cture serving to
retain the prey. Considerable diversity of opinion exists with respect
to the composition of the mouth parts, and by some authors the "palpi"
have been termed the side pieces of the lower lip. The prothorax is
extremely small, consisting of only a narrow ring. The rest of the
thorax is very large, and consolidated into a single piece with
oblique sutures on the sides beneath the wings.

The abdomen varies excessively in form, the two extremes being the
filiform structure observable in most _Agrionidae_, and the very broad
and depressed formation seen in the familiar British _Libellula
depressa_. It consists of ten distinct segments, whereof the basal two
and those at the apex are short, the others elongate, the first being
excessively short. In a slit on the under side of the second in the
male, accompanied by external protuberances, are concealed the genital
organs: on the under side of the eighth in the female is a scale-like
formation, indicating the entrance to the oviduct. The tenth is always
provided in both sexes with prominent appendages, differing greatly in
form, and often furnishing the best specific (and even generic)
characters.

The legs vary in length and stoutness, but may, as a rule, be termed
long and slender. The anterior pair probably assist in capturing and
holding insect prey, but the greatest service all the legs render is
possibly in enabling the creature to rest lightly, so that it can quit
a position of repose in chase of passing prey in the quickest possible
manner. The coxa is short and stout, followed by a still shorter
trochanter; the femora and tibiae long and slender, almost invariably
furnished on their under surface with two series of strong spines, as
also are the tarsi, which consist of three slender joints, the last
having two long and slender claws.

The wings are always elongate, and furnished with strong longitudinal
neuration and dense transverse nervules strengthening the already
strong (although typically transparent) membrane. In the _Agrionidae_
both pairs are nearly equal, and are carried vertically and
longitudinally in repose, and the neuration and membrane are less
strong; hence the species of this family are not so powerful on the
wing as are those of the other groups in which the wings are
horizontally extended in a position ready for instant service. The
neuration is peculiar,