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Descent of Man [ 1871]

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Bullseye
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« Reply #120 on: February 09, 2009, 03:07:43 pm »

* See also Mr. Bates's paper in Proc. Ent. Soc. of Philadelphia,
1865, p. 206. Also Mr. Wallace on the same subject, in regard to
Diadema, in Transactions, Entomological Society, London, 1869, p. 278.

  In the genus Papilio, all the species of the Aeneas group are
remarkable for their conspicuous and strongly contrasted colours,
and they illustrate the frequent tendency to gradation in the amount
of difference between the sexes. In a few species, for instance in
P. ascanius, the males and females are alike; in others the males
are either a little brighter, or very much more superb than the
females. The genus Junonia, allied to our Vanessae, offers a nearly
parallel case, for although the sexes of most of the species
resemble each other, and are destitute of rich colours, yet in certain
species, as in J. oenone, the male is rather more bright-coloured than
the female, and in a few (for instance J. andremiaja) the male is so
different from the female that he might be mistaken for an entirely
distinct species.
  Another striking case was pointed out to me in the British Museum by
Mr. A. Butler, namely, one of the tropical American Theclae, in
which both sexes are nearly alike and wonderfully splendid; in another
species the male is coloured in a similarly gorgeous manner, whilst
the whole upper surface of the female is of a dull uniform brown.
Our common little English blue butterflies of the genus Lycaena,
illustrate the various differences in colour between the sexes, almost
as well, though not in so striking a manner, as the above exotic
genera. In Lycaena agestis both sexes have wings of a brown colour,
bordered with small ocellated orange spots, and are thus alike. In
L. oegon the wings of the males are of a fine blue, bordered with
black, whilst those of the female are brown, with a similar border,
closely resembling the wings of L. agestis. Lastly, in L. arion both
sexes are of a blue colour and are very like, though in the female the
edges of the wings are rather duskier, with the black spots plainer;
and in a bright blue Indian species both sexes are still more alike.
  I have given the foregoing details in order to show, in the first
place, that when the sexes of butterflies differ, the male as a
general rule is the more beautiful, and departs more from the usual
type of colouring of the group to which the species belongs. Hence
in most groups the females of the several species resemble each
other much more closely than do the males. In some cases, however,
to which I shall hereafter allude, the females are coloured more
splendidly than the males. In the second place, these details have
been given to bring clearly before the mind that within the same
genus, the two sexes frequently present every gradation from no
difference in colour, to so great a difference that it was long before
the two were placed by entomologists in the same genus. In the third
place, we have seen that when the sexes nearly resemble each other,
this appears due either to the male having transferred his colours
to the female, or to the male having retained, or perhaps recovered,
the primordial colours of the group. It also deserves notice that in
those groups in which the sexes differ, the females usually somewhat
resemble the males, so that when the males are beautiful to an
extraordinary degree, the females almost invariably exhibit some
degree of beauty. From the many cases of gradation in the amount of
difference between the sexes, and from the prevalence of the same
general type of coloration throughout the whole of the same group,
we may conclude that the causes have generally been the same which
have determined the brilliant colouring of the males alone of some
species, and of both sexes of other species.
  As so many gorgeous butterflies inhabit the tropics, it has often
been supposed that they owe their colours to the great heat and
moisture of these zones; but Mr. Bates* has shown by the comparison of
various closely-allied groups of insects from the temperate and
tropical regions, that this view cannot be maintained; and the
evidence becomes conclusive when brilliantly-coloured males and
plain-coloured females of the same species inhabit the same
district, feed on the same food, and follow exactly the same habits of
life. Even when the sexes resemble each other, we can hardly believe
that their brilliant and beautifully arranged colours are the
purposeless result of the nature of the tissues and of the action of
the surrounding conditions.

  * The Naturalist on the Amazons, vol. i., 1863, p. 19.

  With animals of all kinds, whenever colour has been modified for
some special purpose, this has been, as far as we can judge, either
for direct or indirect protection, or as an attraction between the
sexes. With many species of butterflies the upper surfaces of the
wings are obscure; and this in all probability leads to their escaping
observation and danger. But butterflies would be particularly liable
to be attacked by their enemies when at rest; and most kinds whilst
resting raise their wings vertically over their backs, so that the
lower surface alone is exposed to view. Hence it is this side which is
often coloured so as to imitate the objects on which these insects
commonly rest. Dr. Rossler, I believe, first noticed the similarity of
the closed wings of certain Vanessae and other butterflies to the bark
of trees. Many analogous and striking facts could be given. The most
interesting one is that recorded by Mr. Wallace* of a common Indian
and Sumatran butterfly (Kallima) which disappears like magic when it
settles on a bush; for it hides its head and antennae between its
closed wings, which, in form, colour and veining, cannot be
distinguished from a withered leaf with its footstalk. In some other
cases the lower surfaces of the wings are brilliantly coloured, and
yet are protective; thus in Thecla rubi the wings when closed are of
an emerald green, and resemble the young leaves of the bramble, on
which in spring this butterfly may often be seen seated. It is also
remarkable that in very many species in which the sexes differ greatly
in colour on their upper surface, the lower surface is closely similar
or identical in both sexes, and serves as a protection.*(2)

  * See the interesting article in the Westminster Review, July, 1867,
p. 10. A woodcut of the Kallima is given by Mr. Wallace in Hardwicke's
Science Gossip, September 1867, p. 196.
  *(2) Mr. G. Fraser, in Nature, April, 1871, p. 489.
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