THE CASE OF _PAPILIO POLYTES_
Many instances of mimicry are known to-day, but comparatively few of them
have been studied in any detail. Yet a single carefully analysed case is
worth dozens which are merely superficially recorded. In trying to arrive
at some conception of the way in which the resemblance has come about we
want to know the nature and extent of the likeness in the living as well as
in the dead; the relative abundance of model and mimic; what are likely
enemies and whether they could be supposed to select in the way required,
whether the model is distasteful to them; whether intermediate forms occur
among the mimics; how the various forms behave when bred together, etc.,
etc. Probably the form that from these many points of view has, up to the
present, been studied with most care is that of the Swallow-tail, _Papilio
polytes_. It is a common butterfly throughout the greater part of India and
Ceylon, and closely allied forms, probably to be reckoned in the same
species, reach eastwards through China as far as Hongkong. _P. polytes_ is
one of those species which exhibit polymorphism in the female sex. Three
distinct forms of female are known, of which one is like the male, while
the other two are very different. Indeed {76} for many years they were
regarded as distinct species, and given definite specific names. To Wallace
belongs the credit of shewing that these three forms of female are all to
be regarded as wives of the same type of male[38]. He shewed that there
were no males corresponding to two of the females; also that the same one
male form was always to be found wherever any of the females occurred. As
the result of breeding experiments in more recent years Wallace's
conclusions have been shewn to be perfectly sound.
The male of _polytes_ (Pl. V, fig. 1) is a handsome blackish insect with a
wing expanse of about four inches. With the exception of some
yellowish-white spots along their outer margin the fore wings are entirely
dark. Similar spots occur along the margin of the hind wing also, while
across the middle runs a series of six yellowish-white patches producing
the appearance of a broad light band. The thorax and abdomen are full
black, though the black of the head is relieved by a few lighter yellowish
scales. The under surface is much like the upper, the chief difference
being a series of small and slightly reddish lunules running outside the
light band near the margin of the hind wing (Pl. V, fig. 1 _a_). In some
specimens these markings are almost absent. One form of female is almost
exactly like the male (Pl. V, fig. 2), the one slight difference being that
the lunules on the under surface of the hind wing are generally a trifle
larger. For brevity she may be called the M form. The second form of female
{77} differs in many respects from the male and the M female. Instead of
being quite dark, the fore wings are marked by darker ribbed lines on a
lighter ground[39] (Pl. V, fig. 3). The hind wings shew several marked
differences from those of the male. Of the series of six patches forming
the cross band the outermost has nearly disappeared, and the innermost has
become smaller and reddish. The middle four, on the other hand, have become
deeper, reaching up towards the insertion of the wing, and are pure white.
A series of red lunules occurs on the upper surface outside the white band,
and the yellowish-white marginal markings tend to become red. These
differences are equally well marked on the under surface (Pl. V, fig. 3
_a_). The colour of the body, however, remains as in the male. From the
resemblance shewn by this form to another species of Swallow-tail, _Papilio
aristolochiae_ (Pl. V, fig. 5), we shall speak of it as the A form.
The third form of female is again very distinct from the other two. The
fore wings are dark but are broken by an irregular white band running
across the middle (Pl. V, fig. 4), and there is also an irregular white
patch nearer the tips of the wing. The hind wings, on the other hand, are
characterised by having only red markings. The yellowish-white band of the
male is much reduced and is entirely red, while the red lunules are much
larger than in the A form. The under surface (Pl. V, fig. 4 _a_)
corresponds closely with the {78} upper. The body remains black as in all
the other forms. This type of female bears a resemblance to _Papilio
hector_ (Pl. V, fig. 6), and for that reason we shall speak of it as the H
form. It should be added that these three forms of female are quite
indistinguishable in the larval and chrysalis stages.
It was Wallace who first offered an explanation of this interesting case in
terms of mimicry. According to this interpretation _P. polytes_ is a
palatable form. The larva, which feeds on citronaceous plants, and the
chrysalis are both inconspicuous in their natural surroundings. They may be
regarded as protectively coloured, and consequently edible and liable to
persecution. The original coloration is that of the male and the M female.
From this the other two forms of female have diverged in the direction of
greater instead of less conspicuousness, although the presumed edibility of
the insect might have led us to think that a less conspicuous coloration
would have been more to its advantage. But these two females resemble the
two species _Papilio aristolochiae_ and _Papilio hector_, which, though
placed in the same genus as _P. polytes_, belong to a very different
section of it[40]. The larvae of these two species are conspicuously
coloured black and red with spiny tubercles. They feed upon the poisonous
_Aristolochia_ plants. For these reasons and also from the fact that the
butterflies themselves are both conspicuous and plentiful it is inferred
that they are unpalatable. In short, they are the models upon {79} which
the two _polytes_ females that are unlike the male have been built up by
natural selection.
The suggestion of mimicry in this case is supported by the fact that there
is a general correspondence between the areas of distribution of model and
mimic. _P. hector_ is not found outside India and Ceylon, and the H female
of _P. polytes_ is also confined to this area. _P. aristolochiae_, on the
other hand, has a much wider range, almost as wide indeed as that of _P.
polytes_ itself. Generally speaking the A female accompanies _P.
aristolochiae_ wherever the latter species is found. Beyond the range of
_P. aristolochiae_, in northern China, the M female alone is said to occur.
On the other hand, as the matter comes to be more closely studied
exceptions are beginning to turn up. The H female, for instance, is found
on the lower slopes of the Himalayas, far north of the range of _P.
hector_, and there are indications that a careful study of the distribution
in China and Japan may prove of importance.
Moreover, the investigation of a smaller area may also bring to light
points of difficulty. In Ceylon, for example, _P. polytes_ is common up to
several thousand feet, while _P. hector_ is rare at half the height to
which _polytes_ ascends. Nevertheless the H form of female is relatively
just as abundant up-country where _hector_ is rarely found as it is low
down where _hector_ is plentiful[41]. On the other hand, _P. aristolochiae_
may be exceedingly abundant at altitudes where _hector_ is scarce. Yet the
A form of _polytes_ is no more relatively abundant {80} here than elsewhere
on the island. All over Ceylon, in fact, the relative proportions of the
three forms of female appear to be the same, quite irrespective of the
abundance or scarcity of either of the models. As, however, we shall have
to return to this point later, we may leave it for the moment to consider
other features of this case of _P. polytes_.
In collections of insects from India or Ceylon it is not unusual to find
specimens of the A form of female of _polytes_ placed with _P.
aristolochiae_, and the H form with _P. hector_. When the insects are old
and faded and pinned out on cork the mistake is a very natural one. But
after all the enemies of _polytes_ do not hunt it in corked cabinets, and
any estimation of resemblance to be of use to us must be based upon the
living insects. Are the resemblances of the mimics to the models when alive
so close that they might be expected to deceive such enemies[42] as prey
upon them and have no difficulty in distinguishing the male form of
_polytes_ from _P. aristolochiae_ or _P. hector_?
To answer for a bird is a hazardous undertaking. We know so little of the
bird's perceptive faculties whether of taste or sight. But on general
grounds, from the specialization of their visual apparatus, it is probable
that the sense of sight is keen, though whether the colour sense is the
same as our own is doubtful[43]. On the other hand, the olfactory apparatus
{81} is relatively poorly developed in birds, and from this we can only
argue that the senses of smell and taste are not especially acute. Really
we can do little more than to describe how these mimetic resemblances
appear to our own senses, and to infer that they do not appear very
different to the bird. If there is any difference in keenness of perception
we shall probably not be far wrong in presuming that the advantage rests
with the bird. After all if there is any truth in the theory of mimicry the
bird has to depend largely upon its keenness of sight in making its living,
at any rate if that living is to be a palatable one. If natural selection
can bring about these close resemblances among butterflies it must
certainly be supposed to be capable of bringing the bird's powers of vision
to a high pitch of excellence.
Returning now to the case of _P. polytes_, there is not the least doubt
that to the ordinary man accustomed to use his eyes the A form of female is
easily distinguishable from _P. aristolochiae_, as also is the H form from
_P. hector_. The two models have a feature in common in which they both
differ from their respective mimics. In both of them the body and head are
largely of a brilliant scarlet, whereas neither of the mimics has a touch
of red on the body. In the living insect when the body is swelled by its
natural juices the effect is very striking[44]. It gives at once a
"dangerous" look {82} to the insect when settled, even at a distance of
several yards, and this although one may be perfectly familiar with its
harmless nature. The mimics on the other hand with their sombre-coloured
bodies never look otherwise than the inoffensive creatures that they are.
The "dangerous" look due to the brilliant scarlet of the body and head of
_hector_ and _aristolochiae_ is reinforced by the quality of the red on the
markings of the wings. In both models it is a strong clamorous red
suggestive of a powerful aniline dye, whereas such red as occurs in the
mimics is a softer and totally distinct colour. The difference in quality
is even more marked on the under than on the upper surface (Pl. V, figs. 3
_a_--6 _a_), and the net result is that when settled, with wings either
expanded or closed, there is no possibility of an ordinarily observant man
mistaking mimic for model in either case, even at a distance of several
yards.
It may, however, be argued that it is not when at rest but during flight
that the mimetic resemblance protects the mimic from attack. Actually this
can hardly be true, for the mode of flight constitutes one of the most
striking differences between model and mimic. _P. hector_ and _P.
aristolochiae_ fly much in the same way. They give one the impression of
flying mainly with their fore wings, which vibrate rapidly, so that the
course of the insect, though not swift, is on the whole sustained and even.
The flight of all the different forms of _polytes_ is similar and quite
distinct from that of the models. It is a strong but rather {83} heavy and
lumbering up-and-down flight. One gets the impression that all the wing
surface is being used instead of principally the fore wings as appears in
_P. hector_ and _P. aristolochiae_. The difference is difficult to put into
words, but owing to these peculiarities of flight the eye has no difficulty
in distinguishing between model and mimic even at a distance of 40 to 50
yards. Moreover, colour need not enter into the matter at all. It is even
easier to distinguish model from mimic when flying against a bright
background, as for instance when the insect is between the observer and a
sunlit sky, than it is to do so by reflected light. I have myself spent
many days in doing little else but chasing _polytes_ at Trincomalee where
it was flying in company with _P. hector_, but I was never once lured into
chasing the model in mistake for the mimic. My experience was that whether
at rest or flying the species are perfectly distinct, and I find it
difficult to imagine that a bird whose living depended in part upon its
ability to discriminate between the different forms would be likely to be
misled. Certainly it would not be if its powers of discrimination were
equal to those of an ordinary civilised man. If the bird were unable to
distinguish between say the A form of female and _P. aristolochiae_ I think
that it would be still less likely to distinguish between the same A form
and the male or the M form of female. For my experience was that at a
little distance one could easily confuse the A form of _polytes_ with the
male. Except when one was quite close the red on the A form was apt to be
lost, the {84} white markings on the hind wing were readily confused with
those of the male, and one had to depend entirely on the lighter fore wing.
Unless the bird were keener sighted than the man the A form would be more
likely to be taken in mistake for its unprotected relative than avoided for
its resemblance to the presumably unpalatable model. On the other hand, if
the bird were sufficiently keen sighted never to confuse the A female with
the male form its sight would be too keen to be imposed upon by such
resemblance as exists between the A female and _P. aristolochiae_.
These, however, are not the only criticisms of the theory of mimicry which
the study of this species forces upon us. _Papilio polytes_ is one of the
few mimetic species that has been bred, and in no other case of
polymorphism is the relation between the different forms so clearly
understood. For this result we are indebted mainly to the careful
experiments of Mr J. C. F. Fryer, who recently devoted the best part of two
years to breeding the different forms of this butterfly in Ceylon[45].
Fryer came to the conclusion that an explanation of this curious case is
possible on ordinary Mendelian lines. At first sight the breeding results
appear complicated, for any one of the three forms of female can behave in
several different ways. For the sake of simplicity we may for the moment
class together the A and H females as the mimetic females, the non-mimetic
being represented by the M or male-like females. {85} The different kinds
of families which each of the three females can produce may be tabulated as
follows:--
([alpha]) The M form may give either:--
(1) M only.
(2) M and mimetics in about equal numbers.
(3) Mimetics only.
([beta]) The A form may give either:--
(1) M and mimetics in about equal numbers.
(2) M and mimetics in the ratio of about 1:3.
(3) Mimetics only.
([gamma]) The H form may give either:--
(1) M and mimetics in about equal numbers.
(2) M and mimetics in the ratio of about 1:3.
(3) Mimetics only.
The males are in all cases alike to look at but it must nevertheless be
supposed that they differ in their transmitting powers. In fact the
evidence all points to there being three different kinds of male
corresponding to the three different kinds of female. But they cannot shew
any difference outwardly because there is always present in the male a
factor which inhibits the production of the mimetic pattern even though the
factor for that pattern be present.
Returning now to the records of the females it will be noticed that
although the M form may breed true the mimetics never give the M form
alone. Where they give the M form among their progeny they produce mimetics
and non-mimetics either in the ratio 1:1 or of 3:1. This at once suggests
that the {86} non-mimetic is recessive to the mimetic forms--that the
mimetics contain a factor which does not occur in the non-mimetics. If this
factor, which may be called X, be added to the constitution of a
non-mimetic female it turns it into a mimetic. If X be added to a male such
an individual, though incapable of itself exhibiting the mimetic pattern
owing to the inhibitory factor always present in that sex, becomes capable
of transmitting the mimetic factor to its offspring. Expressed in the usual
Mendelian way the formulae for these different butterflies are as
follows:--
M[F] = iixx | Iixx = [M] (1)
Mimetic } = { iiXX | IiXX = [M] (2)
[F][F] } { or iiXx | IiXx = [M] (3)
where X stands for the mimetic factor and I for the factor which inhibits
the action of X. All males are heterozygous for I, but during the
segregation of characters at some stage in the formation of the families
only the male-producing sperms come to contain the factor I. It is lacking
in all the female-producing sperms formed by the male.
[M] (1) does not contain the factor for the mimetic condition and gives
only daughters of the M form when mated with an M[F]. [M] (2) on the
other hand is homozygous for the factor X, and consequently all of his germ
cells contain it. This is the male that gives nothing but mimetic daughters
with whatever form of female he is bred. [M] (3) is heterozygous for X;
that is to say, one half of his germ cells contain it, the other half not.
With the M[F] he must give equal numbers {87} of offspring with and
without X, _i.e._ half of his daughters will be mimetic and the other half
non-mimetic. With a heterozygous mimetic female (iiXx), which is also
producing germ cells with and without X in equal numbers, he may be
expected to give the usual result, viz. dominants and recessives in the
ratio 3:1; or in other words mimetic and non-mimetic females in the ratio
3:1.
One of Fryer's experiments may be given here in illustration of the nature
of the evidence upon which the above hypothesis depends.
H[F](wild) H[F](wild)
| |
+--------+----------+ +--+------+----------+
| | | | | |
18[M][M] 10M[F][F] 7H[F][F] 26[M][M] 7M[F][F] 26H[F][F]
| |
+---------+ +------+
| |
M[F] × [M]
|
+---------+
| |
7[M][M] 12H[F][F]
| |
+-----+ |
| \ |
| +---------+--------------+
| | \ |
[M] × H[F] ------------- [M] × H[F]
| |
+---------+--+-----+ +---------+----------+
| | | | | |
14[M][M] 6H[F][F] 1M[F] 8[M][M] 10H[F][F] 2M[F][F]
Families were reared from the two wild H females of whom nothing was known
either as to ancestry or husband. The first family contained 10 M and 7 H
females. Hence the original wild mother was probably iiXx and had mated
with a male of the constitution {88} Iixx. The family from the second wild
H female contained 26 H and 7 M females; _i.e._ the ratio in which these
two forms appeared was not far from 3:1. Hence the wild female was probably
iiXx and her husband IiXx. If this were so some of the 26 [M][M] should
receive the X factor from both parents and consequently be IiXX in
constitution. This was almost certainly so in the case of the single male
in this brood tested by mating with an M female from the other brood. All
of his 12 daughters were of the H form, as should have been the case had
his constitution been IiXX. Supposing this to be so, all his offspring, of
both sexes, must be heterozygous for X. Consequently any pair mated
together should give both H and M females in the ratio of three of the
former to one of the latter. In Mr Fryer's experiment two males and two
females chosen at random were mated together. In the one case six H and one
M female were produced, in the other ten H and two M females. As was
expected both classes of female appeared, and the looked-for ratio of three
H to one M was, in view of the smallness of the numbers, not departed from
widely in either instance.
In the experiments selected as an illustration, the mimetic females happen
to be all of the H form. In other experiments, however, both the H form and
the A form occurred. As the result of his experiments Mr Fryer came to the
conclusion that here again the difference is one of a single hereditary
factor. All mimetic females contain the X factor, but the H {89} females
contain in addition a factor which we may call Y. The function of the Y
factor is to carry the change made by the X factor a step further, and to
turn the A form of female into the H form. Y is a modifier of X, but unless
X is present Y can produce no effect. All the different individuals which
are to be found among _P. polytes_ in Ceylon may be represented as
follows:--
[M][M] | M[F][F] | A[F][F] | H[F][F] |
| | | |
IixxYY | iixxYY | -- | -- |
IixxYy | iixxYy | -- | -- |
Iixxyy | iixxyy | -- | -- |
IiXxYY | -- | -- | iiXxYY |
IiXxYy | -- | -- | iiXxYy |
IiXxyy | -- | iiXxyy | -- |
IiXXYY | -- | -- | iiXXYY |
IiXXYy | -- | -- | iiXXYy |
IiXXyy | -- | iiXXyy | -- |
In this way is offered a simple explanation in terms of three Mendelian
factors which serves at once to explain the various results of the breeding
experiments, and the fact that intermediates between the different forms of
female are not found.
The only other experiments comparable with these on _P. polytes_ are some
made by Jacobsen on _Papilio memnon_ in Java[46]. Here again there are
three forms of female, one of which, _laomedon_, is something like the
male, while the other two, _agenor_ and _achates_, are quite distinct. Of
these three _achates_, unlike the male and the other two females, is
tailed, and resembles {90} the species _Papilio coon_ which belongs to the
same presumably distasteful group as _P. aristolochiae_. These experiments
of Jacobsen's are not so complete as the series on _P. polytes_, but
Professor de Meijere and Mr Fryer have both pointed out that they are
capable of being interpreted on the same simple lines.
Another instance of experimental breeding involving polymorphism and
mimicry in the female sex is that of the African _Papilio dardanus_, but
the case is here complicated by the greater number of female forms (cf. pp.
30-33). The data, too, are far more scanty than in the other two cases, but
so far as they go there is nothing to preclude an explanation being
eventually arrived at on similar lines[47].
And now we may consider briefly the bearing of these experiments on the
theory of mimicry. Throughout the work no individuals intermediate between
the three well-marked forms of _polytes_ were met with. There is no
difference in appearance between the heterozygous and the homozygous
mimetic insects, whether they belong to the A or to the H form. The factor
X, whether inherited from both parents, or from one only, produces its full
effect, and the same is also true of the action of the factor Y. Now the
most generally accepted hypothesis as to the formation of these mimetic
resemblances supposes that they have been brought about through the gradual
operation of natural selection accumulating slight variations. {91}
Professor Poulton, for example, a prominent exponent of this school,
considers that the A form of female was first evolved gradually from the M
form, and later on the H form came by degrees from the A form. If this be
true we ought, by mingling the M germ plasm with the H germ plasm and by
subsequently breeding from the insects produced, to get back our series of
hypothetical intermediates, or at any rate some of them. We ought as it
were to reverse the process by which the evolution of the different forms
has taken place. But as is shewn by the experiment of Mr Fryer, which was
quoted above, nothing of the sort happens.
From experiments with cultivated plants such as primulas and sweet peas, we
have learnt that this discontinuous form of inheritance which occurs in _P.
polytes_ is the regular thing. Moreover, we have plenty of historical
evidence that the new character which behaves in this way is one that has
arisen suddenly without the formation of intermediate steps. The dwarf
"Cupid" form of sweet pea, for instance, behaves in heredity towards the
normal form as though the difference between them were a difference of a
single factor. It is quite certain that the "Cupid" arose as a sudden sport
from the normal without the intervention of anything in the way of
intermediates. And there is every reason to suppose that the same is true
for plenty of other characters involving colour and pattern as well as
structure, both in the sweet pea, the primula, and other species. Since the
forms of _polytes_ female behave in breeding like the various {92} forms of
sweet pea and primula there is every reason to suppose that they _arose_ in
the same way, that is to say, as sudden sports or mutations and not by the
gradual accumulation of slight differences.
But if we take this view, which is certainly most consonant with the
evidence before us, we must assign to natural selection a different rôle
from that which is generally ascribed to it. We cannot suppose that natural
selection has played any part in the _formation_ of a mimetic likeness. The
likeness turned up suddenly as a sport quite independently of natural
selection. But although natural selection may have had nothing to do with
its production, it may nevertheless have come into play in connection with
the _conservation_ of the new form. If the new form possesses some
advantage over the pre-existing one from which it sprang, is it not
conceivable that natural selection will come into operation to render it
the predominant form? To this question we shall try to find an answer in
the next chapter.
* * * * *
{93}