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Descent of Man [ 1871]

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

* Mr. Wallace in Harwicke's Science Gossip, September, 1867, p. 193.
  *(2) See also, on this subject, Mr. Weir's paper in Transactions,
Entomological Society, 1869, p. 23.
  *(3) Westminster Review, July, 1867, p. 16.

  Display.- The bright colours of many butterflies and of some moths
are specially arranged for display, so that they may be readily
seen. During the night colours are not visible, and there can be no
doubt that the nocturnal moths, taken as a body, are much less gaily
decorated than butterflies, all of which are diurnal in their
habits. But the moths of certain families, such as the Zygaenidae,
several Sphingidae, Uraniidae, some Arctiidae and Saturniidae, fly
about during the day or early evening, and many of these are extremely
beautiful, being far brighter coloured than the strictly nocturnal
kinds. A few exceptional cases, however, of bright-coloured
nocturnal species have been recorded.*

  * For instance, Lithosia; but Prof. Westwood (Modern Class. of
Insects, vol. ii., p. 390) seems surprised at this case. On the
relative colours of diurnal and nocturnal Lepidoptera, see ibid.,
pp. 333 and 392; also Harris, Treatise on the Insects of New
England, 1842, p. 315.

  There is evidence of another kind in regard to display. Butterflies,
as before remarked, elevate their wings when at rest, but whilst
basking in the sunshine often alternately raise and depress them, thus
exposing both surfaces to full view; and although the lower surface is
often coloured in an obscure manner as a protection, yet in many
species it is as highly decorated as the upper surface, and
sometimes in a very different manner. In some tropical species the
lower surface is even more brilliantly coloured than the upper.* In
the English fritillaries (Argynnis) the lower surface alone is
ornamented with shining silver. Nevertheless, as a general rule, the
upper surface, which is probably more fully exposed, is coloured
more brightly and diversely than the lower. Hence the lower surface
generally affords to entomologists the more useful character for
detecting the affinities of the various species. Fritz Muller
informs me that three species of Castnia are found near his house in
S. Brazil: of two of them the hind-wings are obscure, and are always
covered by the front-wings when these butterflies are at rest; but the
third species has black hind-wings, beautifully spotted with red and
white, and these are fully expanded and displayed whenever the
butterfly rests. Other such cases could be added.

  * Such differences between the upper and lower surfaces of the wings
of several species of Papilio may be seen in the beautiful plates to
Mr. Wallace's "Memoir on the Papilionidae of the Malayan Region," in
Transactions of the Linnean Society, vol. xxv., part i., 1865.

  If we now turn to the enormous group of moths, which, as I hear from
Mr. Stainton, do not habitually expose the under surface of their
wings to full view, we find this side very rarely coloured with a
brightness greater than, or even equal to, that of the upper side.
Some exceptions to the rule, either real or apparent, must be noticed,
as the case of Hypopyra.* Mr. Trimen informs me that in Guenee's great
work, three moths are figured, in which the under surface is much
the more brilliant. For instance, in the Australian Gastrophora the
upper surface of the fore -wing is pale greyish-ochreous, while the
lower surface is magnificently ornamented by an ocellus of
cobalt-blue, placed in the midst of a black mark, surrounded by
orange-yellow, and this by bluish-white. But the habits of these three
moths are unknown; so that no explanation can be given of their
unusual style of colouring. Mr. Trimen also informs me that the
lower surface of the wings in certain other Geometrae*(2) and
quadrifid Noctuae are either more variegated or more brightly-coloured
than the upper surface; but some of these species have the habit of
"holding their wings quite erect over their backs, retaining them in
this position for a considerable time," and thus exposing the under
surface to view. Other species, when settled on the ground or herbage,
now and then suddenly and slightly lift up their wings. Hence the
lower surface of the wings being brighter than the upper surface in
certain moths is not so anomalous as it at first appears. The
Saturniidae include some of the most beautiful of all moths, their
wings being decorated, as in our British emperor moth, with fine
ocelli; and Mr. T. W. Wood*(3) observes that they resemble butterflies
in some of their movements; "for instance, in the gentle waving up and
down of the wings as if for display, which is more characteristic of
diurnal than of nocturnal Lepidoptera."

  * See Mr. Wormald on this moth: Proceedings of the Entomological
Society, March 2, 1868.
  *(2) See also an account of the S. American genus Erateina (one of
the Geometrae) in Transactions, Ent., Soc., new series, vol. v.,
pls. xv. and xvi.
  *(3) Proc Ent. Soc. of London, July 6, 1868, p. xxvii.

  It is a singular fact that no British moths which are brilliantly
coloured, and, as far as I can discover, hardly any foreign species,
differ much in colour according to sex; though this is the case with
many brilliant butterflies. The male, however, of one American moth,
the Saturnia io, is described as having its forewings deep yellow,
curiously marked with purplish-red spots; whilst the wings of the
female are purple-brown, marked with grey lines.* The British moths
which differ sexually in colour are all brown, or of various dull
yellow tints, or nearly white. In several species the males are much
darker than the females,*(2) and these belong to groups which
generally fly about during the afternoon. On the other hand, in many
genera, as Mr. Stainton informs me, the males have the hind-wings
whiter than those of the female- of which fact Agrotis exclamationis
offers a good instance. In the ghost-moth (Hepialus humuli) the
difference is more strongly marked; the males being white, and the
females yellow with darker markings.*(3) It is probable that in
these cases the males are thus rendered more conspicuous, and more
easily seen by the females whilst flying about in the dusk.

  * Harris, Treatise, &c., edited by Flint, 1862, p. 395.
  *(2) For instance, I observe in my son's cabinet that the males
are darker than the females in the Lasiocampa quercus Odonestis
potatoria, Hypogymna dispar, Dasychira pudibunda, and Cycnia
mendica. In this latter species the difference in colour between the
two sexes is strongly marked; and Mr. Wallace informs me that we
here have, as he believes, an instance of protective mimicry
confined to one sex, as will hereafter be more fully explained. The
white female of the Cycnia resembles the very common Spilosoma
menthrasti, both sexes of which are white; and Mr. Stainton observed
that this latter moth was rejected with utter disgust by a whole brood
of young turkeys, which were fond of eating other moths; so that if
the Cycnia was commonly mistaken by British birds for the Spilosoma,
it would escape being devoured, and its white deceptive colour would
thus be highly beneficial.
  *(3) It is remarkable, that in the Shetland Islands the male of this
moth, instead of differing widely from the female, frequently
resembles her closely in colour (see Mr. MacLachlan, Transactions,
Entomological Society, vol. ii., 1866, p. 459). Mr. G. Fraser suggests
(Nature, April, 1871, p. 489) that at the season of the year when
the ghost-moth appears in these northern islands, the whiteness of the
males would not be needed to render them visible to the females in the
twilight night.
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