Handbook of medical entomology . hese have originated in widely separated families, theAcarina form an especially favorable group for study of the origin ofparasitism. Such a study has been made by Ewing (1911), who hasreached the following conclusions: We have strong evidence indicating that the parasitic habit hasoriginated independently at least eleven times in the phylogeny of theArarina. Among the zoophagous parasites, the parasitic habit hasbeen developed from three different types of free-living Acarina:(a) predaceous forms, (b) scavengers, (c) forms living upon the juicesof plants. Ewi
Handbook of medical entomology . hese have originated in widely separated families, theAcarina form an especially favorable group for study of the origin ofparasitism. Such a study has been made by Ewing (1911), who hasreached the following conclusions: We have strong evidence indicating that the parasitic habit hasoriginated independently at least eleven times in the phylogeny of theArarina. Among the zoophagous parasites, the parasitic habit hasbeen developed from three different types of free-living Acarina:(a) predaceous forms, (b) scavengers, (c) forms living upon the juicesof plants. Ewing also showed that among the living forms of Acarina we cantrace out all the stages of advancing parasitism, semiparasitism,facultative parasitism, even to the fixed and permanent type, andfinally to endoparasitism. Of the many parasitic forms, there are several species which areserious parasites of man and we shall consider the more important ofthese. Infestation by mites is technically known as acariasis. Acarina, or Mites 59. 43. Effect of the harvest inites on the skin of man. Photograph byJ. C. Bradley. 6o Parasitic Arthropods The Trombidiidae, or Harvest Mites In many parts of this country it is impossible for a visitor to gointo the fields and, particularly, into berry patches and among tallweeds and grass in the summer or early fall withQut being affected byan intolerable itching, which is followed, later, by a breaking out ofwheals, 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 or later, the victim finds that it is due to none of these, butto the attacks of an almost microscopic red mite, usually calledjigger or chigger in this country. As the term chigger isapplied to one of the true fleas, Dermatophilus penetrans, of the tropics, these forms are morecorrectly known asharvest of an infeste
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