148 
Sarcoptes of Man 
and from man. In shape I can detect no differences provided that a sufficient 
number of scales be examined. Scales are slightly concave or slightly convex 
at the side, or concave on one side, convex on the other; these variations 
occur quite irrespectively of the host from which the specimen was derived. 
Canestrini and Kramer state that the spines of S. scabiei var. hominis are 
long and pointed, those of var. equi blunt. This distinction is borne out by 
measurements, and I have measured the second spine of the inner row, and the 
second spine of the outer row of a series of ovigerous females. That of the 
inner row measures 31-33 microns in Sarcoptes from the horse, that of the 
outer row 25-28 microns: corresponding figures for S. scabiei var. hominis 
are 35 microns and 29-30 microns. There is a further small and rather incon¬ 
stant difference in the greater stoutness of many of the spines of the equine 
Figs. 3 and 4. Sarcoptes scabiei var. scabiei-crustosae (Fiirst.), adult $. Second spine of inner (3) 
and outer (4) rows. 
Figs. 5 and 6. Sarcoptes scabiei var. hominis, adult $. Second spine of inner (5) and outer (6) 
rows. 
Figs. 7 and 8. Sarcoptes scabiei var. equi, adult $. Second spine of inner (7) and outer (8) rows. 
All the figures are drawn to the same scale with a Zeiss camera lucida. 
Sarcoptes. The slight curve in Fig. 6 is due to the spine being viewed later¬ 
ally; all spines are slightly curved, and the curvature cannot be used for 
separating races. I can detect no other differences whatever between 
S. scabiei var. hominis and var. equi. I have examined with particular care 
the cones, the trochanteric claw, and the chaetotaxy of the capitulum, body 
and legs; taxonomic differences have been described in all these structures, 
but I can find no differences whatever. I have examined adult males and 
females, but not the immature stages. 
I conclude that the Sarcoptes of the horse and the common species found 
on man cannot invariably be separated. Certain minute differences exist in 
scales and spines but they are not constant, and the measurements overlap. 
Other characters which have been used for separating these forms appear to 
be wholly useless. It is convenient to regard these forms as varieties, as many 
