Motion is one of the principal phenomena of nature; it is as it were the soul of our system, and is as admirable in the smallest animal, as in the universe at large. It is the principal agent in producing all that diversity and change which perpetually affect every object in the creation. The motions of animals are proportioned to their weight and structure, a flea can leap to the distance of at least two hundred times its own length; were an elephant, a camel, or an horse to leap in the same proportion, their weight would crush them to atoms. The same remark is applicable to spiders, worms, and other insects; the softness of their texture, and the comparative smallness of their specific gravity, enable them to fall without injury from heights that would prove fatal to larger and heavier animals.[59]
[59] The parts of some of the larger animals are, however, so admirably constructed for swiftness, as to enable them to perform surprizing acts of agility; for instance, the Siberian jerboa, mus saliens, Pennant; this animal springs forward by successive leaping so very nimbly, that it is said to be very difficult for a man well mounted to overtake it; it is about the size of a large rat. The kanguroo, opossum of Pennant, macropus giganteus, Shaw, leaps to so uncommon a height, and to so great a distance, as to outstrip the swiftest greyhound; its size is that of a full-grown sheep. Accurate coloured figures of both these extraordinary animals are given in that elegant work, the Naturalist’s Miscellany. EDIT.
Many insects can only move the thigh in a vertical direction, while others can move it in a variety of ways. The progressive motion of insects, and the various methods employed to effect it, will be found a very curious and important subject, and well worthy the attention of the naturalist. The intelligent mechanic will not find it lost labour if he bestow some time on the same subject. Very little has been done on this head, and that principally by Reaumur, in his excellent Memoires; and by M. Weiss, in a Memoir published in the Journal de Physique for 1771. The reader may also consult Borelli de Motu Animalium.
Cauda, the tail, terminates the abdomen, and is constructed in a wonderful manner for answering the purposes for which it is formed, namely, to direct the motion of the insect, to serve as an instrument of defence, or for depositing the eggs; the figure and size thereof varying in each genus and its families. In most insects it is simple, simplex, and yet capable of being extended or drawn back at pleasure; in others, elongata, elongated, as in the crab and scorpion; setacea, shaped like a bristle, as in the raphidia; triseta, with three appendages like bristles, as in the ephemera; in some it is forked, furcata, as in the podura; and in others it is furnished with a pair of forceps, forcipata, as in the forficula; in the blatta, grylli, and others, it is foliosa, or like a leaf; in the scorpion and panorpa it is telifera, furnished with a dart or sting. Further particulars may be obtained from the Philosophia Entomologica of Fabricius.