and in many respects without precise analogy in
other groups of insects, but it is not necessary here to enter into
more than some special points. The arrangement of the nervures at the
base of the wing is very singular, and slight differences in it form
useful aids to classification. In the _Aeschnidae_ and _Libellulidae_
this arrangement results in the formation of a triangular space (known
as the "triangle"), which is either open or traversed by nervules; but
in many _Agrionidae_ this space, instead of being triangular, is
oblong or elongately quadrate, or with its upper edge partly straight
and partly oblique. This fixitude of type in neuration is not one of
the least important of the many peculiarities exhibited in these
insects.

The internal structure is comparatively simple. The existence of
salivary glands, denied by L. Duprix, has been asserted by O.
Poletajewa. The rest of the digestive apparatus consists of an
elongate canal extending from mouth to anus, comprising the
oesophagus, stomach and intestine, with certain dilatations and
constrictions; the characteristic Malpighian vessels are stated to
number about forty, placed round the posterior extremity of the
stomach. Dragon-flies eat their prey completely, and do not content
themselves by merely sucking its juices; the harder portions are
rejected as elongate, nearly dry, pellets of excrement.

_Pairing._--But the most extraordinary feature in the economy--one
which has attracted the attention of naturalists from remote times--is
the position of the genital organs, and the corresponding anomalous
manner in which the pairing of the sexes and impregnation is effected.
In the male the intromittent organ is situated in a slit on the under
surface of the second abdominal segment; it is usually very crooked or
sinuous in form, and is accompanied by sheaths, and by external hooks
or secondary appendages, and also by seminal vessels. But the ducts of
the vessels connected with the testes unite and open on the under
surface of the ninth segment; hence, before copulation can take place,
it is necessary that the vessels in the second segment be charged
from this opening, and in the majority of cases this is done by the
male previously to seeking the female. In the latter sex the entrance
to the oviduct and genital organs is on the under surface of the
eighth abdominal segment. The act of pairin