Fig. 21.—A, larva of Ephydra californica: a, b, c, pupa.
There is good reason to suppose that such limbs arose from dynamical causes, similar to those exciting the formation of secondary adaptations such as are to be seen in the prop or supporting legs of certain dipterous larvæ, as the single pair of Chironomus (Fig. 20) and Simulium, or the series of unjointed soft tubercles of Ephydra (Fig. 21), etc., which are armed with hooks and claws, and are thus adapted for dragging the insect through or over vegetation or along the ground.
Now by frequent continuous use of such unjointed structures, the cuticle would tend to become hard, owing to the deposit of a greater amount of chitin between the folds of the skin, until finally the body being elongated and homonomously segmented, the movements of walking or running would be regular and even, and we would have homonomously jointed legs like those of the trilobites, or of the most generalized Crustacea and of Myriopoda.
In the most primitive arthropods,—and such we take it were on the whole the trilobites, rather than the Crustacea,—the limbs were of nearly the same shape, being long and slender and evenly jointed from and including the antennæ, to the last pair of limbs of the abdominal region. In these forms there appear to be, so far as we now know, no differentiation into mandibles, maxillæ, maxillipedes, and thoracic legs, or into gonopoda. The same lack of diversity of structure and function of the head-appendages has survived, with little change, in Limulus. In the trilobites (Fig. 1) none of the limbs have yet been found to end in claws or forceps; being in this respect nearly as primitive as in the worms. Secondary adaptations have arisen in Limulus, the cephalic appendages being forcipated, adapted as supports to the body and for pushing it onward through the sand or mud, while the abdominal legs are broad and flat, adapted for swimming and bearing the broad gill-leaves.
It is thus quite evident that we have three stages in the evolution of the arthropodan limb; i.e. 1, the syllid stage, of simple, jointed, soft, yielding appendages not used as true supports (Fig. 19); 2, the trilobite stage, where they are more solid, evenly jointed, but not ending in claws; and by their comparatively great numbers (as in the trilobite, Triarthrus) fully supporting the body on the bottom of the sea. In Limulus they are much fewer in number, thicker, and acting as firm supports, the cephalic limbs of use in creeping, and ending in solid claws. 3, The third stage is the long slender swimming head-appendages of the nauplius stage of Crustacea.
As regards the evolution of limbs of terrestrial arthropods, we have the following stages: 1, the soft unjointed limbs of Tardigrades, ending in two claws, and those of Peripatus, and the pseudo- or prop-legs of certain dipterous larvæ; 2, finally the evolution of the long, solid, jointed limbs of Pauropus and other primitive myriopods, the legs forming solid, firm supports elevating the body, and enabling the insect to drag itself over the ground or to walk or run. When the body is elongated and many-segmented, the legs are necessarily numerous; but when it is short, the legs become few in number, i.e. six, in the hexapodous young of myriopods and in insects, or eight in Arachnida. Whenever the legs are used for walking, i.e. to raise and support the body, they end in a solid point or in a pair of forceps or claws. On the other hand, as in phyllopods, where the legs are used mainly for swimming, they are unarmed and are soft and membranous, or, as in the limbs of the nauplius or zoëa stage of crustaceans, end in a simple soft point, which often bears tactile setæ.
The tarsal joints are more numerous in order to give greater flexibility to the limb in seizing and grasping objects, both to drag the body forwards and to support it.
Unlike those of the Crustacea, the limbs of insects are not primitively biramose, but single, the three-lobed first maxillæ, and secondarily bilobed second maxillæ being the result of adaptation. Embryology on the whole proves the truth of this assumption; the maxillæ of both pairs are at first single buds, afterwards becoming lobed. All the appendages of the body, including the ovipositor or sting, are modified limbs, as shown by their embryological development.
It is noticeable that in the crab, where the body is raised by the limbs above the bottom, it is much shorter and more cephalized than in the shrimps. Also in the simply walking and running spiders, the hind-body is shorter than in scorpions, while in the running and flying insects, such as the Cicindelidæ, and in the swiftly flying flies and bees, there is a tendency to a shortening of the body, especially of the abdomen. The long body of the dragon-fly is an impediment to flight, but compensated for by the action of the large wings.
The arthropodan limb is a compound leverage system. It is, says Graber, a lateral outgrowth of the trunk, which repeats in miniature that of the main trunk, its single series of joints or segments forming a jointed dermo-muscular tube. Yet the lateral appendages of an insect differ from the main trunk in two ways: (1) they taper to the end which bears the two claws, and (2) their segments are in the living animal arranged not in a straight line, but at different angles to each other. The basal joint turning on the trunk acts as the first of a whole series of levers. The second joint, however, is connected with the musculature of the first or basal joint, and thus each succeeding joint is moved on the one preceding. Each lever, from the first to the last, is both an active and a passive instrument. (Graber.)