Batesian and Mullerian Mimicry

It is not always necessary, as has been generally assumed, that a mimetic pattern should be beneficial to the species. Certain butterflies which have developed such patterns in the presence of Danaine or Acraeine models are equally abundant, but non mimetic, in other areas where the models are absent. It is clear that, in these species, the variations which promote the resem blance have been selected, but it is equally clear that the average number of survivors in each generation was not thereby increased. It is the proportion of survivors with and without the mimetic pattern that has been changed, not the total number of surviving individuals of the species. For this process, J. S. Huxley has sug gested the term intra-specific selection.

One of the chief objections urged against the interpretation of mimicry as a resemblance perfected by selection is the belief that birds rarely attack butterflies. Some naturalists of great experi ence state that they have never witnessed such an attack. Never theless those who have paid special attention to this aspect of the subject have recorded numbers of instances from their own obser vation. Fig. 1 on Plate II. represents an injury seen by F. Muir to be inflicted by a bird upon a butterfly at Durban. The insect was at rest and the bird, attacking it from the rear, has shorn through both hind-wings symmetrically. Now this is a common type of injury and it is reasonable to suppose that it is generally if not always caused in the manner observed by Muir and others. Again Swynnerton in Rhodesia and Lamborn in Nyasaland have found the impress of a beak upon the wings which they have picked up of ter seeing them torn off by a bird. And such beak-marks are often found on specimens by those who look for them.

The remaining figures on Plate

II. illustrate mimicry in other insects, especially the species resembling the most formidable of all models, the bees and wasps, among which, it must be added, Miillerian combinations are especially prevalent. Striking examples of the carpenter bees (Xylocopidae) and their fly mimics are rep resented in figures 9-12A, fig. 9 being a black Indian species (Xylocopa tenuiscapa) mimicked by the fly (Hyperechia xylo shown in fig. 9A. The following three examples are African, fig. so the white-banded bee X. inconstans and i oA its fly mimic H. bifasciata; fig. i 1, the reddish-brown banded X.

flavorufa, with 'IA H. marshalli, the model and mimic here rep resented having been taken within a few yards of each other at Mt. Mlanje, Nyasaland; fig. 12, the white-marked X. nigrita, with I2A, H. consimilis. It has been proved in recent years that the larvae of these flies burrow into the tunnels made by the larvae of their models and devour the occupants. It might appear at first sight that the object of the mimetic resemblance is to enable the flies to approach the nest and lay their eggs near or within the opening, but no such aid is necessary, for the mimics are more powerful fliers and more alert than their models. They are also more formidable and sometimes have been known to prey upon the bees themselves although generally upon other insects; but with all their other powers they have not the dreaded sting and are therefore protected by resembling the much commoner bees among which they live.

Wasps and bees are also mimicked by many other insects of varied groups. Fig. 2 represents a Brazilian sand-wasp (Pepsis) the model of 2A, a bug (Spiniger), and 2B a long-horned grass hopper (Scapkura). The movements in life of these two mimics— both members of unwasp-like groups—are known to promote the resemblance. The wasp's yellow-tipped antennae are seen to be short and thick as compared with those of its mimics but in both the section near the base is thickened and that beyond of hair-like fineness so as to be invisible except on close inspection. The end of the thick section is yellow in the grasshopper like the tip of the wasp's antennae. Similarly in certain beetles which mimic others with shorter knobbed antennae, a thickening at the corre sponding distance from the base, or in some species the appearance of a thickening produced by a tuft of hairs, brings about a super ficial likeness to the model. The close resemblance to a powerful curved spine on the wing-cases of the model, a distasteful Bornean beetle (an Endomychid), fig. 14, is similarly brought about by a curved pencil of hairs on its Longicorn mimic (Zelota) shown in 14A. Examples such as these afford strong evidence that the like ness has been achieved by the selection of any variation which led in the right direction, and as a result some feature in the mimic which seems to the eye to be similar to that of the model is often in its essential structure entirely different.