The representatives of this group undergo a marked metamorphosis. Commonly, the larvæ on hatching from the egg, possess but three pairs of legs, and hence are called _hexapod larvæ_. After a molt, they transform into nymphs which, like the adult, have four pairs of legs and are called _octopod nymphs_. These after a period of growth, molt one or more times and, acquiring external sexual organs, become adult. Most of the mites are free-living, but there are many parasitic species and as these have originated in widely separated families, the Acarina form an especially favorable group for study of the origin of parasitism. Such a study has been made by Ewing (1911), who has reached the following conclusions: "We have strong evidence indicating that the parasitic habit has originated independently at least eleven times in the phylogeny of the Ararina. Among the zoophagous parasites, the parasitic habit has been developed from three different types of free-living Acarina: (a) predaceous forms, (b) scavengers, (c) forms living upon the juices of plants." Ewing also showed that among the living forms of Acarina we can trace out all the stages of advancing parasitism, semiparasitism, facultative parasitism, even to the fixed and permanent type, and finally to endoparasitism. Of the many parasitic forms, there are several species which are serious parasites of man and we shall consider the more important of these. Infestation by mites is technically known as _acariasis_. [Illustration: 43. Effect of the harvest mites on the skin of man. Photograph by J. C. Bradley.] The Trombidiidæ, or Harvest Mites In many parts of this country it is impossible for a visitor to go into the fields and, particularly, into berry patches and among tall weeds and grass in the summer or early fall without being affected by an intolerable itching, which is followed, later, by a breaking out of wheals, or papules, surrounded by a bright red or violaceous aureola, (fig. 43). It is often regarded as a urticaria or eczema, produced by change of climate, an error in diet, or some condition of general health. Sooner or later, the victim finds that it is due to none of these, but to the attacks of an almost microscopic red mite, usually called "jigger" or "chigger" in this country. As the term "chigger" is applied to one of the true fleas, _Dermatophilus penetrans_, of the tropics, these forms are more correctly known as "harvest mites." Natives of an infested region may be so immune or accustomed to its attacks as to be unaware of its presence, though such immunity is by no means possessed by all who have been long exposed to the annoyance. [Illustration: 44. Harvest mites. (Larvæ of Trombidium). After C. V. Riley.]