268 THE ESSEX NATURALIST.
The latest appearances of the Avocet are also very remarkable.
One was shot near Southend on January 4th, 1936, and another
was seen off Foulness on February 6th, 1937 (British Birds,
V, xxx, pp. 50, 95, 37S). It will be remembered that this is a
migratory species, which flies southwards from Europe at the
close of the nesting season.
In presenting these broad features the present writer has
used the latest information available to him. The field covered
is a vast one, however, and it is possible that changes may have
occurred which are unknown to him.
NOTE ON THE DIFFERENCE IN THE
COLORATION OF THE HEAD IN ASELLUS
AQUATICUS AND A. MERIDIANUS.
By D. J. SCOURFIELD, I.S.O., F.L.S., F.Z.S.
(With Two Text Figures.)
WHEN examining specimens of Water Wood-lice from
ponds, etc., in Epping Forest some years ago I noticed
that the two species, Asellus aquaticus Linn, and A. meridianus
Racovitza, appeared to be characterised by a different arrange-
ment of the brown pigment on the head. I have since examined
many further specimens from Epping Forest and other localities
and have found this character to be constant; and one or two
friends, to whom I have mentioned the matter, have told me
that they also have found the character a reliable one in the
separation of the two species. It seems, therefore, that the
difference in the coloration of the head may be regarded as an
easy way of recognising these species, and this should be of
some value in any piece of ecological work in which they may
be involved.
The necessity for a reliable naked-eye or simple-lens character
for such work may be judged from the fact that from the time of
Linnaeus until 1919 the two species had always been confused
and recorded under the one name A. aquaticus. (See references
at end. No. 2 includes a Plate with figures of the chief morpho-
logical differences between the two species.) The two are, in
fact, so similar in general appearance that, apart from the