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Descent of Man [ 1871]

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Bullseye
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« Reply #120 on: February 09, 2009, 03:08:55 pm »

Mimicry.- This principle was first made clear in an admirable
paper by Mr. Bates,* who thus threw a flood of light on many obscure
problems. It had previously been observed that certain butterflies
in S. America belonging to quite distinct families, resembled the
Heliconidae so closely in every stripe and shade of colour, that
they could not be distinguished save by an experienced entomologist.
As the Heliconidae are coloured in their usual manner, whilst the
others depart from the usual colouring of the groups to which they
belong, it is clear that the latter are the imitators, and the
Heliconidae the imitated. Mr. Bates further observed that the
imitating species are comparatively rare, whilst the imitated
abound, and that the two sets live mingled together. From the fact
of the Heliconidae being conspicuous and beautiful insects, yet so
numerous in individuals and species, he concluded that they must be
protected from the attacks of enemies by some secretion or odour;
and this conclusion has now been amply confirmed,*(2) especially by
Mr. Belt. Hence Mr. Bates inferred that the butterflies which
imitate the protected species have acquired their present marvellously
deceptive appearance through variation and natural selection, in order
to be mistaken for the protected kinds, and thus to escape being
devoured. No explanation is here attempted of the brilliant colours of
the imitated, but only of the imitating butterflies. We must account
for the colours of the former in the same general manner, as in the
cases previously discussed in this chapter. Since the publication of
Mr. Bates's paper, similar and equally striking facts have been
observed by Mr. Wallace in the Malayan region, by Mr. Trimen in
South Africa, and by Mr. Riley in the United States.*(3)

  * Transact. Linn. Soc., vol. xxiii., 1862, p. 495.
  *(2) Proc. Entomological Soc., Dec. 3, 1866, p. xlv.
  *(3) Wallace, Transact. Linn. Soc., vol. xxv., 1865 p. i.; also,
Transact. Ent. Soc., vol. iv., 3rd series: 1867, p. 301. Trimen, Linn.
Transact., vol. xxvi., 1869, p. 497. Riley, Third Annual Report on the
Noxious Insects of Missouri, 1871, pp. 163-168. This latter essay is
valuable, as Mr. Riley here discusses all the objections which have
been raised against Mr. Bates's theory.

  As some writers have felt much difficulty in understanding how the
first steps in the process of mimicry could have been effected through
natural selection, it may be well to remark that the process
probably commenced long ago between forms not widely dissimilar in
colour. In this case even a slight variation would be beneficial, if
it rendered the one species more like the other; and afterwards the
imitated species might be modified to an extreme degree through sexual
selection or other means, and if the changes were gradual, the
imitators might easily be led along the same track, until they
differed to an equally extreme degree from their original condition;
and they would thus ultimately assume an appearance or colouring
wholly unlike that of the other members of the group to which they
belonged. It should also be remembered that many species of
Lepidoptera are liable to considerable and abrupt variations in
colour. A few instances have been given in this chapter; and many more
may be found in the papers of Mr. Bates and Mr. Wallace.
  With several species the sexes are alike, and imitate the two
sexes of another species. But Mr. Trimen gives, in the paper already
referred to, three cases in which the sexes of the imitated form
differ from each other in colour, and the sexes of the imitating
form differ in a like manner. Several cases have also been recorded
where the females alone imitate brilliantly-coloured and protected
species, the males retaining "the normal aspect of their immediate
congeners." It is here obvious that the successive variations by which
the female has been modified have been transmitted to her alone. It
is, however, probable that some of the many successive variations
would have been transmitted to, and developed in, the males had not
such males been eliminated by being thus rendered less attractive to
the females; so that only those variations were preserved which were
from the first strictly limited in their transmission to the female
sex. We have a partial illustration of these remarks in a statement by
Mr. Belt;* that the males of some of the Leptalides, which imitate
protected species, still retain in a concealed manner some of their
original characters. Thus in the males "the upper half of the lower
wing is of a pure white, whilst all the rest of the wings is barred
and spotted with black, red and yellow, like the species they mimic.
The females have not this white patch, and the males usually conceal
it by covering it with the upper wing, so that I cannot imagine its
being of any other use to them than as an attraction in courtship,
when they exhibit it to the females, and thus gratify their
deep-seated preference for the normal colour of the Order to which the
Leptalides belong."

  * The Naturalist in Nicaragua, 1874, p. 385.

  Bright Colours of Caterpillars.- Whilst reflecting on the beauty
of many butterflies, it occurred to me that some caterpillars were
splendidly coloured; and as sexual selection could not possibly have
here acted, it appeared rash to attribute the beauty of the mature
insect to this agency, unless the bright colours of their larvae could
be somehow explained. In the first place, it may be observed that
the colours of caterpillars do not stand in any close correlation with
those of the mature insect. Secondly, their bright colours do not
serve in any ordinary manner as a protection. Mr. Bates informs me, as
an instance of this, that the most conspicuous caterpillar which he
ever beheld (that of a sphinx) lived on the large green leaves of a
tree on the open llanos of South America; it was about four inches
in length, transversely banded with black and yellow, and with its
head, legs, and tail of a bright red. Hence it caught the eye of any
one who passed by, even at the distance of many yards, and no doubt
that of every passing bird.
  I then applied to Mr. Wallace, who has an innate genius for
solving difficulties. After some consideration he replied: "Most
caterpillars require protection, as may be inferred from some kinds
being furnished with spines or irritating hairs, and from many being
coloured green like the leaves on which they feed, or being
curiously like the twigs of the trees on which they live." Another
instance of protection, furnished me by Mr. J. Mansel Weale, may be
added, namely, that there is a caterpillar of a moth which lives on
the mimosas in South Africa, and fabricates for itself a case quite
indistinguishable from the surrounding thorns. From such
considerations Mr. Wallace thought it probable that conspicuously
coloured caterpillars were protected by having a nauseous taste; but
as their skin is extremely tender, and as their intestines readily
protrude from a wound, a slight peck from the beak of a bird would
be as fatal to them as if they had been devoured. Hence, as Mr.
Wallace remarks, "distastefulness alone would be insufficient to
protect a caterpillar unless some outward sign indicated to its
would-be destroyer that its prey was a disgusting morsel." Under these
circumstances it would be highly advantageous to a caterpillar to be
instantaneously and certainly recognised as unpalatable by all birds
and other animals. Thus the most gaudy colours would be serviceable,
and might have been gained by variation and the survival of the most
easily-recognised individuals.
  This hypothesis appears at first sight very bold, but when it was
brought before the Entomological Society* it was supported by
various statements; and Mr. J. Jenner Weir, who keeps a large number
of birds in an aviary, informs me that he has made many trials, and
finds no exception to the rule, that all caterpillars of nocturnal and
retiring habits with smooth skins, all of a green colour, and all
which imitate twigs, are greedily devoured by his birds. The hairy and
spinose kinds are invariably rejected, as were four
conspicuously-coloured species. When the birds rejected a caterpillar,
they plainly shewed, by shaking their heads, and cleansing their
beaks, that they were disgusted by the taste.*(2) Three conspicuous
kinds of caterpillars and moths were also given to some lizards and
frogs, by Mr. A. Butler, and were rejected, though other kinds were
eagerly eaten. Thus the probability of Mr. Wallace's view is
confirmed, namely, that certain caterpillars have been made
conspicuous for their own good, so as to be easily recognised by their
enemies, on nearly the same principle that poisons are sold in
coloured bottles by druggists for the good of man. We cannot, however,
at present thus explain the elegant diversity in the colours of many
caterpillars; but any species which had at some former period acquired
a dull, mottled, or striped appearance, either in imitation of
surrounding objects, or from the direct action of climate, &c., almost
certainly would not become uniform in colour when its tints were
rendered intense and bright; for in order to make a caterpillar merely
conspicuous, there would be no selection in any definite direction.

  * Proceedings, Entomological Society, Dec. 3, 1866, p. xlv., and
March 4, 1867, p. lxxx.
  *(2) See Mr. J. Jenner Weir's Paper on "Insects and Insectivorus
Birds," in Transact. Ent. Soc., 1869, p. 21; also Mr. Butler's
paper, ibid., p. 27. Mr. Riley has given analogous facts in the
Third Annual Report on the Noxious Insects of Missouri, 1871, p.
148. Some opposed cases are, however, given by Dr. Wallace and M. H.
d'Orville; see Zoological Record, 1869, p. 349.

  Summary and Concluding Remarks on Insects.- Looking back to the
several Orders, we see that the sexes often differ in various
characters, the meaning of which is not in the least understood. The
sexes, also, often differ in their organs of sense and means of
locomotion, so that the males may quickly discover and reach the
females. They differ still oftener in the males possessing diversified
contrivances for retaining the females when found. We are, however,
here concerned only in a secondary degree with sexual differences of
these kinds.
  In almost all the Orders, the males of some species, even of weak
and delicate kinds, are known to be highly pugnacious; and some few
are furnished with special weapons for fighting with their rivals. But
the law of battle does not prevail nearly so widely with insects as
with the higher animals. Hence it probably arises, that it is in
only a few cases that the males have been rendered larger and stronger
than the females. On the contrary, they are usually smaller, so that
they may be developed within a shorter time, to be ready in large
numbers for the emergence of the females.
  In two families of the Homoptera and in three of the Orthoptera, the
males alone possess sound-producing organs in an efficient state.
These are used incessantly during the breeding-season, not only for
calling the females, but apparently for charming or exciting them in
rivalry with other males. No one who admits the agency of selection of
any kind, will, after reading the above discussion, dispute that these
musical instruments have been acquired through sexual selection. In
four other Orders the members of one sex, or more commonly of both
sexes, are provided with organs for producing various sounds, which
apparently serve merely as call-notes. When both sexes are thus
provided, the individuals which were able to make the loudest or
most continuous noise would gain partners before those which were less
noisy, so that their organs have probably been gained through sexual
selection. It is instructive to reflect on the wonderful diversity
of the means for producing sound, possessed by the males alone, or
by both sexes, in no less than six Orders. We thus learn how effectual
sexual selection has been in leading to modifications which sometimes,
as with the Homoptera, relate to important parts of the organisation.
  From the reasons assigned in the last chapter, it is probable that
the great horns possessed by the males of many lamellicorn, and some
other beetles, have been acquired as ornaments. From the small size of
insects, we are apt to undervalue their appearance. If we could
imagine a male Chalcosoma (see fig. 16), with its polished bronzed
coat of mail, and its vast complex horns, magnified to the size of a
horse, or even of a dog, it would be one of the most imposing
animals in the world.
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