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Descent of Man [ 1871]

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Author Topic: Descent of Man [ 1871]  (Read 2881 times)
Bullseye
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« Reply #120 on: February 09, 2009, 03:07:56 pm »

Although the obscure tints both of the upper and under sides of many
butterflies no doubt serve to conceal them, yet we cannot extend
this view to the brilliant and conspicuous colours on the upper
surface of such species as our admiral and peacock Vanessae, our white
cabbage-butterflies (Pieris), or the great swallowtail Papilio which
haunts the open fens- for these butterflies are thus rendered
visible to every living creature. In these species both sexes are
alike; but in the common brimstone butterfly (Gonepteryx rhamni),
the male is of an intense yellow, whilst the female is much paler; and
in the orange-tip (Anthocharis cardamines) the males alone have
their wings tipped with bright orange. Both the males and females in
these cases are conspicuous, and it is not credible that their
difference in colour should stand in any relation to ordinary
protection. Prof. Weismann remarks,* that the female of one of the
Lycaenae expands her brow wings when she settles on the ground, and is
then almost invisible; the male, on the other hand, as if aware of the
danger incurred from the bright blue of the upper surface of his
wings, rests with them closed; and this shows that the blue colour
cannot be in any way protective. Nevertheless, it is probable that
conspicuous colours are indirectly beneficial to many species, as a
warning that they are unpalatable. For in certain other cases,
beauty has been gained through the imitation of other beautiful
species, which inhabit the same district and enjoy an immunity from
attack by being in some way offensive to their enemies; but then we
have to account for the beauty of the imitated species.

  * Einfluss der Isolirung auf die Artbildung, 1872, p. 58.

  As Mr. Walsh has remarked to me, the females of our orange-tip
butterfly, above referred to, and of an American species (Anth.
genutia) probably show us the primordial colours of the parent-species
of the genus; for both sexes of four or five widely-distributed
species are coloured in nearly the same manner. As in several previous
cases, we may here infer that it is the males of Anth. cardamines
and genutia which have departed from the usual type of the genus. In
the Anth. sara from California, the orange-tips to the wings have been
partially developed in the female; but they are paler than in the
male, and slightly different in some other respects. In an allied
Indian form, the Iphias glaucippe, the orange-tips are fully developed
in both sexes. In this Iphias, as pointed out to me by Mr. A.
Butler, the under surface of the wings marvellously resembles a
pale-coloured leaf; and in our English orange-tip, the under surface
resembles the flower-head of the wild parsley, on which the
butterfly often rests at night.* The same reason which compels us to
believe that the lower surfaces have here been coloured for the sake
of protection, leads us to deny that the wings have been tipped with
bright orange for the same purpose, especially when this character
is confined to the males.

  * See the interesting observations by T. W. Wood, the Student,
Sept., 1868, p. 81.

  Most moths rest motionless during the whole or greater part of the
day with their wings depressed; and the whole upper surface shaded and
coloured in an admirable manner, as Mr. Wallace has remarked, for
escaping detection. The front-wings of the Bombycidae,* when at
rest, generally overlap and conceal the hind-wings; so that the latter
might be brightly coloured without much risk; and they are in fact
often thus coloured. During flight, moths would often be able to
escape from their enemies; nevertheless, as the hind-wings are then
fully exposed to view, their bright must generally have been
acquired at some little risk. But the following fact shews how
cautious we ought to be in drawing conclusions on this head. The
common yellow under-wings (Triphoena) often fly about during the day
or early evening, and are then conspicuous from the colour of their
hind-wings. It would naturally be thought that this would be a
source of danger; but Mr. J. Jenner Weir believes that it actually
serves them as a means of escape, for birds strike at these brightly
coloured and fragile surfaces, instead of at the body. For instance,
Mr. Weir turned into his aviary a vigorous specimen of Triphoena
pronuba, which was instantly pursued by a robin; but the bird's
attention being caught by the coloured wings, the moth was not
captured until after about fifty attempts, and small portions of the
wings were repeatedly broken off. He tried the same experiment, in the
open air, with a swallow and T. fimbria; but the large size of this
moth probably interfered with its capture.*(2) We are thus reminded of
a statement made by Mr. Wallace,*(3) namely, that in the Brazilian
forests and Malayan islands, many common and highly-decorated
butterflies are weak flyers, though furnished with a broad expanse
of wing; and they "are often captured with pierced and broken wings,
as if they had been seized by birds, from which they had escaped: if
the wings had been much smaller in proportion to the body, it seems
probable that the insect would more frequently have been struck or
pierced in a vital part, and thus the increased expanse of the wings
may have been indirectly beneficial."
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