BIOLOGICAL NOTES ON DRAGON-FLIES. 293
metamorphosis incomplete, there being no pupal or resting
stage, but they undergo a great change at. the final ecdysis.
If one looks at a collection of these insects one notices that
there are two forms, a thick bodied and a thin bodied form.
The more they are studied, the more distinct these two forms
appear. In the thick bodied form the eyes are large and occupy
the major part of the head, often meeting in a line in front ;
the wings are unequal, the hind-wings being larger than the
front wings. This sub-order is called Anisoptera. The other
sub-order, the thin bodied forms, is the Zygoptera ; in these the
eyes are small, wide apart, and the wings are equal. In the
early stages there are also many differences ; we shall deal
with these when we come to the larva.
Look at the head of one of the Anisoptera : Aeschna cyanea
will do. You will notice that the head consists very largely
of the compound eyes, these meet in a line at the top of the
head and the line is carried down in front. Between the eyes
is a triangular portion of the epicranium upon which are the
three simple eyes and the antennae. Below this we see the
frons and clypeus. The lower portion of the head is occupied
by the mouth, it consists of a lower lip called the labium, an
upper lip or labrum, and between these are a pair of mandibles
and a pair of maxillae. The labium and labrum work like our
lips, the maxillae and mandibles do not work parallel with them
but at right angles to them. The labium of the dragonflies is
very specialised, it consists of three lobes, a median lobe and
two lateral lobes. The median lobe in A. grandis is simple, but
in some forms it is divided almost into two by a cleft at the
centre of its width. The lateral lobes have a fixed hook on their
inner border and just exterior to this is a moveable one. These
three lobes are not in one plane, the two lateral lobes make an
angle with the median lobe. The mandibles are used for
chewing the food and are hard, strong and thick, with a number
of teeth ; those in the front are sharp pointed, and may be
looked upon as incisors, those farther back as molars. The
maxillae are much more slender and less powerful; the outer
end of each is divided into an outer hinged palp and an
inner portion, upon which are six spines on the outer part, and
on the inner is a lobe clothed with sensory hairs. The upper lip
or labrum is quite simple. If we compare this head with one