LIBRARY

282
COLLEGE ENTOMOLOGY
venation are shown;n the accompanying illustrations. Classification is based
very largely upon venation, and a thorough knowledge can be acquired only
by a careful study of the specimens and the work of competent odonatologists.
The greatly elongated abdomen is either cylindrical, somewhat flattened
dorsally. or enlarged posteriorly. There are 10 well-defined segments and two
rudimentary ones at the anal end. In the ANISOPTERA the males with angulate
hind wings have small protuberances, auricles, on the second tergite, and
in some species the third segment is constricted. The tergites extend downwards
so as to conceal the spiracles and plural membranes and even to partially overlap
the sternites. The cerci or superior appendages in both sexes are short and
unsegmented. The supra-anal plate or inferior appendage is present in the
males of ANISOPTERA and vestigial in males of ZYGOPTERA and in all
females. Paraprocts are strongly chitinized in the males of ZYGOPTERA to
form the inferior appendages. The females of all ZYGOPTERA and certain
ANISOPTERA have a well-developed ovipositor while those in other members
of the ANISOPTERA have the ovipositor aborted or vestigial. Although the
male gonopore is situated on the ninth sternite, the penis with accessory armature
is situated in a groove of the second sternite and arises from the penis
vesicle on the anterior portion of the third sternite - a characteristic unknown
in any other order of insects. In general the males may be distinguished
by the copulatory groove on the second abdominal segment and by three or
four caudal appendages and the females by the presence of only two caudal
appendages. The males are also often more brightly colored. The two sexes
are often seen pairing on the wing, either in tandem flight in which case the
male holds the female damselfly by the prothorax or the female dragonfly by
the back of the head by means of his caudal appendages, or in copulation in
which case the female brings her abdomen forward to connect with the second
sternite of the male. There are eight pairs of spiracles located on adbominal
segments I to VIII and segment II of the thorax.
The nymphs are grotesque creatures that look like miniature prehistoric
reptiles. They are robust or elongated with a rather thin, leather-like integument
which may be rough and horny and are wholly aquatic, living on the
bottoms in mud and sand or clinging to rocks, logs, and other places of lodgment.
In coloration they are mostly somber or cryptic, and those species which
inhabit the aquatic plants are often covered with alga! or overgrown with bryozoans,
The first-born are peculiar in being still enclosed in a thin membranous
shroud for a few seconds or minutes, during which brief period they are designated
as pronymphs. After shedding this membrane they are in the second
instar and begin their active life of preying upon all kinds of water animals
that can be overpowered. The most remarkable thing about these insidious
creatures is the unusually developed labium, which is long, jointed near the
base, and extensile like a human arm. At the apex the two palps are provided
with movable hooks and spines and act like a hand in grasping and holding
living prey. The labium may be darted forwards with great speed and pre-