i6 



ARACHNIDA — ARANEAE 



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 



ch o 



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 prevailinr,"- arrangement is to 

 liave 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 

 backwards), or " recurved " (with the con- 

 vexity forwards). Thus, in Fig. 

 anterior row is slightly, and the 



177, the 

 posterior 



Fig. 173. — Diagrammatic 

 dorsal view of a Spider. 

 ch, Chelicera ; /, me- 

 dian fovea ; n, normal 



marking ; o, ocular j-qw considerably " recurved." 



srstria.^' (The clotted Sometimes there is a marked difference 



line should reach the {y^ the colour of the eycs, two or more being 



radial marking on the ,,, i-ii -i i i-. 



cephaiothorax. ) 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. 

 1 77, is 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 cephaiothorax is protected l)y 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 stei'iii." 



The labium and the maxillary lobes of the palpi more or less 



