LIBRARY

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COLLEGE ENTOMOLOGY
Great numbers of Post-Permian fossil dragonflies have been discovered in
many parts of the world and are admirably treated by Handlirsch (1908).
Although the immature stages are strictly aquatic the adults, especially of
the strong-fiying dragonfiies, are often seen great distances from water.
The eggs, which are usually rounded in exophytic species and elongated in
endophytic species, are deposited in several ways. They may be dipped and
washed into the surface of the water by the females in flight or deposited in
long gelatinous strings or masses and attached to objects just under the surface
of the water or on the undersides of chips or pieces of bark floating on the
water or they may be inserted into logs or mud or in the tissues of living plants
growing near or in the water. In the last case the females, sometimes accompanied
by the males, may crawl down the stems under the water where
they remain for a considerable length of time, apparently indifferent to their
complete submergence and in no hurry to renew their supply of air. The
females may also oviposit in such plants by merely extending the abdomen
under the water. A single dragonfly female has been known to lay more than
800 eggs in a single mass. The nymphs or naiads can hardly be called beautiful
and are not at all to be compared with the adults in vivacity. They are unattractive
in form and movements, but their cryptic and often changing colors
make them remarkably well adapted to the particular life they lead. They are
to be found in fresh, or rarely brackish, water, living among the water plants,
clinging to rocks or logs, or buried in the sand or mud at the bottoms of pools,
lakes, and streams, and are occasionally seen in open water. All are highly
predacious upon whatever forms of aquatic animal life can be overpowered,
and they are equally given to cannibalism. Mayfly naiads and mosquito and
gnat larvre form a large part of their diet. After 10 to 15 instars, over a period
of from 1 to 5 years the nymphs are fully grown. Crawling out of the water,
they then attach themselves to some suitable object by means of their feet and
emerge as adulLs through a slit in the dorsum of the head and thoracic region
to take on a life of aerial predacity. Their prey is taken on the wing, caught
and held in a sort of a trap formed by the forward position of the legs. Their
food consists of a remarkably long and complete run of insects including many
of the noxious gnats, midges, mosquitoes, and fiies, and they thereby confer
some benefit upon the human race in lessening the numbers of its tormentors
and of disease-bearing insects. The winter is usually passed in the nymphal
stage, but adults also hibernate or are active throughout most of the year in
the warmer regions. On occasions dragonflies migrate in large numbers over
long distances. The widely distributed north temperate species, Libellula
quadrimaculala Linnreus, often appears in great numbers in parts of Europe
while Hemicordulia tau Selys acts similarly in Australia. Although most
species are diurnal and sun-loving, certain tropical species fiy and feed at night.
External anatomy. - The adult has already been briefly characterized.
The head is wide because of the large, laterally placed compound eyes which
occupy most of the space and is pivoted on a very slender neck, giving it freedom
of movement. Three ocelli occur on the dorsum just in front of the COffi-