Chap. VIII. 
SEXUAL SELECTION. 
255 
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 hermaphrodite form, and are destitute of a mouth 
and prehensile limbs. In these cases it is the male 
which has been modified and has lost certain import- 
ant organs, which the other members of the same 
group possess. In other cases it is the female which 
has lost such parts ; for instance, the female glowworm 
is destitute of wings, as are many female moths, some 
of which never leave their cocoons. Many female para- 
sitic crustaceans have lost their natatory legs. In 
some weevil-beetles (Cureulionidoe) there is a great 
difference between the male and female in the length 
of the rostrum or snout ; 2 but the meaning of this and 
of many analogous differences, is not at all understood. 
Differences of structure between the two sexes in rela- 
tion to different habits of life are generally confined to 
the lower animals ; but with some few birds the beak 
of the male differs from that of the female. No doubt 
in most, but apparently not in all these cases, the dif- 
ferences are indirectly connected with the propagation 
of the species : thus a female which has to nourish a 
multitude of ova will require more food than the male, 
and consequently will require special means for procur- 
ing it. A male animal which lived for a very short 
time might without detriment lose through disuse its 
organs for procuring food ; but he would retain his 
locomotive organs in a perfect state, so that he might 
reach the female. The female, on the other hand, 
might safely lose her organs for flying, swimming, 
regard to the statement about Tanais, mentioned below, I am indebted 
to Fritz Muller. 
2 Kirby and Spence, 1 Introduction to Entomology/ vol. iii. 1826, 
p. 309. 
