P. A. Buxton 
147 
of female S. scabiei, and 450-500 by 310-370 microns as those of the sarcopt 
of the horse. Gerlach separated his “S . equi ” from S. scabiei on its greater 
size and greater relative length in the ovigerous female. I have shown ( loc . cit.) 
that variation in size and proportions occurs even when mites are measured 
living under standard conditions. The relation between width and breadth 
is altered by pregnancy. My material of the common S. scabiei was collected 
by Mr J. E. M. Mellor during the war from a large number of different cases 
both civil and military, therefore the possibility is excluded that we are dealing 
with a case of equine scabies in man. I have been unable to obtain sufficient 
living material from man to give the measurements of a series of specimens, 
but I do not believe that differences of size are of the least use in separating 
species and varieties of Sarcoptes. The plastron in adult females from man is 
frequently, but by no means always, less highly pigmented than it is in those 
from the horse. There is no difference to be detected in the three plastra of 
the male. The bare areas (rugose areas) vary in their development among 
^ -,r 
_- ___ __ _ ^ 
Fig. 1. Sarcoptes scabiei var. hominis, adult $. Scales from left side immediately behind the 
antero-median cone. 
Fig. 2. S. scabiei var. equi, adult $. Scales from same area. The two figures drawn to the same 
scale with a Zeiss camera lucida. r, integumentarj^ ridges; sc, scales. 
specimens from man just as they do among specimens from the horse. In 
ovigerous females from either host either none or one or two may be present, 
or a large one partly divided by a row or two of scales; if two are present one 
or other may be very small. Neither the plastron nor the bare areas are of 
any systematic importance. The scales in specimens from man are said to be 
pointed and longer than broad; in those from the horse, small and scarcely 
longer than broad (Canestrini and Kramer). Hirst (1920), speaking of speci¬ 
mens from man, refers to the “ dorsal scales being distinctly longer as compared 
with their breadth than in Sarcoptes scabiei var. equi” I find that the differ¬ 
ences are so slight, and subject to such individual variation, that they are use¬ 
less for purposes of discrimination. Scales vary greatly in size on any one 
mite, but the largest on ovigerous females from the horse do not exceed 
8*5 microns in length. In specimens from man they reach 10-11 microns, as 
a maximum, but on many mites no scale longer than 9*5 can be found. Figs. 1 
and 2 show scales from the same part of the dorsum of females from the horse 
10—2 
