Chapter IX
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THE ENEMIES OF BUTTERFLIES
The theory of mimicry demands that butterflies should have enemies, and further that those enemies should exercise a certain discrimination in their attacks. They must be sufficiently observant to notice the difference between the mimetic and the non-mimetic form; they must be sufficiently unobservant to confuse the mimetic form with the unpalatable model. And, of course, they must have enough sense of taste to dislike the unpalatable and to appreciate the palatable varieties. What these enemies are and whether they can be supposed to play the part required of them we may now go on to consider.
Butterflies are destroyed in the imago state principally by three groups of enemies—predaceous insects, lizards, and birds. It is known that monkeys also devour butterflies to some extent, but such damage as they inflict is almost certainly small in comparison with that brought about by the three groups already mentioned. In view of the very different nature of these groups it will be convenient to consider them separately.
What is true for mantids is probably also true for the other groups of predaceous insects. Dragon-flies and wasps have been recorded as attacking the distasteful as well as butterflies of unprotected groups. Among the most serious enemies of butterflies must probably be reckoned the blood-sucking Asilids. These powerful and ferocious flies seize butterflies on the wing with their strong claws and plunge their proboscis into the thorax. Apparently they inject some swift poison, for the butterfly is instantly paralysed, nor is there any sign of struggle. The Asilid flies off with its victim, sucking the juices as it goes. There can be no doubt in the mind of any one who has watched these creatures hawking butterflies that their natural gifts are such as to enable them to exercise discrimination in their food. Most insect life is at their mercy but they appear to exercise no choice, seizing and devouring the first flying thing that comes within easy reach. Certainly as regards butterflies palatability or the reverse makes no difference, and they are known to feed indiscriminately both upon the evil-flavoured and upon the good. Taking it all together the evidence is such that we cannot suppose predaceous insects to pay any attention to warning colours, and, therefore, we cannot regard them as playing any part in connection with mimetic resemblance.
It seems clear from these various observations and experiments that certain lizards devour butterflies freely, but that they do not exercise any discrimination in the species which they attack. All are caught and devoured indiscriminately, so that in spite of the fact that such lizards are among the most serious enemies of butterflies we cannot suppose them to play any part in establishing a mimetic resemblance.
Observations on birds attacking butterflies where mimetic forms occur have been made almost entirely in certain parts of Africa, in India, and in Ceylon. For Africa, Marshall has collected some forty-six observations of which almost half are concerned with Pierines. The remainder include four instances of attacks on species of Acraea, a genus which on the mimicry theory must be regarded as among the most unpalatable of butterflies.
The records from the Indo-Malayan region (principally India and Ceylon) are somewhat more numerous and here again more than one-third of them refer to Pierines. Among the others are records of the distasteful forms Euploea core, E. rafflesii, Acraea violae, and Papilio hector being taken and devoured.
"Coimbatore, 6. 6. 10.... I have frequently seen the common green bee-eater (Merops viridis) and the king-crow (Buchanga atra) take butterflies on the wing, the butterflies being Catopsilia pyranthe, C. florella, Terias hecabe and Papilio demoleus. The bee-eater I have also seen taking Danais chrysippus and Danais septentrionis, and I remember to have been struck with their taste for those latter...."
For North America Marshall records over 80 cases of birds attacking butterflies. Among them is an interesting record of a bird seizing and rejecting a specimen of Anosia plexippus (= Danais archippus), one of the few Danaines found in this region.
It must be admitted that the data at present available with regard to the attacks of birds upon butterflies under natural conditions are too meagre to allow of our coming to definite conclusions on the points at issue. It is safe to say that a number of species of birds have been known to attack butterflies—that a few out of the number feed upon butterflies systematically—that some of the most persistent bird enemies devour the presumably protected forms as freely as the unprotected—but that in a few instances there is some reason for supposing that the bird discriminates. Beyond this it is unsafe to go at present.
A few experiments on feeding birds with South African butterflies are recorded by Marshall. A young Kestrel (Cerchneis naumanni) was fed from time to time with various species of butterflies. In most cases the butterflies offered were eaten even when they were species of Acraea. On the other hand Danais chrysippus was generally rejected after being partly devoured. When first offered this unpalatable species was taken readily and it was only after it had been tasted that the bird rejected it. When offered on several subsequent occasions it was partly eaten each time, and the behaviour of the Kestrel did not suggest that it associated a disagreeable flavour even with this conspicuous pattern. Another young Kestrel (Cerchneis rupicoloides) was also used for experiment. At first it would not take butterflies and at no time did it shew any fondness for them. Indeed it is doubtful from the way in which they seem to have shaped at the insects whether either of these Kestrels had had any experience of butterflies before the experiments began.
A Ground Hornbill with which Marshall also experimented ate various species, including Acraea, but, after crushing it, refused the only Danais chrysippus offered. It is hardly likely that this large omnivorous bird operates as a selecting agent in cases of mimicry.
On the whole these experiments of Hess convey a strong suggestion that the colour perceptions of birds may be quite different from our own, more especially where blue is concerned. Great caution is needed in discussing instances of mimicry in their relation to the bird, for we have no right to assume that the bird sees things as we do. On the other hand, it is a matter of much interest to find that in general blue plays relatively little part in cases of mimetic resemblance among butterflies; some combination of a dark tint with either red, white, brown, or yellow being far more common.
It will probably be admitted by most people that the evidence, taken all together, is hardly sufficient for ascribing to birds that part in the establishing of a mimetic likeness which is required on the theory of mimicry. That birds destroy butterflies in considerable numbers is certainly true, but it is no less true that some of the most destructive birds appear to exercise no choice in the species of butterfly attacked. They simply take what comes first and is easiest to catch. It is probably for this reason that the Wood-swallow feeds chiefly on Euploeines and Danaines (cf. p. 112). It is probably for this reason also that such a large proportion of the records of attacks on butterflies under natural conditions refer to the Pierines; for owing to their light colour it is probable that the "Whites" are more conspicuous and offer a better mark for a bird in pursuit than darker coloured species.
The experiments with the baboons were more extensive. Two species of Acraea, halali and axina, were recognised when first offered and refused untasted. Danais chrysippus, on the other hand, was tasted on being offered for the first time, and then rejected. This species was twice offered subsequently and tasted each time before being rejected. When offered the fourth time it was rejected at sight. The baboon evidently learned to associate an unpleasant taste with the chrysippus pattern. At this stage it would have been interesting to have offered it some well-known mimic of chrysippus, such as the female of Hypolimnas misippus or the trophonius form of Papilio dardanus, but this experiment was unfortunately not made. Marshall did, however, offer it at the same time a specimen each of Byblia ilithyia (a Vanessid) and of Acraea axina to which it bears a general resemblance. The baboon took the former but neglected the latter altogether. The general resemblance between the two species was not sufficiently close to deceive it.