(a.) The itch-mite, acarus scabiei, sarcoptes hominis, punctiform, from a quarter to half a-millimetre long, ovoid, garnished with transverse, bandlike, dorsal striae, and with central, acuminate warts; anteriorly a bristled proboscis, prolonged inferiorly to a band upon the thorax; four bristly fore-feet terminating in disk-plate, whilst the four hind-feet taper into lengthy bristles.

It burrows in the epidermis, often boring beneath it a canal several lines long, at the termination of which the acarus is, on a narrow inspection, discoverable as a minute whitish speck, marked with a brown point. When the said canals penetrate to the cutis, they engender the itch-vesicles and pustules.

Researches into the natural history of this mite, together with the results of extended experience, prove beyond a doubt its relation to itch as its sole cause.

The follicle mite, acarus commedonum sive folliculorum, an elongated acarus, from one-fifth to one-third of a millimetre long, and about one-twentieth broad, the head having two lateral antennae and an intermediate proboscis. The head passes immediately into the anterior part of the body, which occupies about one-fourth of the entire mite. From it project four pair of very short, thick, conoid, three-jointed feet, each furnished with three toes. The anterior body passes without break into the posterior, which gradually tapers, but is rounded off at the extremity, is transversely striated, and contains a finely granular, brownish mass.

It inhabits singly or numerously the hair sacs and sebaceous follicles on various parts of the person. Amongst other anomalies, it occasionally displays only six feet, which no doubt implies an earlier state of its development. Its presence is probably often of little moment. Occasionally, however, it may, by stimulating the secretion, engender commedones, or set up inflammation, and thus give rise to the acne pustule.