the reproductive glands, it is scarcely possible to decide which ought
to be called primary and which secondary.
The female often differs from the male in having organs for the
nourishment or protection of her young, such as the mammary glands of
mammals, and the abdominal sacks of the marsupials. In some few cases
also the male possesses similar organs, which are wanting in the
female, such as the receptacles for the ova in certain male fishes, and
those temporarily developed in certain male frogs. The females of most
bees are provided with a special apparatus for collecting and carrying
pollen, and their ovipositor is modified into a sting for the defence
of the larvae and the community. Many similar cases could be given, but
they do not here concern us. There are, however, other sexual
differences quite unconnected with the primary reproductive organs, and
it is with these that we are more especially concerned such as the
greater size, strength, and pugnacity of the male, his weapons of
offence or means of defence against rivals, his gaudy colouring and
various ornaments, his power of song, and other such characters.
Besides the primary and secondary sexual differences, such as the
foregoing, the males and females of some animals differ in structures
related to different habits of life, and not at all, or only
indirectly, to the reproductive functions. Thus the females of certain
flies (Culicidae and Tabanidae) are blood-suckers, whilst the males,
living on flowers, have mouths destitute of mandibles. (1. Westwood,
Modern Classification of Insects, vol. ii. 1840, p. 541. For the
statement about Tanais, mentioned below, I am indebted to Fritz
Muller.) The males of certain moths and of some crustaceans (e.g.
Tanais) have imperfect, closed mouths, and cannot feed. The
complemental males of certain Cirripedes live like epiphytic plants
either on the female or the hermaphrodite form, and are destitute of a
mouth and of prehensile limbs. In these cases it is the male which has
been modified, and has lost certain important organs, which the females
possess. In other cases it is the female which has lost such parts; for
instance, the female glow-worm is destitute of wings, as also are many
female moths, some of which never leave their cocoons. Many female
parasitic crustaceans have lost their natatory legs. In some
weevil-beetles (Curculionidae) there is a great difference between the
male and female in the length of the rostrum or snout (2. Kirby and
Spence, Introduction to Entomology, vol. iii. 1826, p. 309.); but the
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