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 procurved.