Page:EB1911 - Volume 18.djvu/522

 and leaf-particle. The legs and lower part of the body are dark coloured, but the dorsal surface of the thorax and abdomen is coloured green and is raised so as to form a crest with jagged edges exactly reproducing the irregular margin of a fragment of leaf cut out by the mandibles of the ant. In Borneo the Homopteron Issus bruchoides mimics a species of Curculionid beetle of the genus Alcides.

In the Hemipterous group of the Rhynchota ant-mimicry is illustrated by the larva of a British species of Reduviidae (Nabis lativentris) in which the forepart of the abdomen is furnished on each side with a patch of white hairs leaving a central narrow dark portion in imitation of the waist of the ant; and also by an East African species (Myrmoplasta mira) which in its general form exhibits a close resemblance to an ant (Polyrrhacis gagates) which occurs in the same neighbourhood. Another instance in this group is supplied by a Bornean species of Reduviidae which mimics a species of the genus Bracon, one of the parasitic Hymenoptera.

Typical dipterous insects (flies) closely resemble in general form aculeate Hymenoptera belonging to the families of bees and wasps. The changes in colour and structure required to complete the resemblance to particular species are comparatively slight and much less complicated than those needed to produce a likeness to other protected insects. Hence we find that the majority of flies that mimic insects of other orders have bees or wasps for their models. Many of the Syrphidae are banded black and yellow and present a general resemblance to wasps, especially when they alight, the resemblance being enhanced by a twitching action of the abdomen imitating the similar action so familiar in species of stinging hymenoptera. These flies are characterized by a peculiar method of flight. They commonly hang poised in the air, then dart with lightning swiftness to another spot and poise themselves again. This habit, the origin of the name “hover-flies,” is probably connected with their mimetic coloration. If they flew like ordinary flies their resemblance to Hymenoptera would be obscured by the rapidity of their flight and they might be caught on the wing by insectivorous birds or other insects; but when poised they display their coloration. When the latter is lost during flight, the rapidity of their movement defies pursuit. The particular likeness to a honey-bee presented by one member of this family, the drone-fly (Eristalis tenax), has been already referred to. But the likeness probably goes deeper than superficial resemblance that appeals to the eye; for spiders which distinguish flies from bees by touch and not by sight, treat drone-flies after touching them, not in the fearless way they evince towards blue-bottles (Calliphora), but in the cautious manner they display towards bees and wasps, warily refraining from coming to close quarters until their prey is securely enswathed in silk. This forcibly suggests that the drone-fly mimics a honey-bee not only in appearance but also in the feel of its hairs or the nature of its buzz. Other flies of the genus Volucella, larger and heavier in build than Eristalis, resemble humble-bees in colour and form, and it was formerly supposed that the purpose of this similarity was to enable the flies to enter with impunity the nests of the humble-bees and to lay their eggs amongst those of the latter insects. But it has been ascertained that the species of Volucella which behave in this manner also visit for a like purpose the nests of wasps, which they do not resemble. Hence it is probable that this case of mimicry is purely of a protective and not of an aggressive nature and serves to save the flies from destruction by insectivorous enemies. The same explanation no doubt applies to the mimicry, both in Borneo and South Africa, of hairy bees of the family Xylocopidae by Asilid flies of the genus Hyperechia, and also to other cases of mimicry of Hymenoptera as well as of inedible beetles of the family Lycidae by Diptera. Numerous other cases of mimicry between Diptera and Hymenoptera might be cited.

The Lepidoptera furnish more instances of mimicry, both Batesian and Müllerian, than any other order of insects. In the majority of cases both model and mimic belong alike to the Lepidoptera, and it is often uncertain whether both are inedible (Müllerian mimicry) or whether inedibility is the attribute only of the model (Batesian mimicry). A large number of cases that were formerly regarded as belonging to the latter category are now suspected of belonging rather to the former. Sometimes Lepidoptera mimic protected members of other orders of insects—such as Coleoptera, Hymenoptera and Hemiptera; but perhaps the most singular illustrations of the phenomenon known in the order are exemplified by the larvae of the hawk-moth Chaerocampa, which imitate the heads of snakes. Professor Poulton long ago suggested, and supported the suggestion by experimental evidence on a lizard, that the larvae of two British species, C. elpenor and C. porcellus, are protected by the resemblance to the heads of snakes presented by the anterior extremities of their bodies which are ornamented with large eye-like spots. When the larvae are disturbed the similarity is produced with startling suddenness by the telescopic contraction of the anterior segments in such a manner as to suggest a triangular, pointed head with two large dorsal eyes. Subsequent observers (A. Weismann, Lady Verney) have shown by experimenting upon birds that this suggestion is correct; and Guy Marshall found that baboons which are afraid of snakes are also afraid of the snake-like larva of the South African Chaerocampa osiris. Finally Shelford states that the anterior end of a Bornean species

(C. myodon) offers a striking and detailed resemblance to the head of a snake (Dendrophis picta).

Instances of ant-mimicry in this order are sometimes confined to the larval stage. The early larval stage of the “Lobster Moth” (Stauropus fagi) for example, presents a general resemblance, due to a combination of shape, colour, attitude and movements, to black ants, the swollen head and the caudal disk with its two tentacles representing respectively the abdomen and antenna-bearing head of the model. A parallel case of mimicry exists at Singapore between the larva of a Noctuid moth and the common red tree-ant (Oecophylla smaragdina). In this case also the posterior end of the larva represents the anterior end of the ant. Another instance of mimicry affecting the larval form is supplied by the moth Endromis versicolor, the caterpillars of which resemble the inedible larvae of saw-flies. The resemblance that certain moths—e.g. Trochilium apiforme, crabroniforme—present to bees and wasps is effected in the main by the loss of the scales from the wings, leaving these organs transparent. It is important to note that the scales are present when the moths first emerge from the pupa-case, but are loosely attached and fall off with the first flight.

Of the multitudes of cases of mimicry between different species of Lepidoptera, a few only can be selected for description. These cases, however, have a peculiar interest and importance for they have been studied in fuller detail than any others and the discovery of a particular instance in South America first suggested to Bates the theoretical explanation of this bionomical phenomenon. On the Amazons and in other parts of South America there are butterflies of the group Ithomiinae which are distasteful and have all the characters of specially protected species, being conspicuously coloured, slow of flight, careless of, exposure and abundant in individuals. The wings are transparent and are black-bordered and black-barred, the anterior wing having two black bars and the posterior one. This type of colouring is also found in genera of quite distinct sub-families of butterflies, namely in Danainae and Pierinae, as well as in some diurnal moths, all of which occur in the same district as the Ithomiinae. The following species may be cited as instances of this type of pattern: Methona confusa, Thyridia psidii, Eutresis imitatrix and Dirgenna dero (Ithomiinae); Itura ilione and I. phenarete (Danainae); Dismorphia orise (Pierinae); Anthomyza buckleyi (moth of the family Pericopidae) and Castnia linus (moth of the family Castniidae). So alike in form, colour and mode of flight are those Lepidoptera that when on the wing it is almost or quite impossible to distinguish one from the other, and the resemblance between members belonging to different sub-families cannot be assigned to affinity. Microscopical examination of the wings, moreover, has shown that the transparency of the wings, common to all, has been acquired by a different modification of the scales in each of the genera exhibiting the Ithomiine type of coloration. That the Danaine and Ithomiine species are distasteful is known. Itura, for example, belonging to the former, has protrusible scent-emitting processes at the end of the abdomen; and Thyridia has scent-producing tufts of hair on the edge of the posterior-wing. Bates offered no satisfactory explanation of the resemblance between these two genera and others of the same protected sub-families; but he did not hesitate to ascribe the resemblance to them presented by the Pierine, Dismorphia (Leptalis) orise, to mimicry, believing Dismorphia to be unprotected and noting that it departed widely in the matter of coloration from typical members of the sub-family to which it belongs. Although mimicry in the Lepidoptera has been carried to a greater extreme in South America than in any other country of the world, remarkable instances of it have taken place in the Ethiopian and Oriental regions. A classical and highly complex case first investigated and explained by R. Trimen is that of Papilio dardanus which is widely distributed in Africa and is represented by several sub-species or geographical races. The most primitive of these is antinorii from Abyssinia, which is non-mimetic and has the two sexes nearly alike. The males, of the other subspecies are much like the males of antinorii; but the females are widely different and mimic various species of inedible butterflies belonging to the protected groups of the Danainae and Acraeinae. One of these sub-species, merope, which ranges from the west coast to Victoria Nyanza, is polymorphic and occurs under three forms, namely (a) hippocoon, which mimics the Danaine Amauris niavius; (b) trophonius, which mimics the Danaine Limnas chrysippus; (c) planemoides, which mimics the Acraeine Planema poggei. Oddly enough one or more of these forms may occur in other sub-species. For example, the sub-species cenea which occurs in south and south-east Africa not only has the form cenea mimicking two Danaines, Amauris echeria and A. albimaculata, but also the hippocoon form which resembles a local race of Amauris niavius, known as dominicanus. The sub-species polytrophus from the Kikuyu Escarpments also has the planemoides and cenea forms and another form trimeni, which is intermediate between the unmodified female of antinorii and hippocoon, and like the latter is mimetic of Amauris niavius dominicanus. Finally the sub-species tibullus from the east coast has the cenea-form, the trimeni-form and probably the planemoides-form. The study of this intricate case is not yet completed and it is at present unknown whether it is an instance of Batesian or Müllerian mimicry. Special attention may be drawn to two phenomena connected with it, both of not uncommon occurrence in