CHAPTER IV.
MUTUALISTS.
In this chapter we bring together animals which live on each other, without being either parasites or messmates; many of them are towed along by others; some render each other mutual services, others again take advantage of some assistance which their companions can give them; some afford each other an asylum, and some are found which have sympathetic bonds which always draw them together. They are usually confounded with parasites or messmates.
Many insects shelter themselves in the fur of the mammalia, or in the down of birds, and remove from the hair and the feathers the pellicle and epidermal débris which encumber them. At the same time they minister to the outward appearance of their host, and are of great utility to him in a hygienic point of view.
Those which live in the water have other guardians: instead of insects, we find a number of crustaceans which establish themselves on fishes, and if there are no scales of the epidermis which annoy them, there are mucosities which are incessantly renewed in order to protect the skin from the continual action of the water.
We find many on the surface of the scales, and others which conceal themselves at the bottom of mucous canals. We have brought together only a few examples, and there are certain others which are mentioned elsewhere, but which ought more properly to be placed here.
The insects long known under the name of Ricini, and to which many other appellations have been given, deserve to figure in the first rank in this group. They have always perplexed entomologists, who seem to consider them as parasites allied to acaridæ and lice. It has, however, been long known that they have no trunk to suck with, and that they have two small scaly teeth, which rather serve for the purpose of biting. A long time since, the examination of their stomach proved that they contain only morsels of skin instead of blood. This has induced many entomologists to place them in the same order as grasshoppers, that of Orthoptera.
Lyonet has given figures of several of those which he studied with the care which he so well knew how to employ in his anatomical investigations; and in 1818 Nitzsch, a professor at Göttingen, had brought together so great a number of them, that it required several days to examine his collection; he began the publication of his catalogue, but has not had time to finish it. Several other entomologists and anatomists have since taken up the subject.
We owe the description of several hundred species to Mr. Denny. Mons. F. Rudow has lately made known a great number of species which he has collected from
the skins of birds coming from Japan, Australia, Africa, and the two Americas.