296
THE ESSEX NATURALIST.
so that the young naiad can slip easily out. The time taken from
the pushing up of the cap to the emergence of the naiad is only a
few seconds.
The naiad is generally slow in its movements, crawling along
the bottom of the pond or stream or else climbing the water
plants in search of other living animals for food. In the
Zygoptera the naiads possess at the tail end three caudal plates ;
these are sometimes called gills, but though they may be used
as gills, the naiad also obtains air by other means, for it often
loses all three and yet seems none the worse for it. These plates
are also used for propelling the naiad, much in the same way as
a man uses an oar at the stern of a boat. The Anisopterid
naiad does not possess these plates, but in their place are five
pointed appendages; in these the air is obtained by rectal
branchiae. The water is drawn in between the five appendages,
which are then closed and the air extracted, after this the water
is expelled. This expulsion of the water is often used as a
means of locomotion, it causes the creature to dart forward
with a rapid motion. The labium of the naiad is a most
interesting structure ; it is referred to as the mask, as it hides the
lower part of the head. The mask is long and is attached to
the head just below the mouth, and at the centre of its length
is a hinge. When at rest the mask is folded in such a way that
the hinge is held back between the legs and its front is close to
the mouth. At the front of the mask are hinged lateral lobes
that have a moveable hook and by these the food is grasped.
The naiad stalks its prey, and when near enough the mask is
thrust out, the food captured and then the mask is drawn back
to its resting position ; the food is thus brought to the mouth.
The number of times a naiad moults seems to be very uncertain,
some moulting more than others ; each time there is a gradual
increase in size and the wing-buds get larger, those of the hind-
wings gradually overlapping those of the fore-wings. The mask
also undergoes some changes, more spines being developed upon
the median and lateral lobes. The antenna;, which at first
consist of three joints, are increased to seven joints. The
sexual organs also make their appearance, and it is quite easy
to distinguish the male from the female. When full grown the
naiad climbs out of the water up the stem of a plant, or, failing
this, the bank. When it reaches a satisfactory position it