OCR Text |
Show 1891.] EXAMPLE OF PROTECTIVE MIMICRY. 463 There is little doubt that we have to do with a palatable insect much relished by insect-eating foes, which defended itself, like the great majority of its allies, by Protective Resemblance (Procryptic Colouring), in this case by a likeness to leaves. The green colour and compressed body were probably evolved in response to the need for concealment. But as the increasing acuteness of foes, also subject to natural selection, enabled them to see through a disguise which is so universally adopted by palatable insects, it became of advantage to certain hard-pressed forms to resemble something which was positively objectionable to their enemies rather than merely useless and uninteresting. And in this case the transition from Protective Resemblance to Protective Mimicry (Pseudaposematic Colouring) would he especially easy, for it would be brought about bv comparatively insignificant modifications of colour and form. Such an easy transition into so marvellous a method of defence is of course due to the accident that specially defended insects generally associated with pieces of leaf are well known in the same locality. M y friend Col. Swinhoe has objected to this interpretation that, it is extremely difficult to believe that the habit of the ant has been continued sufficiently long for the change to have been produced in the Homopterous insect. But the habit seems to be very ancient, inasmuch as it is common to a group of species inhabiting tropical America, and presumably dates from the time of their origin in a common ancestor. Furthermore it has been shown that, detailed as the resemblance certainly is, it was probably prepared for by an earlier resemblance to leaves alone. It must also be remembered that several species of the genus Stegaspis are known in tropical America, and that the published figures and descriptions of these show that the same shape and outline are characteristic of the whole. It is therefore extremely probable that Mr. Sclater's interpretation applies to a group of closely allied Homopterous insects inhabiting a district where the various species of leaf-carrying ants are well known and abundant. In the preparation of the Plate accompanying this paper I have to thank Miss Kelly for kindly lending me some sketches of living Cooshie Ants bearing leaves ; these sketches, drawn upon the spot, were of especial value. Miss Horman Fisher also made for me a careful drawing of a leaf-carrying Ant in the Natural History Museum. Mr. H. M . J. Underhill has prepared the coloured figures from this material. DESCRIPTION OF PLATE XXXVI. Fig. 1. X about 6"5. Immature form of an unknown species of Stegaspis (Membracidee) as seen from the right side. Drawn with the camera lucida. 2. A representation of the Stegaspis (on the right) beside the leaf-carrying Ant (on the left). The fragment of leaf carried by the latter is generally gnawed along its upper edge. The figure of the ant was chiefly adapted from Miss Horman Fisher's drawing of a specimen in the Natural History Museum. |