SOCIAL AND COMMUNAL LIFE, COMMENSALISM AND PARASITISM

Social life and gregariousness.—Technical Note.—Students should refer to examples of gregariousness from their own observations of animals. The roosting together of crows and of blackbirds; the gathering of swallows preparatory to migration; the flocking of geese and ducks, with leaders, in their migratory flights, all can be readily observed. From observation or general reading students will be more or less familiar with prairie-dog villages, beaver-dams and marshes, the one-time great herds of bison, etc.

The struggle for existence is always operative; but in some cases one or more phases of it may be ameliorated. For example, the amelioration of the struggle among individuals of one species obtains in a lesser or greater degree in the case of those animals which exhibit a social life, of which mutual aid and mutual dependence are the basis. The honey-bee and the ants are familiar examples of animals which show a high degree of social life. They live, indeed, a truly communal life, where the fate of the individual is bound up in the fate of the community. But there are many animals which show a much lower degree of mutual aid and a far less coherent society. The simplest form of social life exists among those animals in which many individuals of one species keep together, forming a great band or herd. In this case there is not nearly so much mutual aid or mutual dependence as in that of the honey-bee, and the safety of the individual is not wholly bound up in the fate of the herd. Such animals are said to be gregarious in habit, and this gregariousness is undoubtedly advantageous to the individuals of the band. The great herds of reindeer in the North, and of the bison or buffalo which once ranged over the Western American plains are examples of a gregariousness in which mutual protection from enemies, like wolves, seems to be the principal advantage gained. The bands of wolves which hunted the buffalo show the advantage of mutual help in aggression as well as in protection. Prairie-dogs live in great villages or communities which spread over many acres. By shrill cries they tell each other of the approach of enemies, and they seem to visit each other and to enjoy each other's society a great deal, although that they are thus afforded much actual active help is not apparent. The beavers furnish a well-known and very interesting example of mutual help; they exhibit a communal life although a simple one. They live in villages or communities, all helping to build the dam across the stream which is necessary to form the marsh or pool in which the nests or houses are built.

Communal life.—Technical Note.—See technical notes, pp. [212] et seq, for directions for work in connection with the study of the communal life of ants, bees, and wasps.

When many individuals of a species live together in a community in which the different kinds of work are divided more or less distinctly among the different members and where each individual works primarily for the whole and not for himself; where there is, in other words, a thorough mutual help and mutual dependence among the members of the community accompanied by a division of labor, the life of the species is truly communal. Those animals which show the most elaborate and specialized communal life are the termites or white ants, the social bees and wasps, and the true ants. Of these the ants and honey-bees stand first. As already explained (see pp. [220] et seq), there are among these communal insects several different kinds of individuals in each species. With most animals there are two kinds only, males and females, which may or may not show differences in color, form, etc., so that they are readily distinguishable. Among all the communal insects, however, there are always three kinds of individuals, males, females, and workers, these last being infertile individuals. With the social wasps and social bees the workers are all infertile females and are smaller than the fertile forms; with the termites there are besides the fertile males and females, which are winged, workers which are wingless, and also peculiar wingless individuals called soldiers which have very large jaws and whose business it is to fight off attacking enemies of the community. Among the ants the workers are also wingless, while the males and females are winged. The worker ants in many species are of two kinds, so-called worker majors and worker minors, differing markedly in size. All the ant workers are good soldiers, but with some the fighting is done almost wholly by certain especially large-headed and large-jawed ones which may be called soldier-workers.

Thus among all strictly communal animals there is a specialization or differentiation of individuals accompanying the division of labor. Special individuals have a certain part of the work of the community to do, and they are specially modified in structure to do this work. This structural modification may make them incapable of performing certain other labor or work which is necessary for their living and which must be done for them, therefore, by others. Thus the mutual interdependence of the individuals composing a colony is very real. The worker honey-bees cannot perpetuate the species; honey-bees would die out were it not for the males and females. But the males and females have given up the functions of food-getting and of caring for their young; did not the workers do these things for them, the community would die out quite as soon.

The advantages of communal or social life, of co-operation and mutual aid are real. Those animals that have adopted such a life are among the most successful of all in the struggle for existence. The termite worker is one of the most defenseless and for those animals that prey on insects one of the most toothsome insects, and yet the termite is one of the most abundant and successfully living insect kinds in all the tropics. Ants are everywhere and are everywhere successful. The honey-bee is a popular type of successful life. The artificial protection afforded it by man may aid it in its struggle for existence, but it gains this protection because of certain features of its communal life, and in nature the honey-bee takes care of itself well. Co-operation and mutual aid are among the most important factors which help in the struggle for existence.

Commensalism.—Technical Note.—Examine ants' nests to find myrmecophilous insects. If on the seashore search for hermit-crabs with sea-anemones on shell. If inland, try to have some preserved specimens showing the crabs and sea-anemones.

The phases of living together and mutual help just discussed concerned in each instance a single species of animal. All the members of a pack of wolves or of a honey-bee community belong to a single species. But there are numerous instances known of the mutually advantageous association of individuals of two different species. Such an association is called commensalism or symbiosis.

The hermit-crabs live, as has been learned (p. 154), in the shells of molluscs, most of the body of the crab being concealed within the shell, only the head and the grasping and walking legs protruding. In some species of hermit-crabs there is always to be found on the shell near the opening a sea-anemone. "This sea-anemone is carried from place to place by the crab, and in this way is much aided in obtaining food. On the other hand, the crab is protected from its enemies by the well-armed and dangerous tentacles of its companion. On the tentacles there are many thousand long slender stinging threads, and the fish that would eat the hermit-crab must first deal with the stinging anemone." If the sea-anemone be torn away from the shell the crab will wander about seeking another anemone. When he finds one, he struggles to loosen it from the rock to which it is attached, and does not rest until he has torn it loose and placed it on his shell.

In the case of the hermit-crab and the sea-anemone there is no doubt of the mutual advantage derived from their communal life. But this mutual advantage is not so obvious in some cases of commensalism, where indeed most or all of the advantage often seems to lie with one of the animals, while the other derives little or none, but on the other hand suffers no injury. For example, "small fish of the genus Nomeus may often be found accompanying the beautiful Portuguese man-of-war (Physalia) as it sails slowly about on the ocean's surface. These little fish lurk underneath the float among the various hanging thread-like parts of the man-of-war which are provided with stinging cells. They are protected from their enemies by their proximity to these stinging threads, but of what advantage to the man-of-war their presence is is not understood." Similarly in the nests of the various species of ants and termites many different kinds of other insects have been found. "Some of these are harmful to their hosts, in that they feed on the food-stores gathered by the industrious and provident ant, but others appear to feed only on refuse or useless substances in the nest. Some may be of help to their hosts by acting as scavengers. Over one thousand species of these myrmecophilous (ant-loving) and termitophilous (termite-loving) insects have been recorded by collectors as living habitually in the nests of ants and termites."

Parasitism.—Technical Note.—As examples of temporary external parasites find and examine fleas and ticks on dogs and cats, red mites on house-flies and grasshoppers (at the bases of the wings), etc. As examples of permanent external parasites find bird-lice on pigeons or domestic fowls or on other birds. Note the absence of wings and the peculiarly modified body shape of these parasites. Examine a bird-louse under the microscope; note the absence of compound eyes (it has simple eyes) and absence of wings; note bits of feathers, its food, in stomach, showing through the body. Find, as examples of internal parasites, intestinal worms or flukes. Examine trichinized pork to see Trichinæ in muscles. Examine preserved specimens of tapeworms. Collect pupæ of some common butterfly or moth and keep them in the schoolroom until either the butterflies or ichneumon flies issue. Some will surely be parasitized, and yield ichneumon flies (parasites) instead of a butterfly. As examples of degeneration by quiescence examine barnacles (found on outer rocks of seashore at low tide; easily obtained as preserved specimens by inland schools) and the females of scale-insects. These insects may be found on oleanders (the black scale, Lecanium oleæ) or fruit-trees (the San Jose scale, Aspidiotus perniciosus). Note the great degeneration of the adult female of the San Jose scale; it has no eyes, antennæ, wings, or legs. The young may be found crawling about at certain times of the year; they have eyes, antennæ and legs.

In addition to the various ways of living together among animals, already described, namely, the social and communal life of individuals of a single species and the commensal and symbiotic life of individuals of different species, there is another and very common kind of association among animals. This is the association of parasite and host; the association between two sorts of animals whereby one, the parasite, lives on or in the other, the host, and at the expense of the host. In this association the parasite gains advantages great or small, sometimes even obtaining all the necessities of life, while the host gains nothing, but suffers corresponding disadvantage, often even the loss of life itself. Parasitism is a phenomenon common in all the large groups of animals, though the parasites themselves are mostly invertebrates. There are parasitic Protozoa, worms, crustaceans, insects, and molluscs, and a few vertebrates.

Some parasites, like the fleas and lice, live on the surface of the body of the host. These are called external parasites. Others, as the tapeworms, live exclusively inside the body; such are called internal parasites. Again, some, as the bird-lice, which are external parasites feeding on the feathers of birds, spend their whole lifetime on the host; they are called permanent parasites. Others, as a flea, which leaps on or off its host as caprice directs, or like certain parasites which as young live free and active lives, finally attaching themselves to some host and remaining fixed there for the rest of their lives, are called temporary parasites. Such a grouping is purely arbitrary and exists simply for the sake of convenience. It is not rigid, nor does it class parasites in their proper natural groups.

When various parasites are examined it will be noted that practically in all cases the body of a parasite is simpler in structure than the body of other animals closely related to it; that is, species which live parasitically, obtaining their food from and being carried about by a host, have simpler bodies than related forms that live free active lives, competing for food with other animals about them. This simplicity is not primitive, but results from the loss or atrophy of the structures which the special mode of life of the parasite renders useless. Many parasites are attached firmly by hooks or suckers to their host, and do not move about independently of it. They have no need of the power of locomotion, and accordingly are usually without wings, legs, or other locomotory organs. Because they have no need of locomotion they have no need of organs of orientation, those special sense organs like the eyes, ears, and feelers which serve to guide and direct the moving animal; and most fixed parasites will be found to have no eyes, or any of those organs accessory to locomotion, and which serve for the detection of food or of enemies. Because these important organs, which depend for their successful activity on a well-organized nervous system, are lacking, the nervous system of parasites is usually very simple. Again, because the parasite usually feeds on the already digested food or the blood of its host, most parasites have a very simple alimentary canal, or even none at all. Finally, as the fixed parasite leads a wholly sedentary and inactive life, the breaking down and rebuilding of tissue in its body goes on very slowly and in minimum degree, so that there is little need of highly developed respiratory and circulatory systems; and most fixed and internal parasites have these systems of organs decidedly simplified. Altogether the body of a fixed permanent parasite is so simplified and so wanting in all those special structures which characterize the active, complex animals that it often presents a very different appearance from those forms with which we know it to be nearly related. This simplicity due to loss or reduction of parts is called degeneration. Such simplicity of body-structure due to degeneration is, however, essentially different in its origin from the simplicity of the lower simpler animals. In them the simplicity of body is primitive; they are generalized animals; the simplicity of degeneration is acquired; it is really an adaptation, or specialization.

An excellent example of body degeneration due to the adoption of a parasitic habit is that of Sacculina (fig. [159]), a crustacean parasitic on other crustaceans, namely, crabs. The young Sacculina is an active, free-swimming larva essentially like a young prawn or crab. After a short period of independent existence it attaches itself to the abdomen of a crab, and lives as a parasite. It completes its development under the influence of this parasitic life, and when adult bears absolutely no resemblance to such a typical crustacean as a crab or crayfish. Its body external to the host crab is simply a pulsating tumor-like sac, with no mouth-parts, no legs, and internally hardly any well-developed organs except those of reproduction. Degeneration here is carried very far.

Fig. 159.—Sacculina, a parasitic crustacean; A, attached to a crab, the root-like processes of the parasite penetrating the body of the host; B, the active larval condition; C, the adult removed from its host. (After Haeckel.)

Various parasites have been referred to in Part II under their proper branch and class. The worms include an unusually large number of them, such as the tapeworms, trichinæ and other intestinal forms, all of which live as internal parasites in the alimentary canal or in other organs of higher animals, especially the vertebrates. Many crustaceans are parasitic, usually living, like the fish-lice, as fixed external parasites on fishes, other crustaceans, etc., but with a free and active larval stage. Among the insects, on the contrary, many of the parasitic forms (as the ichneumon flies) are free and active in the adult stage, but live as internal grubs or maggots in the larval stage. The ichneumon flies (of the order Hymenoptera) are four-winged, slender-bodied insects which lay their eggs either on or in (by means of a sharp piercing ovipositor) some caterpillar or beetle grub, into the body of which the young grub-like ichneumon larvæ burrow on hatching. The parasites feed on the body-tissues of the host, not attacking, however, such organs as the heart or nervous system, which would produce the immediate death of the host. The caterpillar lives with the ichneumon grubs within it usually until nearly time for its pupation. Often, indeed, it pupates with the parasite still in its body. But it never comes to maturity. The larval ichneumons pupate either within the body of its host, or in a tiny silken cocoons outside of its body (fig. [160]). From the cocoons the winged adult ichneumons issue; and after mating the females find another caterpillar on whose body to lay their eggs.

Fig. 160.—Larva of a sphinx-moth,
with cocoons of a parasitic
ichneumon fly. (From
specimen.)

Degeneration can be produced by other causes than parasitism. It is evident that if for any other reason an animal should adopt an inactive fixed life it would degenerate. The barnacles (see fig. [37]) are excellent examples of degeneration through quiescence. They are crustaceans related most nearly to the crabs and shrimps. The young barnacle just from the egg is a six-legged, free-swimming larva (nauplius) with a single eye, greatly like a young prawn or crab. It develops during its independent life two compound eyes and two large antennæ. But soon it attaches itself to some stone or shell, or pile or ship's bottom, giving up its power of locomotion, and its further development is a degeneration. It loses its compound eyes and antennæ, and acquires a protecting shell. Its swimming feet become modified into grasping organs, and it loses most of its outward resemblance to the typical members of its class. The Tunicata or ascidians compose a whole group of animals which are fixed in their adult condition and have thus become degenerate. They have been likened to a "mere rooted bag with a double neck." In their young stage they are free-swimming, active, tadpole-like or fish-like larvæ, possessing organs much like those of the adult simplest fish or fish-like animals. Their larval structure reveals, however, the relationships of the ascidians to the vertebrates, a relationship which is not at all apparent in the degenerate adults. Certain insects live sedentary or fixed lives. All the members of one large family, the Coccidæ, or scale-insects (figs. [62] and [63]), have females which as adults are wingless and in some cases have no legs, eyes, or antennæ, while the males are all winged and have legs and the special sense organs. The males lead a free active life, but the females have nearly or quite given up the power of locomotion, attaching themselves by means of their sucking beak to some plant, where they obtain a sufficient food-supply (plant-sap) and lay their eggs. In both males and females the larvæ are little active crawling six-legged creatures with legs, eyes, and antennæ.

We are accustomed perhaps to think of degeneration as necessarily implying a disadvantage in life. It is true that a blind, footless, degenerate animal could not cope with the active, keen-sighted, highly organized non-degenerate in free competition. But free competition is exactly what the degenerate animal has nothing to do with. Certainly the Sacculina and the scale-insects live well; they are admirably adapted to the kind of life they lead. A parasite enjoys certain obvious advantages in life, and even extreme degeneration is no drawback (except as we shall see later), but gives it a body which demands less food and care. As long as the host is successful in eluding its enemies and avoiding accident and injury the parasite is safe. Its life is easy as long as the host lives. But the disadvantages of parasitism and degeneration are nevertheless obvious. The fate of the parasite is bound up with the fate of the host. "When the enemy of the host crab prevails, the Sacculina goes down without a chance to struggle in its own defence. But far more important than the disadvantage in such particular or individual cases is the fact that the parasite cannot adapt itself in any considerable degree to new conditions. It has become so modified, so specialized to adapt itself to the very special conditions under which it now lives, it has gone so far in giving up organs and functions, that if present conditions change and new ones come to exist the parasite cannot adapt itself to them. The independent free-living animal holds itself, one may say, able and ready to adapt itself to any new conditions of life. The parasite has risked everything for the sake of a sure and easy life under the present existing conditions. Change of conditions means its extinction."

Fig. 161.—Young fur seals, Callorhinus ursinus, of the Tolstoi rookery, St. Paul Island, Bering Sea, killed by a parasitic intestinal worm, Uncinaria sp. (Photograph by the Fur Seal Commission.)

For an elementary account of commensalism and parasitism see Jordan and Kellogg's "Animal Life," pp. 172-200. The account here given is based on the author's previously written account in "Animal Life." See also Van Beneden's "Animal Parasites and Messmates."

[CHAPTER XXXI]