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HORSE-FLIES AND WASPS.
passed over without even a cloud to relieve the deep blue of the sky or temper the rays of the sun.
The vegetation on the plains was almost entirely composed of thorny plants and shrubs; acacias, cacti, and bromeliæ were the most abundant. Animal life was scarce; there were a few flycatchers amongst the birds, and armadilloes were the only mammals, Horseflies (Tabanus) were too numerous, and drops of blood trickled down our mules' faces where they had feasted. In some parts large, banded black and yellow wasps (Monedula surinamensis, Fabr.) came flying round us and had a threatening look as they hovered before our faces, but they were old acquaintances of mine in Brazil, and I knew that they were only searching about for the horse-flies with which they store their nests, just as other wasps do with spiders, first benumbing them with their sting. I noted here another instance of the instinctive dread that insects have of their natural enemies. The horse-flies were so bloodthirsty that we could kill them with the greatest ease with our hands on the mules' necks, or if we drove them away they would return immediately. As soon, however, as a wasp came hawking round, the flies lost their sluggish apathy and disappeared amongst the bushes, and I do not think that excepting when gorged with blood they would easily fall a prey to their pursuers.
We were joined on the road by a storekeeper on his way to Teustepe. He was armed with pistols, which it is the fashion to carry in Nicaragua, though many travellers bave nothing more formidable in their holsters than a spirit flask and some biscuits. He talked as usual of threatened revolutionary risings, but these form
the staple conversation throughout Central America amongst the middle classes, and until they really do break out it is best not to believe in them. He told us also that the drought had been very great around Teustepe, and that the crops were destroyed by it.
About three we reached the town, and after buying some provisions to take with us, pushed on again. Below Teustepe we crossed the river Malacatoyo which empties into the Lake of Nicaragua, and beyond it the road passed over a wild alluvial flat with high trees, amongst which we saw a troop of white-faced monkeys.
On the leaves of the bushes there were many curious species of Buprestidæ, and I struck these and other beetles off with my net as I rode along. After one such capture I observed what appeared to be one of the black stinging ants on the net. It was a small spider that closely resembled an ant, and so perfect was the imitation that it was not until I killed it that I determined that it was a spider and that I had needlessly feared its sting. What added greatly to the resemblance was that, unlike other spiders, it held up its two forelegs like antennæ, and moved them about just like an ant. Other species of spiders closely resemble stinging ants; in all of them the body is drawn out long like an ant, and in some the maxillary palpi are lengthened and thickened so as to resemble the head of one.
Ant-like spiders have been noticed throughout tropical America and also in Africa.* The use that the deceptive resemblance is to them has been explained to be the facility it affords them for approaching ants on
* See “Nature,” vol. iii. p. 508.
which they prey. I am convinced that this explanation is incorrect so far as the Central American species are concerned. Ants, and especially the stinging species, are, so far as my experience goes, not preyed upon by any other insects. No disguise need be adopted to approach them, as they are so bold that they are more likely to attack a spider than a spider them. Neither have they wings to escape by flying, and generally go in large bodies easily found and approached. The real use is, I doubt not, the protection the disguise affords against small insectivorous birds. I have found the crops of some humming-birds full of small soft-bodied spiders, and many other birds feed on them. Stinging ants, like bees and wasps, are closely resembled by a host of other insects; indeed, whenever I found any insect provided with special means of defence I looked for imitative forms, and was never disappointed in finding them.
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.* 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.
* Amongst the longicorn beetles of Chontales, Mallocera spinicollis, Neoclytus (Esopus, and Diphyrama singularis,,Bates, all closely resemble stinging ants when moving about on fallen logs.
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 fond of them, and I have already mentioned a wasp that caught them to store its nest with.
Amongst the beetles there is a family that is just as much mimicked as the Heliconii are amongst the butterflies. These are the Lampyridæ, to which the fireflies belong. Many of the genera are not phosphorescent, but all appear to be distasteful to insectivorous mammals and birds. I found they were invariably rejected by the monkey, and my fowls would not touch them.
The genus Calopteron belonging to this family is not phosphorescent. In some of the species, as in C. basalis, (Klug), the wing-covers are widened out behind in a peculiar manner. This and other species of Calopteron are not only imitated in their colour and markings by other families of beetles but also in this peculiar widening of the elytra. Besides this, the Calopteron when walking on a leaf raises and depresses its wing cases, and I observed exactly the same movement in a longicorn beetle (Evander nobilis, Bates), which is evidently a mimetic form of this genus. In addition to being mimicked by other families of beetles, Calopteron is closely resembled by a species of moth (Pionia lycoides, Walker). This