348 The American Naturalist. [April, 
lateral margins. Skin provided with shallow, undulating, 
transverse furrows. Dorsal surface covered with numerous 
small and large appendages of a clavate or conical form, or 
resembling scales, and with ‘spines resting on papillary eleya- 
tions. Head apparently distinct from the trunk, with four 
pairs of jaws, and two strong, three-jointed palpi, placed near 
the jaws, and of the same length. Legs eight in number, five 
jointed ; the first and second pairs provided with pedunculated 
suckers—the peduncles of the same length as the legs them- 
selves, and having no joints; the third and fourth pairs in the 
female terminating in long bristles. In the male, the first, 
second, and fourth pairs of legs are all provided with suckers, 
the third pair alone haying bristles; the epimera of the first 
pair united. Larva presenting six legs—the first and second 
pair with suckers, the third pair terminating in long bristles.” 
In regard to size, Hebra says: “The adult acarus varies in 
size. On an average, its length is 0.45 mm.; its breadth, at 
the fourth ring of the thorax, 0.35 mm. The male is consid- 
erably smaller than thefemale. In length it measures 0.23 mm..; 
in breadth, 0.19 mm. The eggs of the itch mite are of an ovid 
form, and are 0.16 mm. long, 0.11 mm. broad.” It is, perhaps, 
unnecessary to say that the size of the animal is entirely out 
of proportion to the commotion it occasions when its presence 
becomes known. 
The presence of the mite on the human body is character- 
ized by certain appearances and symptoms known as ‘scabies 
or ‘itch’. Scabies comes from scabere, to scratch. “The 
name of ‘scabies’ is, at the present day, used to designate the 
numerous appearances produced on the skin by the presence 
of acari which dwell within it, and which, in obtaining food 
for themselves, and in propagating their species, give rise to a 
continual irritation of its component tissues. The morbid 
appearances themselves resemble, in many respects, those 
caused by other irritants, but, taken as a whole, they never- 
theless present certain peculiarities, which justify our making 
of them a Seance form of disease g ' Hebra. 
“Though is the essential cause of scabies, 
it must be remembered that EES almost, if not wholly. 
? Hebra, “ Diseases of the Skin.” P. 192. 
