316 ARACHNIDA—-ARANEAE CHAP. 
The number, relative size, and particular arrangement of these 
eyes are of considerable systematic importance. Their disposition 
varies very greatly, but it is generally possible to regard them as 
forming two transverse rows, an anterior and 
a posterior, each possessing a pair of median 
and a pair of lateral eyes. 
In many spiders all the eyes have a 
dorsal aspect, but in some groups (Attidae, 
Lycosidae) the prevailing arrangement is to 
have the anterior eyes directed forwards and 
the posterior upwards. In other spiders, 
again, a dorsal view may only show the eyes 
in profile, all having their axes directed 
forwards or sideways, or they may be 
mounted on turrets, and thus command a 
wide range of view. The rows are described 
as straight, “ procurved ” (with the convexity 
Tia 173. Soiseennmatic backwards), or “ recurved” (with the con- 
dorsal view ofa Spider. vexity forwards). Thus, in Fig. 177, the 
ch, Chelicera ; J; me anterior TOW is slightly, and the posterior 
dian fovea ; 7, normal 
marking ; 0, ocular row considerably “ recurved.” 
Pana j Oras Sometimes there is a marked difference 
line should reach the jy the colour of the eyes, two or more being 
radial marking on the : 5 : 
cephalothorax. ) black, while the remainder are pearly white. 
In other cases they are homogeneous, either 
of the black or the white type. Simon considers the black eyes 
to be diurnal and the white nocturnal, but the evidence for this is 
indirect and not altogether satisfactory. The portion of the caput 
occupied by the eyes is often alluded to as the “ ocular area.” The 
space between the ocular area and the chelicerae, well shown in Fig. 
177, 1s known as the “ clypeus.” It is usually more or less vertical, 
but in the Aviculariidae (see p. 386) it is horizontal and dorsal. 
The under surface of the cephalothorax is protected by the 
“sternum ” or “ plastron,” a large plate of variable shape, usually 
notched at either side for the reception of the legs, and having in 
front a small plate, generally hinged, but sometimes soldered to 
it, known as the “labium.” This has no homology with the 
labium of insects, but is a true sternite, more correctly described 
as “pars labialis sterni.” 
The labium and the maxillary lobes of the palpi more or less 
ech 
