LIBRARY

264 COLLEGE ENTOMOLOGY
animals and fowls, and agricultural crops. A few are helpful in destroying injurious
mites and other insects. They vary in size from the minute phylloxeras,
scarcely visible to the unaided eye, to the giant water bugs attaining a length
of over 472 in. In shape they are equally variable. A great many are shieldshaped
or oval and flattened; some are robust, globular, hemispherical, or disklike;
and still others are elongated, slender, cylindrical, and thread-like. The
appendages are often modified to accentuate the form.
The range and combinations of colors are almost inexhaustible. In the
cooler regions, colors for the most part are dull and uninteresting, but in the
FIG. 89. External anatomy of HEMIPTERA. A, box-elder bug, Leptocorjs trivittatus (Say),
witil tile left hemelytron and wing spread; B, same, side view of ilead and thorax; C, hemelytron
of the tarnished plant bug. LygllS pratensis (Linn.) (From Insects oj Western North
America.)
warmer temperate zones, and especially in the tropics, they are brilliant and
striking beyond description, with opaque, transparent, and metallic hues.
The biology will be discussed under the suborders, families, and species.
Suffice it here to say that the metamorphosis, though generally simple, becomes
quite complex in the males of the coccids, and the life histories are exceedingly
complicated in the aphids, phylloxerids, and adelgids. Bisexuality and sexual
dimorphism are the rule, though parthenogenesis is very common. True sexes
are unknown in certain species of aphids in some regions.
The great majority of the species are phytophagous and feed upon the juices