Stinging ants are not only closely copied in form and movements by spiders but by species of Hemiptera and Coleoptera, and the resemblance is often wonderfully close.* (* Amongst the longicorn beetles of Chontales, Mallocera spinicollis, Neoclytus Oesopus, and Diphyrama singularis, Bates, all closely resemble stinging ants when moving about on fallen logs.) All over the world wasps are imitated in form and movements by other insects, and in the tropics these mimetic forms are endless. In many cases the insect imitating is so widely removed, in the normal form of the order to which it belongs, from that of the insect imitated, that it is difficult to imagine how the first steps in the process of imitation took place. Looking however at the immense variety of insect life in the tropics, and remembering that in early tertiary times nearly the whole world was in the same favourable condition as regards temperature (vegetation, according to Heer, extending to the poles), and must have supported a vast number of species and genera that were destroyed during the glacial period, we must suppose that, in that great variety of forms, it sometimes occurred that two species belonging to distinct orders somewhat resembled each other in form or colouration, and that the resemblance was gradually increased, when one species had special means of protection, by the other being benefited the more nearly it approached it in appearance. It is to be remarked that the forms imitated have always some kind of defence against insectivorous birds or mammals; they are provided with stings or unpleasant odours or flavours, or are exceedingly swift in flight; excepting where inanimate nature is imitated for concealment. Thus I had an opportunity of proving in Brazil that some birds, if not all, reject the Heliconii butterflies, which are closely resembled by butterflies of other families and by moths. I observed a pair of birds that were bringing butterflies and dragon-flies to their young, and although the Heliconii swarmed in the neighbourhood and are of weak flight so as to be easily caught, the birds never brought one to their nest. I had a still better means of testing both these and other insects that are mimicked in Nicaragua. The tame white-faced monkey I have already mentioned was extremely fond of insects, and would greedily munch up beetle or butterfly given to him, and I used to bring to him any insects that I found imitated by others to see whether they were distasteful or not. I found he would never eat the Heliconii. He was too polite not to take them when they were offered to him, and would sometimes smell them, but invariably rolled them up in his hand and dropped them quietly again after a few moments. There could be no doubt, however, from the monkey's actions, that they were distasteful to him. A large species of spider (Nephila) also used to drop them out of its web when I put them into it. Another spider that frequented flowers seemed to be