106 Sarcoptes 
and are therefore more noticeable than is usually the case; this is not a char- 
acter of males in general. The first and second pair of legs resemble those of 
the female but the chitinous thickenings on the various joints are more 
strongly developed, so that the division of the leg into joints is more apparent. 
The chaetotaxy of the male and female anterior leg is identical except for the 
greater development of the minute seta P 4 in the male. As regards the spur, 
the claws and the ambulacrum, the male cannot be distinguished from the 
female. The third and fourth legs resemble those of the female in general 
structure as can be seen f^oin Fig. 14, but differ in a number of small points. 
The seta H 1 of the third leg is extremely long, equal to the total length of 
head and body , H 2 is longer and stouter than in the female; in addition there 
are three setae, H 3, 4, and 5, which do not occur on the female; of these H 3 
is a very small and fine seta, lying lateroventrally at the base of the lateral 
claw; H 4 and H 5 are short fine setae lying distal to H 2 and H 3 respectively. 
There is also present a minute papilla (pap) on the mesial side of the joint, 
distal to H 2. The fourth leg can at once be distinguished from that of all 
other stages by its possession of an ambulacrum in the place of the terminal 
seta; this ambulacrum is about half the size of the ambulacrum on the 
anterior legs. The setae H 2 and H 3 are present in the same relative positions 
as on the third leg; H 4 and H 5 are not present. The mesial claw is absent. 
In its place there is a large soft papilla. 
Between the fourth epimeres of the two sides lies the complex genital 
apparatus. The most anterior part of this is the median rod of the epiandrium 
(mea), the “sternite” of Robin (1860). The anterior end of this rod is forked, 
and the limbs of the fork articulate on each side with a flat surface on the 
united third and fourth epimeres. In Fig. 14 the epimere and epiandrial rod 
have been separated by pressure in order to display the articular surfaces. 
The posterior end of the epiandrium consists of two curved epiandrial limbs 
(eal), springing from the median epiandrial rod. These limbs are the episternite 
of Robin; they diverge and pass backwards to terminate as two transversely 
placed hammer-heads (hli). The whole of this structure, the median rod, the 
limbs and the hammer head form the epiandrium (ea, Fig. 14), an organ which 
is comparable to an epimere; that is to say it is a rod-like internal thickening 
of the chitinous investment of the body, and its function is to supply a firm 
base for the erection of the genital organs themselves. From the concavity 
between the epiandrial limbs arises a flat lamella, the genital apron ( ga ), in 
shape something like a finger nail. This apron fills nearly the wfiole space 
between the epiandrial limbs, and exceeds that space posteriorly. It is an 
extremely thin, colourless sheet of chitin, bearing on its ventral surface a pair 
of small setae (F 5). Though it is quite transparent I have omitted from Figs. 
14 and 17 the structures which lie beneath it, in order to simplify the figures. 
In nature the structures shown in Fig. 17 would be superimposed on these in 
Fig. 18. The parts which lie beneath the genital apron are shown in Fig. 18. 
In the concavity of the epiandrial limbs lies a horse-shoe shaped structure, 
