NEW-WORLD MIMICS
Of all the continents South America affords the greatest wealth of
butterfly life, and it is in the tropical part of this region that many of
the most beautiful and striking cases of mimicry are to be found. Viewed as
a whole the butterfly population presents several features which serve to
mark it off from that of the other two great tropical areas. In the first
place the proportion of gaily coloured forms is higher. Bright red, yellow
or fulvous brown contrasted with some deep shade approaching black form the
dominant notes. Sombre coloured species are relatively scarcer than in the
Oriental and African regions. In the second place when looking over
collections from this part of the world one cannot help being struck by the
frequency with which similar colour combinations occur over and over again
in different as well as in the same groups. Now it is a simple scheme of
black with an oblique scarlet band upon the fore wings--now an arrangement
with alternating stripes of bright brown and black relieved with patches of
clear yellow--now again a scheme of pure transparency and black. {38} Gay
and pleasing as are the designs turned out the palette is a small one and
invention is circumscribed. Under such conditions it might well be supposed
that instances of close resemblance between different species would be
numerous, and this in effect is what we find.
As in Asia with its Euploeines and Danaines, and in Africa with its
Danaines and Acraeines, so in S. America are the fashions set by two
dominant groups of models. These are the Heliconinae and the Ithomiinae,
both peculiar to this region and both characterised, like the Old-world
Danaids, by slow flight and great tenacity of life. Both live on poisonous
plants--the Heliconines on Passifloras and the Ithomiines on Solanaceae. In
both groups, but more especially in the Ithomiinae, the species are
numerous, and the number of individuals in a species often beyond
computation. From the point of view of mimicry these two groups have so
much in common that they may conveniently be considered together.
It was from among the Ithomiines, as already pointed out, that the models
came for the Pierine mimics of the genus _Dismorphia_ upon which Bates
founded the theory of mimicry. Though the Pierine mimics are the most
striking the Heliconines and Ithomiines are mimicked by members of other
groups. A few Papilios (Pl. X, fig. 8), certain Nymphalines such as
_Protogonius_ (Pl. X, fig. 9), _Eresia_, _Phyciodes_ and _Colaenis_ (Pl.
XI, fig. 4), together with various day-flying moths, more particularly of
the genera {39} _Castnia_ and _Pericopis_, are among the well-known mimics
of this group of models. The models themselves are very variable in
appearance. In one locality the predominant pattern is black with a warm
red-brown diagonal bar occupying rather more than a third of the fore wing
(Pl. XV, fig. 5), in another it consists of parallel bands of black and
fulvous brown with clear yellow patches at the tips of the fore wings (cf.
Pl. X, fig. 7), while in yet another locality it is different again.
Different localities often have their own peculiar pattern and this affects
the various mimics as well as the Ithomiine and Heliconine models.
These groups of different species, some belonging to palatable and some to
unpalatable groups, all exhibiting a close resemblance in colour and
pattern, are far more strikingly developed in S. America than in either
Asia or Africa, and it is not uncommon for eight or ten species to enter
into such an association. A group of this sort which possesses unusual
interest is the so-called "Transparency Group" from certain parts of the
Amazon region. It was originally described by Bates with seven species
belonging to six different genera. To-day it is said that no less than 28
species of this peculiar facies are known, though some are excessively
rare. The majority are Ithomiines, but two species of the Danaine genus
_Ituna_, the Pierine _Dismorphia orise_ (Pl. XII, fig. 2), the Swallow-tail
_Papilio hahneli_, and several species of diurnal moths belonging to
different families (cf. Pl. XII, fig. 4) also enter into the combination.
{40} In connection with it there is a feature of peculiar interest in that
the transparent effect is not always produced in the same way. In the
Ithomiines such as _Thyridia_, where there are normally two kinds of
scales, the wider ones for the most part lose their pigment, become much
reduced in size and take on the shape of a stumpy V (Pl. XIV, fig. 3). Also
they stand out for the most part more or less at right angles to the
wing[19], and the neck by which they are joined to the wing membrane is
very short. The longer and narrow form of scales also tend to lose their
pigment and become reduced to fine hairs. In _Dismorphia_ the scales, which
are of one sort, are also reduced in size though apparently not in number.
Like the wider scales of the _Thyridia_ they tend sometimes to project at
right angles to the wing membrane, though not to the same extent as in the
Ithomiine: possibly because the neck of the scale is not so short. As in
_Thyridia_ these reduced scales lose their pigment except in the transition
region round the borders of the transparent patches. In _Ituna_ there is a
difference. The scales are not reduced to the same extent in point of size.
Their necks are longer as in normal scales and they lie flat on the wing
membrane. The majority of the scales, as in the preceding cases, lose their
pigment, but mixed up with them is a certain proportion, about one-quarter,
{41} in which the pigment is retained. In _Castnia_ and in _Anthomysa_ the
scales on the transparent parts which are without pigment are also somewhat
reduced in size, being stumpier than the normal ones. At the same time they
tend to stand out at right angles to the wing membrane[20]. The neck here
again is shorter in the transparent than in the pigmented scales. A good
deal of stress has been laid upon this case by some supporters of the
theory of mimicry, since it is supposed to shew that a similar effect can
be brought about in a variety of ways; consequently the existence of this
assembly of similar transparent forms belonging to various families cannot
be put down as due to the effect of similar conditions, but must be
regarded as having arisen in each instance in a different manner through
the independent action of natural selection[21]. It is doubtful, however,
whether such a conclusion necessarily follows from the facts. In all of the
cases the process would appear to be similar: loss of pigment, reduction in
the size of the scales, and eventually a tendency for the scales to stand
at right angles to the wing--this last part of the process apparently
depending upon the reduction of the neck of the scale. It has been said
that greater transparency is brought about by the scales standing out at
right angles in this way, but as the scales {42} themselves are already
transparent there would appear to be no reason why this should be so. Of
course the process has not proceeded in all of the forms to the same
extent. There is least change in _Ituna_ where the scales are not much
reduced in size and where a fair proportion are still pigmented. There is
probably most in an Ithomiine such as _Thyridia_, where the scales are not
only small and entirely without pigment, but also are for the most part
neckless so that they stand out at right angles to the wing. Having regard
to the fact that several widely separate genera with different types of
scaling formed the starting points, the final results do not seem to
preclude the supposition that the transparency has arisen through a similar
process in all of them.
It is somewhat remarkable that no Satyrine exhibits mimicry in S. America,
in spite of the fact that transparency of the wings, as in so many of the
butterflies of this region, is quite common in the group. On the other hand
the relatively large number of more or less mimetic Pierines is a striking
feature of S. America. For the most part they belong to the genera
_Dismorphia_ and _Perrhybris_, and resemble the yellow, black, and brown
Heliconines and Ithomiines, though some of the former genus are mimics of
the small transparent Ithomiines. Some of the species of _Pereute_ with
their dark ground colour and the bright red bar across the fore wing (Pl.
XI, fig. 6) resemble _Heliconius melpomene_, as also does _Papilio
euterpinus_. But some of the most interesting Pierine {43} mimics are
several forms belonging to the genus _Archonias_ (Pl. XI, fig. 10) which
exhibit the simple and striking arrangement of black, red and white so
characteristic of the Swallow-tail Poison-eaters of S. America. They form
one of the rare instances of a Pharmacophagus Papilio being mimicked by a
butterfly which does not belong to the Swallow-tail group.
As everywhere in the tropics the Papilios of S. America supply a goodly
proportion of the mimicry cases. A few, such as _P. zagreus_ (Pl. X, fig.
8), enter into the black-brown and yellow Ithomiine-Heliconine combination;
_P. euterpinus_ resembles _Heliconius melpomene_ (Pl. XI, fig. 5); _P.
pausanias_ is like _Heliconius sulphurea_ (Pl. XI, figs. 1 and 2). But this
practically exhausts the list of Papilios which mimic Heliconines and
Ithomiines. The great majority of mimicking Swallow-tails in S. America
find their models among the Poison-eaters of their own family, offering in
this respect a contrast to those of Asia where the majority of models are
among the Danaines and Euploeines, and of Africa where they are exclusively
Acraeines or Danaines.
The Poison-eaters of S. America fall into two well-marked groups which we
may call the red-spotted and the dark green groups respectively. The red
spotted group form a remarkably compact and uniform assemblage. The general
ground colour is a deep black-brown (Pl. XI, figs. 8 and 9), the hind wings
are almost invariably marked with red near the centre or towards the outer
margin, and the fore wing may {44} or may not bear a patch which is
generally whitish in the female, though often of a brilliant blue or green
in the male. This simple colour scheme with variations runs throughout
about three-quarters (some 40 species) of the Poison-eaters. The same
general colour scheme is also found in about two dozen species of the
unprotected Swallow-tails. As the total number of the unprotected species
is placed by Seitz at less than 100 this means that fully one-quarter of
them fall into the general colour scheme adopted by the majority of the
Poison-eaters. In many cases the resemblance between mimic and model is so
close as to have deceived the most expert entomologists before the
structural differences between the groups had been appreciated (cf.
Appendix II). The matter is further complicated by the fact that
polymorphism is not uncommon, especially among the females of the mimetic
forms. _Papilio lysithous_ for instance has no less than six distinct forms
of female, which differ chiefly in the extent and arrangement of the white
markings on the wings, one form lacking them entirely. Several of these
forms may occur together in a given locality, and may resemble as many
distinct species of Poison-eaters. Thus the three forms _lysithous_, with
white on both wings, _rurik_, with white on the fore wing only, and
_pomponius_ without any white, all fly together in Rio Grande do Sul and
respectively mimic the three distinct Pharmacophagus species _nephalion_,
_chamissonia_, and _perrhebus_ (Pl. XIII). It is worthy of note that mimics
are provided by both unprotected {45} groups of Swallow-tails in S.
America, whereas in Asia the Cosmodesmus division never provides mimics for
Pharmacophagus models (cf. Appendix II).
In the second and smaller group of the Pharmacophagus Swallow-tails the
general colour scheme is a more or less dark metallic blue-green with a
tendency towards the obliteration of light markings. Some idea of their
appearance may be got from the figure of the Central and N. American _P.
philenor_ on Pl. XVI, fig. 1. Though one or two unprotected Papilios in S.
America fall more or less into this colour scheme, the group, from the
point of view of mimicry, is not nearly so important as the red-spotted
one.
Nevertheless the blue-green Pharmacophagus group as represented by _P.
philenor_ is supposed to play a considerable part in mimicry in N. America.
_P. philenor_ is found throughout the greater part of the Eastern United
States, straggling up as far as the Canadian border. On the west it is also
found reaching up to North California. Over considerable parts of its range
are three other Swallow-tails, belonging to the unprotected Papilios, which
are regarded by Professor Poulton and others as mimics of _philenor_[22].
One of these, _P. troilus_, is dark brown with a dusting of blue scales
over the hind wing (Pl. XVI, fig. 2). The sexes here are more or less
alike. _Troilus_ stretches up into North-west Canada some way beyond the
limits reached by its model. _P. glaucus_ is a black and yellow
Swallow-tail with two forms of female. {46} One of these resembles the male
while the other is darker and is said to mimic _philenor_. It is known as
the _turnus_ form and is found more commonly in the southern part of the
range of the species, _i.e._ in the country where _philenor_ is more
plentiful. The third species, _P. asterius_, has a more southerly
distribution. Its female is darker and nearer to _philenor_ than the male.
It must, however, be admitted that none of the three species bears a very
close resemblance to _philenor_. It is suggested that this is because _P.
philenor_ is a tropical form which has only recently invaded N. America.
The crossing of _philenor_ has, as it were, induced the three mimicking
_Papilios_ to turn dark, but the model has not been long enough in contact
with them for the likeness to become a close one. The explanation, however,
hardly accounts for the fact that the best mimic of the three, _P.
troilus_, in which both sexes are dark, is found far north of _philenor_.
Either the dark colour was established without the influence of the
Pharmacophagus model, or else the species rapidly extended its range
northwards after having been modified under the influence of _philenor_ in
the south. But in that case the critic may ask why it does not revert to
the original pattern now that it has got beyond the model's sphere of
influence. On the whole it seems at present quite doubtful whether any
relation of a mimetic nature exists between _P. philenor_ and these three
species of _Papilio_.
_P. philenor_ is also regarded as serving as a model {47} for two
Nymphaline butterflies in the United States. One of these is the large
Fritillary _Argynnis diana_ of which the dark female has a markedly blue
tint on the upper surface (Pl. XVI, fig. 3). The other is a _Limenitis_[23]
related to our own White Admiral. This form, _L. astyanax_ (Pl. XVI, fig.
5), is a dark form with a bluish iridescence on the upper surface. It is
found, like _P. philenor_, over the greater part of the Eastern States,
while to the north, near the Canadian boundary, its place is taken by _L.
arthemis_ with prominent white bar across both wings (Pl. XVI, fig. 4).
There is reason for believing that where the two overlap there is
occasional inbreeding, and that the hybrid is the form known as
_proserpina_, resembling _astyanax_ more than _arthemis_. It must be
admitted that in general appearance _L. astyanax_ and _Argynnis diana_ are
more like _Papilio troilus_ than _P. philenor_. In explanation it has been
suggested that all the mimics are on the way to resembling _P. philenor_,
and consequently we should expect them at certain stages to shew more
resemblance to one another than to the form they have all as it were set
out to mimic. On this view they will all arrive at a close resemblance to
_philenor_ in time. Another explanation is that favoured by Professor
Poulton on which it is assumed that we are here dealing with a case of
Müllerian Mimicry, all of the species in question being distasteful with
the exception perhaps of _A. diana_. Thus _troilus_ and _astyanax_ though
distasteful are less so than {48} _philenor_. Hence it is of advantage to
them to have even a chance of being mistaken for the more obnoxious
_philenor_, and so the one has come from the black and yellow Swallow-tail
pattern and the other from the white-banded _arthemis_ form to what they
are, _i.e._ more alike to one another than to _philenor_. They now form a
Müllerian combination for mutual protection along with the dark females of
_glaucus_ and _asterius_. But they are themselves still moderately
distasteful so that it is to the advantage of the female of _Argynnis
diana_ to mimic them. Whether they are all on the way to resembling
_philenor_ more closely, or whether they have sufficiently vindicated their
inedible properties and are now stationary, it is for the future to reveal
to posterity. Lastly we have the view that these different species have
attained their present coloration entirely independently of one another,
and that we are not here concerned with mimicry at all. Since the sole
evidence available at present is that based on general appearance and
geographical distribution, the view taken of this case must rest largely
upon personal inclination.
Though the cases just quoted are only very problematically mimetic, N.
America has yet several examples of resemblance between distantly related
forms as close as any that occur in the tropics. In this region are found
two species of the genus _Danais_--_D. archippus_ occurring all over the
United States and reaching up northwards into Canada, _D. berenice_ found
in the South-eastern States, _e.g._ in Florida, where it is said to be more
abundant than _archippus_. {49} _D. archippus_ (Pl. XVI, fig. 8) is very
similar to the oriental _D. plexippus_ (Pl. IV, fig. 2), from which perhaps
its most notable difference lies in the extent and arrangement of the white
spots near the tip of the fore wing. _D. berenice_ is not unlike
_archippus_ in its general colour scheme but is smaller and darker (Pl.
XVI, fig. 9).
We have already had occasion to mention the common Nymphaline, _Limenitis
arthemis_ (Pl. XVI, fig. 4) which is found in Canada and the Northeastern
States. Widely spread over N. America is a close ally of this species, _L.
archippus_, which, though so similar in structure and habits, is very
different in external appearance. As appears from Pl. XVI, fig. 6, _L.
archippus_ is remarkably like the Danaid which bears the same specific
name. In the Southern States _L. archippus_ is replaced by a form slightly
different in details of pattern and distinctly darker, _L. floridensis_ (=
_eros_) (Pl. XVI, fig. 7). In Florida occurs also the darker N. American
Danaid, _D. berenice_, to which the colour of _L. floridensis_ approximates
more than to _D. archippus_, and it is of interest that although the last
named is also found in this locality it is said to be much less abundant
than _D. berenice_. Nevertheless it appears to be true that the range of
_L. floridensis_ is much more extensive than that of its model; in other
words, that there are considerable regions where _L. floridensis_ and _D.
archippus_ coexist, and from which _L. archippus_ and _D. berenice_ are
wanting.
* * * * *
{50}