ists they were placed
as forming part of the heterogeneous order _Neuroptera_. J. C.
Fabricius, however, elevated them to the rank of a distinct order, which
he termed _Odonata_; and whatever may be the difference of opinion
amongst authors at the present day, that term is almost universally
employed for the group. W. F. Erichson transferred all the groups of
so-called _Neuroptera_ with incomplete metamorphoses, hence including
the dragon-flies, as a division of _Orthoptera_, which he termed
_Pseudo-Neuroptera_. K. E. A. Gerstacker more recently also retains them
in the _Orthoptera_, terming those groups in which the earlier states
are subaquatic _Orthoptera amphibotica_. All entomologists are agreed in
maintaining the insects as forming a group marked by characters at once
extraordinary and isolated in their nature, and in most modern
classifications they are treated as a distinct order.

The group _Odonata_ is divided into three families, and each of these
again into two subfamilies. The families are the _Agrionidae_,
_Aeschnidae_ and _Libellulidae_--the first including the subfamilies
_Calopterygina_ and _Agrionina_, the second _Gomphina_ and _Aeschnina_,
and the third _Cordulina_ and _Libellulina_.

_Anatomy._--The structure of a dragon-fly being so very remarkable, it
is necessary to enter somewhat extensively into details. The head is
comparatively small, and excavated posteriorly, connected very
slightly with the prothorax, on which it turns almost as on a pivot.
The eyes are, as a rule, enormous, often contiguous, and occupying
nearly the whole of the upper surface of the head, but sometimes
(_Agrionidae_ and _Gomphina_) widely distant; occupied by innumerable
facets, which are often larger on the upper portion. The antennae,
which are smaller in proportion than in almost any other insects,
consist only of two short swollen basal joints and a 5 or 6-jointed
bristle-like thread. The large labrum conceals the jaws and inner
mouth parts. The lower lip, or labium (formed by the conjoined second
maxillae), is attached to a very small chin piece (or mentum), and is
generally very large, often (_Agrionidae_) divided almost to its base
into two portions, or more frequently entire or nearly so; on each
side of it are two usually enormous hypertrophied pieces, which form
the "palpi," and which are often furnished at the tips with an
articulated spine (or terminal joint), the whole stru