P. A. Buxton 
125 
This operculum 1 covers the actual orifice of the tocostome (tc) (“vulve/’ 
Robin; “vulve de ponte,” Neumann), a longitudinal slit between two closely 
opposed slightly thickened lips. These lips lie slightly beneath the integument 
of the body, and are shown most clearly in Fig. 6. They are somewhat thick¬ 
ened in front where they are opposed to one another; further back they are 
more weakly chitinized and may be seen to diverge as they pass deeply into 
the body. In specimens which have been killed slightly before oviposition 
the lips may be seen widely separated to allow for the passage of the egg. 
There is therefore no doubt at all that this orifice is the tocostome, that is to 
say the birth opening or passage through which the egg is laid. 
The earliest and at the same time the most accurate extant figures of the 
genital operculum are those of Robin (1860) and Gudden (1861). Since that 
time it has been figured and described by Delafond and Bourguignon (1862), 
and by Railliet (1895). Other authors seem to have failed to see it, and no 
one so far as I am aware has described the actual longitudinal orifice of the 
tocostome, except Gudden (1861). Railliet, for instance, speaks of “une fente 
transversale,” obviously the line of opening behind the genital operculum. 
It is an extraordinary thing that a writer so recent as Munro (1919) failed to 
observe the genital operculum, and figures only a median structure which 
might be taken to represent either the opening of the tocostome, or the (non¬ 
existent) median seta first figured by Fiirstenberg (1861). 
The Capitulum. 
The structures which comprise the capitulum of Sarcoptes have been inter¬ 
preted in a different way by almost everyone who has seriously tried to 
describe them. Modern technique and a modern microscope have helped me 
to produce the following description, which I believe is an improvement upon 
those previously published. I propose first to describe the structure of the 
capitulum and its component organs, and then to discuss certain points of 
interpretation and homology. 
The capitulum (cap) consists of the chelicerae ( ch ) and palps (pedipalps) 
(pp), and the basis capituli or chitinous capsule to which they are attached. 
The general shape of the capitulum resembles that of a thumb-nail, as can be 
seen from Plate VII and Fig. 7; its length is about 60/z, breadth 55, and depth 
from the dorsal to the ventral surface 50-60 at the base. Seen from the side 
the dorsal surface is flat, the ventral convex, and these approach one another 
from base to apex. The insertion of the basis capituli into the camerostome 
has already been described; the base of the capitulum is overlapped by the 
epistome above, and partly covered by the diverging limbs of the V-shaped 
opening below. The structure of the basis capituli, which is the name given 
to the whole capitulum except the appendages, is as follows. The chitinous 
1 The tocostome is generally overlapped by the margin of the genital operculum. In the 
example which I have chosen for figuring (Figs. 5 and 6) it lies slightly posterior to the margin 
of the operculum and is therefore more clearly seen. 
