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44 COLLEGE ENTOMOLOGY
With certain notable exceptions, insects are for the most part unisexual, there
being corresponding numbers of males and females in each species. Certain
primary peculiarities, such as size, color, vestiture, and activity, may serve to
distinguish the different sexes, and there are also innumerable external variations
commonly known as secondary sexual characters that may modify almost
every part of the body and readily serve to separate the males and females in
what appears to the untrained eye to be a mass of identical individuals known
as a species. On the other hand, sexual variations are sometimes so marked that
B
FIG. 20. The human louse, Pediculus llumanus Linn. A, a portion of the respiratory system;
B, the chief nerve centers or ganglia. (After J. Muller,)
males and females of the same species have frequently been relegated to different
genera.
Certain insects have a sexual cycle followed by cycles of reproduction without
mating (parthenogenesis), and a considerable number reproduce continuously
by the latter method. In fact, in many species of aphids, weeVils, and other
insects, no males have ever been observed though these particular forms have
endured for thousands and perhaps millions of years.
Suppressed sexuality is exhibited by certain individuals of a large number of
species of insects, particularly those which have a form of social life like the
termites, ants, bees, and wasps. In such insects the sexual organs, though