184 THE SPIDERS OF EPPING FOREST. 



With the belief that these two forms are respectively 

 co-existent with diurnal and nocturnal habits I am 

 quite willing to concur ; but whether the eyes of the 

 nocturnal type are specially adapted for viewing objects 

 in a dim light or whether these organs are impaired by 

 disuse, it is rather difficult to determine. Be this 

 as it may, the two forms are in the majority of cases 

 very distinct and are of some use in the separating, 

 at any rate, of the more highly specialised species of 

 several families. As might be expected many forms 

 are found intermediate between the distinct diurnal eye 

 and the nocturnal type. The diurnal eyes are per- 

 fectly, or almost perfectly round, usually of dark colours, 

 or black, and of a strongly convex form. The nocturnal 

 eyes are sometimes round, more often oval or even 

 angular, of a pearly white tint and rather flattened. In 

 some species the eyes are homogeneous [i.e., all of one 

 type, either diurnal or nocturnal) ; but in other cases 

 they are heterogeneous, both types being co-existent in 

 one species. The tract bounded by the eyes is known 

 as the ocular area and the portion between the foremost 

 eyes and the anterior edge of the caput is termed the 

 clypeus. The expression " length of the ocular area" 

 must be regarded as implying the measurement in a 

 direction parallel to the main axis of the body. It will 

 be noticed, therefore, that in the majority of spiders the 

 ocular area is broader than long. Statements as to the 

 straightness or curvature of a row of eyes are based 

 upon measurements made from the centre of the eyes 

 in question. It will be easily seen that the apparent 

 curvature of a row of eyes upon a convex or uneven 

 surface will depend greatly upon the position in which 

 the creature is viewed. It should be understood that 

 details as to the straightness or curvature are, as a rule, 

 founded upon observations made in a line at right angles 

 to the integument at the central portion of the row of 

 eyes in question. The expression recurved indicates 

 that the convexity of the curve is directed forwards, 

 curvature in the opposite direction being denoted by the 

 term pvociivved. 



