123 
P. A. Buxton 
The Ventral Surface. 
The under surface of the body (Fig. 2) is nearly flat, and is crossed normally 
by the indefinite sulcus which divides the notothorax from the notogaster. 
It is covered by ridges of integument, transverse for the most part, but 
sweeping round the articulation of the posterior legs so as to become longi¬ 
tudinal; where the integument is strengthened by the ventral epimeres it is 
not thrown into ridges. At the anterior end of the ventral surface there is a 
deep V-shaped indentation. The sides of the V are the lower margin of the 
camerostome, and pass round the basis capituli somewhat like a collar. There 
are similar V-shaped indentations for the reception of the bases of the legs of 
the first two pairs. The ventral epimeres (el-IV), are the most conspicuous 
objects on the ventral surface of the mite. In sections of the whole organism 
they appear as thickenings of its chitinous integument, extending into the 
body as ridges. They are to be regarded as specialized parts of the exoskeleton 
of the body, and not as parts of the leg; their function is to provide a firm 
point dlappui for the leg, and this they do by stiffening the exoskeleton which 
is elsewhere so soft. Their distal end is forked to receive the obtusely angular 
articular surface of the corresponding leg. One is tempted to think of them 
as ridges for the origin of muscles, as apodemes in fact; I am not prepared 
to say that they never fulfil this function, but it is certainly not their prime 
function. The epimere of the first leg (e I) passes backwards and inwards to 
meet its fellow of the opposite side at a point slightly behind the insertion of 
the basis capituli into the camerostome. From their point of union they pass 
backwards as a single chitinous structure, which terminates bluntly in front 
of the genital operculum ( gop ). This median structure, the product of the 
fusion of the first pair of epimeres, is the sternum of most authors, “sternal 
bar,” and “sternal rod” of Warburton, “pi&ce commune ou sternale” of 
Robin. The name “plastron” is given by Robin (1860) to the point at which 
the first pair of epimeres unite. 
The epimeres of the second pair of legs (e II), resemble those of the first 
pair in size, but differ in the greater development of the mesial limb of the 
fork which receives the articular surface of the basal joint of the leg; this 
limb of the fork is prolonged for a considerable distance up the space which 
separates the first and second legs (Fig. 8). The shaft of the epimere is curved 
towards the middle line, so that its basal part is nearly parallel to the sternum. 
The basal (posterior) end of the epimere is shown in Fig. 5. It bears a minute 
point and shows a very slight tendency to being forked. The epimeres of 
the third and fourth pairs of legs differ from the anterior ones in being 
much shorter. That of the third leg is curved so that its proximal end 
approaches the fourth. On its lateral side it is prolonged out into a thin semi¬ 
circular flange, the free edge of which gradually passes into the cliitin which 
covers the body. The fourth epimere is short, stout and nearly straight; it is 
shallowly forked at its proximal end, and more deeply distally, where it 
articulates with the fourth leg. 
