NOTE ON DIFFERENCE IN COLORATION. 269
Fig. 1.—Asellus Aquaticus Linn.
character to be described, they can only be satisfactorily separated
by the examination of the pleopods and some other small details,
a task requiring the use of fairly high powers or else of dissection.
As will be seen from the accompanying figures the arrangement
of the pigment in A. aquaticus is such that, broadly speaking, two
large lateral clear spaces are left in the posterior part of the
head, whereas in A. meridianus there is one large clear space
extending almost across the posterior part of the head. These
characteristic clear spaces free from pigment are present in both
sexes and are usually quite easy to see with a pocket lens, unless
the specimens are very dirty. The only doubt which may arise
is that, in A. meridianus, the alimentary canal may sometimes
show through as a dark band in the middle, thus producing at
first sight the appearance of two lateral clear spaces as in A.
aquaticus. But when one is on one's guard, this false appearance
Fig. 2.—Asellus Meridianus Racovitza.