forth in the water so as to maintain currents to bring fresh water in contact with them. Young mosquitoes (Fig. 87) do not have gills, but come up to the surface to breathe. The larvæ, or wrigglers, breathe through a special

tube at the posterior tip of the body, while the pupa have a pair of horn-like tubes on the back of the head end of the body.

81. Degree of structural change in adaptations.—While among the higher or vertebrate animals, especially the fishes and reptiles, most remarkable cases of adaptations occur, yet the structural changes are for the most part external, never seriously affecting the development of the internal organs other

than the skeleton. The organization of these higher animals is much

less plastic than among FIG. 86.-Wood-boring beetle larva (Prionus).

the invertebrates. In

general, the higher the type the more persistent and unchangeable are those structures not immediately exposed

to the influence of the struggle for existence. It is thus the outside of an animal that tells where its ancestors have lived. The inside, suffering little change, whatever the surroundings, tells the real nature of the animal.

82. Vestigial organs.-In general, all the peculiarities of animal structure find their explanation in some need of adaptation. When this need ceases, the structure itself tends to disappear or else to serve some other need. In the bodies of most animals there are certain incomplete or rudimentary organs

or structures which serve no distinct useful purpose. They are structures which, in the ancestors of the animals now possessing them, were fully developed functional organs, but which, because of a change in habits or conditions of living, are of no further need, and are gradually dying out. Such organs are called vestigial organs. Examples are the disused ear muscles of man, the vermiform appendix in man, which is the reduced and now useless anterior end of the large intestine. In the lower animals, the thumb or degenerate first finger of the bird with its two or three little quills serves as an example. So also the reduced and elevated hind toe of certain birds, the splint bones or rudimentary side toes of the horse, the rudimentary eyes of blind fishes, the minute barbel or beard of the horned dace or chub, and the rudimentary teeth of the right whales and sword-fish.

FIG. 87.-Young stages of the mosquito. a, larva (wriggler); b, pupa.

Each of these vestigial organs tells a story of some past adaptation to conditions, one that is no longer needed in the life of the species. They have the same place in the study of animals that silent letters have in the study of words. For example, in our word knight the k and gh are no longer sounded; but our ancestors used them both, as the Germans do to-day in their cognate word Knecht. So with the French word temps, which means time, in which both p and s are silent. The Romans, from whom the French took this word, needed all its letters, for they spelled and pronounced it tempus. In general, every silent letter in every word was once sounded. In like manner, every vestigial structure was once in use and helpful or necessary to the life of the animal which possessed it.

publisher of Recreation.

CHAPTER IX

ANIMAL COMMUNITIES AND SOCIAL LIFE

83. Man not the only social animal.-Man is commonly called the social animal, but he is not the only one to which this term may be applied. There are many others which possess a social or communal life. A moment's thought brings to mind the familiar facts of the communal life of the honey-bee and of the ants. And there are many other kinds of animals, not so well known to us, that live in communities or colonies, and live a life which in greater or less degree is communal or social. In this connection we may use the term communal for the life of those animals in which the division of labor is such that the individual is dependent for its continual existence on the community as a whole. The term social life would refer to a lower degree of mutual aid and mutual dependence.

84. The honey-bee.-Honey-bees live together, as we know, in large communities. We are accustomed to think of honey-bees as the inhabitants of bee-hives, but there were bees before there were hives. The "bee-tree" is familiar to many of us. The bees, in Nature, make their home in the hollow of some dead or decaying tree-trunk, and carry on there all the industries which characterize the busy communities in the hives. A honey-bee community comprises three kinds of individuals (Fig. 88)— namely, a fertile female or queen, numerous males or drones, and many infertile females or workers. These three kinds of individuals differ in external appearance sufficiently to be readily recognizable. The workers are

smaller than the queens and drones, and the last two differ in the shape of the abdomen, or hind body, the abdomen of the queen being longer and more slender than that of the

male or drone. In a single community there is one queen, a few hundred drones, and ten to thirty thousand workers. The number of drones and workers varies at different times of the year, being smallest in winter. Each kind of individual has certain work or business to do for the whole community. The queen lays all the eggs from which new bees are born; that is, she is the mother of the entire community. The drones or males have simply to act as royal consorts; upon them depends the fertilization of the eggs. The workers undertake all the food-getting, the care of the young bees, the comb-building, the honey-making all the industries with which we are more or less familiar that are carried on in the hive. And all the work done by the workers is strictly work for the whole community; in no case does the worker bee work for itself alone; it works for itself only in so far as it is a member of the community.

How varied and elaborately perfected these industries are may be perceived from a brief account of the life history of a bee community. The interior of the hollow in the bee-tree or of the hive is filled with "comb "-that is, with wax molded into hexagonal cells and supports for these cells. The molding of these thousands of symmet