mites will be found suitable objects for the study of development, as the process is slow and their eggs do not require much care. The segmentation of the eggs differs; some of the cells are distinguished by their large nuclei, which stain feebly by carmine. During the cleavage of the egg no division of the so-called yolk has been observed, but later on this breaks up into several minute pieces.
Fig 414.—Mouth organs of Sheep-tick.
c. Capitulum; d, e, f, g. Segments of palpi; h. Labial process; i. Spiny beak formed of fused mandibles.—(Warne.)
The accompanying [Fig. 413] shows the under surface of the body and the mouth parts of the common English dog and sheep tick, Ixodes ricinus, with its six formidable legs. The upper surface is shown in [Fig. 415]; the head (capitulum) and mouth organs in [Fig. 414], c, d, e, f, g, together with the four segments of the palpi; h the labial process armed with hooks forming the lower side of the beak, and i indicating the tips of the two mandibles forming the upper side, and projecting beyond the apex of the labium. By means of this beak, which is thrust to its base into the integument, the tick adheres firmly to its host, and in detaching them care must be taken that the head is not left behind buried in the skin. This tick is found in all stages of growth; the females, gorging themselves with blood, swell up to the size of a pea, as seen in [Fig. 413], but the male, formerly regarded as a distinct species, is of a much smaller size. In distribution these pests are almost cosmopolitan, and in tropical countries they grow to much greater dimensions, the females sometimes attaining the size of a large gooseberry.
The family of true mites is that of the Sarcoptidæ; these are either free or parasitic. They have no breathing organs; the palpi are basally fused to the rostrum, the mandibles are pincer-like, and the tarsi are often furnished at their tips with a sucker. The most familiar is the cheese mite, Tyroglyphus, which feeds upon decaying matter.
Fig. 415.
1. Female Sheep-tick; 2. Rat-tick; 3. Head of Cat-flea; 4. Larva of Flea. (The life size is given in circles.)
The well-known cheese mite attains to a size plainly visible to the naked eye, but when first hatched out from the egg (shown in its several stages of development in [Fig. 417]), requires a moderate amount of magnification. Its growth, however, is rapid and the young begin to feed as soon as they leave the egg. The body is partially covered over by setæ, or hairs, and the feet terminate in hooklets, as seen in the full-grown acarus. The mandibles are cutting, but as a rule they prefer soft and partially-decayed kinds of food. It also feeds upon damaged flour, sugar, and other domestic articles. The Dermestes lardarius, one of the minute beetle tribe ([Fig. 418]), commits even greater depredations among insect and other collections during the larval stage of its existence.