To society the most alien relations of two living beings which can be produced are those of the predator and his prey. In general, the predator is bulkier than his prey, since he overcomes him and devours him. Yet smaller ones sometimes attack larger creatures, consuming them, however, by instalments, and letting them live that they themselves may live on them as long as possible. In such a case they are forced to remain for a longer or a shorter time attached to the body of their victim, carried about by it wherever the vicissitudes of its life lead them. Such animals have received the name of parasites. Parasitism forms the line inside of which our subject begins; for if one can imagine that the parasite, instead of feeding on the animal from whom he draws his subsistence, is content to live on the remains of the other's meals, one will find himself in the presence, not yet of an actual society, but of half the conditions of a society; that is to say, a relation between two beings such that, all antagonism ceasing, one of the two is useful to the other. Such is commensalism. However, this association does not yet offer the essential element of all society, co-operation. There is co-operation when the commensal is not less useful to his host than the latter is to the commensal himself, when the two are concerned in living in a reciprocal relation and in developing their double activity in corresponding ways toward a single and an identical goal. One has given to this mode of activity the name of mutualism. Domestication is only one form of it. Parasitism, commensalism, mutualism, exist with animals among the different species. 2. Symbiosis (literally "living together")[82] In gaining their wide and intimate acquaintance with the vegetable world the ants have also become acquainted with a large number of insects that obtain their nutriment directly from plants, either by sucking up their juices or by feeding on their foliage. To the former group belong the phytophthorous Homoptera, the plant lice, scale insects, or mealy bugs, tree-hoppers, lantern flies, and jumping plant lice; to the latter belong the caterpillars of the lycaenid butterflies, the "blues," or "azures," as they are popularly called. All of these creatures excrete liquids which are eagerly sought by the ants and constitute the whole, or, at any rate, an important part of the food of certain species. In return the Homoptera and caterpillars receive certain services from the ants, so that the relations thus established between these widely different insects may be regarded as a kind of symbiosis. These relations are most apparent in the case of the aphids, and these insects have been more often and more closely studied in Europe and America.