(1) That the imitative species occur in the same area and occupy the very same station as the imitated.
This on the whole is generally true. It is well shewn in some of the most striking cases such as those of the Old-World Papilios that mimic Danaines, or of the Dismorphias and their Ithomiine models. In many of these cases the range of neither model nor mimic is a very wide one, yet the mimic is found strictly inside the area inhabited by the model. Papilio agestor, for instance, is only found where Caduga tytia occurs, nor is P. mendax known outside the area frequented by Euploea rhadamanthus. Even more striking in this respect are some of the Ithomiine-Dismorphia resemblances in the New World. The Ithomiine models are as a rule very local though very abundant. Two hundred miles away the predominant Ithomiine often bears quite a distinct pattern, and when this is the case the mimicking Dismorphia is generally changed in the same sense.
But though mimic and model may be found together in the same locality, they do not always occupy the same station in the sense that they fly together. According to Seitz[[25]] the Dismorphias themselves do not fly with the Ithomiines which they mimic. The occurrence of butterflies is largely conditioned by the occurrence of the plants on which the larva feeds, and this is especially true of the female, which, as has already been noticed, is more commonly mimetic than the male. The female of Papilio polytes, for instance, is found flying where are to be found the wild citronaceous plants on which its larva feeds. On the other hand, its so-called models, Papilio hector and P. aristolochiae, are generally in the proximity of the Aristolochias on which their larvae feed. The two plants are not always found together, so that one frequently comes across areas where P. polytes is very abundant while the models are scarce or absent.
Though in the great majority of cases the imitator and the imitated occur in the same locality, this is not always so. The female of the Fritillary Argynnis hyperbius ([Pl. IV], fig. 3), for instance, is exceedingly difficult to distinguish from Danais plexippus when flying, although when at rest the difference between the two is sufficiently obvious. Both insects are plentiful in Ceylon but inhabit different stations. The Danaid is a low-country insect, while the Fritillary is not found until several thousand feet up. The two species affect entirely different stations and hardly
come into contact with each other. Where one is plentiful the other is not found. It has been suggested that migratory birds may have come into play in such cases. The bird learns in the low country that D. plexippus is unpleasant, and when it pays a visit to the hills it takes this experience with it and avoids those females of the Fritillary which recall the unpleasant Danaine.
Migratory birds have also been appealed to in another case where the resembling species are even further removed from one another than in the last case. Hypolimnas misippus is common and widely spread over Africa and Indo-Malaya, and the male ([Pl. IV], fig. 8) bears a simple and conspicuous pattern—a large white spot bordered with purple on each of the very dark fore and hind wings. The same pattern occurs in the males of two other Nymphalines allied to H. misippus, viz. Athyma punctata and Limenitis albomaculata. The two species, however, have a distribution quite distinct from that of H. misippus, being found in China. It has nevertheless been suggested by Professor Poulton[[26]] that the case may yet be one of mimicry. According to his explanation, H. misippus is unpalatable, the well-known association of its female with Danais chrysippus being an instance of Müllerian mimicry. Migratory birds did the rest. Having had experience of H. misippus in the south, on their arrival in China they spared such specimens of Athyma punctata and Limenitis
albomaculata as approached most nearly to H. misippus in pattern, and so brought about the resemblance. The explanation is ingenious, but a simpler view will probably commend itself to most. Other cases are known in which two butterflies bear a close resemblance in pattern and yet are widely separated geographically. Several species of the S. American Vanessid genus Adelpha are in colour scheme like the African Planema poggei which serves as a model for more than one species. The little S. American Phyciodes leucodesma would almost certainly be regarded either as a model for or a mimic of the African Neptis nemetes, did the two occur together. Nevertheless examples of close resemblance between butterflies which live in different parts of the world are relatively rare and serve to emphasise the fact that the great bulk of these resemblance cases are found associated in pairs or in little groups.
(2) That the imitators are always the more defenceless.
In the case of butterflies "defence" as a rule denotes a disagreeable flavour rendering its possessor distasteful to birds and perhaps to other would-be devourers. Feeding experiments with birds (cf. Chapter IX) suggest that certain groups of butterflies, notably the Danaines, Acraeines, Heliconines, Ithomiines and Pharmacophagus Papilios—groups from which models are generally drawn—are characterised by a disagreeable taste, while as a rule this is not true for the mimics. This distasteful quality is frequently accompanied by a more or less conspicuous type of coloration,
though this is by no means always so. Many Euploeas are sombre inconspicuous forms, and it is only some of the Ithomiines that sport the gay colours with which that group is generally associated. The members of the distasteful groups usually present certain other peculiarities. Their flight is slower, they are less wary, their bodies are far tougher, and they are more tenacious of life. The slow flight is regarded as an adaptation for exhibiting the warning coloration to the best advantage, but from the point of view of utility it is plausible to suggest that the insect would be better off if in addition to its warning coloration it possessed also the power of swift flight[[27]]. It is possible that the peculiar slowness of flight of these unpalatable groups is necessitated by the peculiar tough but elastic integument which may present an insufficiently firm and resistant skeletal basis for sharp powerful muscular contraction, and so render swift flight impossible. It is stated that the flight of the mimics is like that of the model, and in some cases this is undoubtedly true. But in a great many cases it certainly does not hold good. Papilio clytia ([Pl. I], figs. 7 and 8) is a strong swift flyer very unlike the Danaine and Euploeine which it is supposed to mimic. The flight of the female of Hypolimnas misippus ([Pl. IV], fig. 7) is quite distinct from that of Danais chrysippus, while the mimetic