mph._--The duration of the subaquatic life of a dragon-fly
is no doubt variable, according to the species. In the smaller forms it
is probably less than a year, but precise evidence is wanting as to the
occurrence of two broods in one year. On the other hand, it is certain
that often a longer period is requisite to enable the creature to attain
its full growth, and three years have been stated to be necessary for
this in the large and powerful _Anax formosus._ Like all insects with
incomplete metamorphoses, there is no quiescent pupal condition, no
sharp line of demarcation between the larval and so-called "nymph" or
penultimate stage. The creature goes on eating and increasing in size
from the moment it emerges from the egg to the time when it leaves the
water to be transformed into the aerial perfect insect. The number of
moults is uncertain, but they are without doubt numerous. At probably
about the antepenultimate of these operations, the rudimentary wings
begin to appear as thoracic buddings, and in the full-grown nymph these
wings overlap about one-half of the dorsal surface of the abdomen. In
structure there is a certain amount of resemblance to the perfect
insect, but the body is always much stouter and shorter, in some cases
most disproportionately so, and the eyes are always separated; even in
those genera (e.g. _Aeschna_) in which the eyes of the imago are
absolutely contiguous, the most that can be seen in the larva is a
prolongation towards each other, and there are no ocelli. The legs are
shorter and more fitted for crawling about water plants and on the
bottom. In the mouth parts the mandibles and maxillae are similar in
form to those of the adult, but there is an extraordinary and unique
modification of the lower lip. This is attached to an elongate and
slender mentum articulated to the posterior portion of the lower surface
of the head, slightly widened at its extremity, to which is again
articulated the labium proper, which is very large, flattened, and
gradually dilated to its extremity; but its form differs according to
group as in the perfect insect. Thus in the _Agrionidae_ it is deeply
cleft, and with comparatively slender side-pieces (or palpi), and
strongly developed articulated spines; in the _Aeschnidae_ it is at the
most notched, with narrow side-pieces and very strong spines; in the
_Libellulidae_ it is entire, often triangular at its apex, and with
enormously developed palpi without spines, b