124 
Sarcoptes 
The following setae are present on the ventral surface: a pair of short 
setae (V 1) on either side of the sternum; a very short pair, the tocostomal 
setae (ts), close to the middle line on the base of the genital operculum; a 
pair of rather stout but short setae (V 2) arising from the bare space between 
the epimeres of the third and fourth legs; a pair of extremely fine setae in¬ 
serted close to the middle line among the integumentary ridges which lie 
between the bases of the last pair of legs (V 3); and a short seta (V 4) on the 
cuff of integument which covers the base of the third leg. 
In addition to these there are in the ventro-lateral region the setae L 1, 
which arise anterior to the dorso-lateral pair (L 2), and on a level with the 
epimere of the third leg. The seta L 1 is longer than any of the ventral setae 
but not so long as L 2. Other setae, which are visible on the ventral surface, 
but belong to the chaetotaxy of the legs are described on p. 130. The median 
ei 
Fig. 6 
Fig. 5. Sarcoptes scabiei var. equi. Ventral view of the genital operculum and surrounding parts 
in an adult female, el, e II, epimeres of first and second legs; gop, genital operculum; 
tc, tocostome; ts, tocostomal seta; V 1, first ventral seta. 
Fig. 6. Ventral view of actual tocostomal orifice; the same structure as shown in dotted line in 
Fig. 5. tc, tocostome. 
tocostomal seta which is figured by Fiirstenberg and others, and apparently 
by Munro (1919), does not exist, at any rate in S. scabiei var. equi. As I shall 
explain below these authors have seen the longitudinal aperture of the toco¬ 
stome, and figured it as a seta. All the setae on the ventral surface are directed 
backwards. 
Three or four scales (sc) can generally be seen from below, on the flank at 
the level of the fourth pair of legs. The tip of the last spine ( sp ) of the outer 
row is also often in sight, projecting over the posterior end of the body, but 
it is of course a dorsal structure. With these exceptions there are no scales 
or spines on the ventral surface. 
Behind the epimeres of the first and second pairs of legs a wide transverse 
flap, very shallow from before backwards, can easily be seen. This is the 
genital operculum (gop, Fig. 5), bearing at its base the tocostomal setae (ts). 
