The greater number of the Mites are too small to strike the eye. Some of them have, however, contrived to attract attention, in no very agreeable manner. Every one knows the Mite popularly called the “Harvest-bug,” but to this day there is some uncertainty as to its identity. It was described as a separate species under the name of Leptus autumnalis, and Mégnin was the first to show that it was the larval form of one of the Trombidiidae (see p. [472]). Most authors have considered it the larva of Trombidium holosericeum, but Murray referred it to the genus Tetranychus. The difficulty is that the minute creature cannot be removed from its victim without such injury as to prevent it from being bred out and the mature form determined. Brucker[[350]] has recently compared a large number of “Harvest-bugs” taken from human beings with the figures and descriptions of the larvae of certain Trombidiidae given by Henking and Berlese, and he determined them as the larvae of T. gynopterorum. Quite possibly, however, more than one genus is concerned in the production of this pest.
That certain skin-diseases are due to Mites (Demodicidae, Sarcoptidae) is a fact which is widely known. The fruit-grower, too, has to take cognisance of the Order, for his trees may suffer from “Red-spider” (Tetranychus telarius), and his black-currant bushes fail under the attack of the “Gall-mite” (Eriophyes or Phytoptus ribis). The curious swellings or galls which disfigure the leaves of many trees are sometimes of insect origin, but they are often due to Mites.
Domestic pets suffer greatly from Acarine parasites. A large number of species confine their attention exclusively to the feathers of birds (Analges, etc., see p. [466]). One curious genus, Syringophilus, is parasitic within the feathers, feeding upon the pith of the quill. Heller of Kiel discovered them in 1879, but the researches of Trouessart first showed their frequent presence and very wide distribution. He found that they entered by the superior umbilicus of the feather, and disappeared by the inferior umbilicus when the feathers moulted or the infested bird died.
It is probable that the comparatively large Mites of the group Ixodoidea (see p. [468]), commonly called “Ticks,” are the most widely known of the order. They attack wild and domestic animals and man, and are nearly always acquired from vegetables, such as brush or herbage. It would seem likely that many of these creatures can never have the chance of attaching themselves to animals, and it has been suggested that animal juices are a luxury but not a necessity to them, and that they can live, if need be, on vegetable sap, but further investigations have quite dispelled this view.
The suspected connection between the North American Tick, Boophilus annulatus, and the cattle disease known as Texas fever or “red water,” since clearly proved by the researches of Smith and Kilborne, led to the careful investigation of the life-history of that creature, and this was undertaken by Curtice.[[351]]
The female Ticks laid eggs a few days after dropping off the cattle, egg-laying lasting a week or more. The eggs took from three to five weeks to hatch, and the larvae attached themselves to cattle, on which they remained a fortnight, becoming mature and fertilised before they again sought the ground. The whole cycle occupied a time varying from six to ten weeks, a period apparently much exceeded by some members of the family.
Lounsbury[[352]] has recently made out the life-history of the South African “Bont” tick, Amblyomma hebraeum.
The eggs are deposited in the soil, ten to twenty thousand eggs in all being laid by one female. The larvae climb neighbouring plants and seize passing animals. After the third day of attachment they begin to distend, and they generally fall off, fully distended, on the sixth day, immediately seeking a place of concealment, where they become torpid. Under natural conditions the nymph does not emerge for at least eleven weeks, and then it behaves in the same way as the larva, again attaching itself to an animal for six days. A new time of torpidity and concealment ensues, again of at least eleven weeks’ duration, when the final moult takes place and the mature tick emerges. The males at once attach themselves to animals, but the females hesitate to fix themselves, except close by a male. For four days after fixation the male appears to exercise no attraction for the female, but after that period he shows great excitement at her approach. She, however, does all the courting, the male remaining fixed in the skin of the host. After pairing, the female distends greatly, attaining her maximum size (nearly one inch in length) in about a week, when she lets go and descends to the earth to lay eggs. If unmated, she detaches herself within a week, and seeks another host. Oviposition lasts from three to nine weeks, and the development of the egg from eleven weeks to six months. At least a year is occupied in the whole cycle. These ticks, and many others, communicate disease[[353]] by inoculation, conveying it from one animal to another.
No poison-glands have been demonstrated in the Acari, the function of the salivary glands of the Ticks being probably to prevent the coagulation of the blood of their victims.
It is an important point in the mode of life of the Ticks that they can live for a long time without food. Mégnin[[354]] states that he kept an Argas alive for four years, entirely without nutriment.