This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(784). In Cimbex lutea (fig. 150, d) the arrangement of the suckers is different, one large and spoon-shaped disk being attached to the extremity of each tarsal joint. Moreover, in this case there is another singular structure: two spur-like organs project from each side of the extremity of the tibia, each being provided with a sucking disk, while the two together form a strong prehensile forceps.
(785). In some Water-beetles (Dytiscidce) the feet are armed with a still more elaborately constructed apparatus of suckers; but in this case, as they are only met with in the male insect, they perhaps ought rather to be looked upon as a provision made for the purpose of securely holding the female during sexual union, than as being specially connected with locomotion.
(786). In the anterior legs of the male Dytiscus the first three joints of the tarsus are excessively dilated, so as to form a broad circular palette: on examining the inferior surface of this expanded portion under a microscope, it is seen to be covered with an immense number of sucking-cups (fig. 150, f), two or three being much larger than the rest; but they form collectively a wonderful instrument of adhesion.
(787). The middle pair of legs of the same beetle (fig. 150, a) exhibit a somewhat similar structure; but in this case the disk upon which the sucking apparatus is placed is much elongated, and the suckers are all of small dimensions.
(788). In the female Dytiscus (fig. 152, c) this configuration of the tarsus is wanting; and moreover the surface of the back is marked with deep longitudinal grooves that do not exist in the male insect, but seem to be an additional provision for facilitating the intercourse of the sexes in these powerful aquatic beetles.
(789). Another mode of progression common among insects is by leaping, to which, from their extraordinary muscular power, these little beings are admirably adapted. The common Flea, for example (Pulece irritans) (fig. 153), will leap two hundred times its own length; and many Orthoptera possess a power of vaulting through the air scarcely less wonderful, of which the Cricket affords a familiar instance. In such insects (fig. 145, a, b) the thighs of the posterior legs are enormously dilated, and the length of these limbs is much greater than that of the anterior pair. When disposed to leap, such insects bend each hind-leg, so as to bring the tibia into close contact with the thigh, which has often a longitudinal furrow, armed on each side with a row of spines, to receive it. The leg being thus bent, they suddenly unbend it with a jerk, when, pushing against the plane of position, they spring into the air*. In many of these saltatorial tribes the tarsus is furnished with very curious appendages, either provided for the purpose of obviating any jar when the animal alights from its lofty leaps1, or else they may act like firm cushions, adapted, by their elasticity, to give greater effect to the spring which raises the insect from the ground. In the magnified view of the tarsus of an Abyssinian Grasshopper (fig. 150, e) the arrangement of these organs is well exhibited.
* Kirby and Spence, Introduction to Entomology. 4 vols. 8vo. 1 Sir E. Home, Phil. Trans. 1816.
(790). The next modification in the structure of the legs is met with in such species as burrow beneath the surface of the ground, of which mode of progression the most remarkable example is seen in the Mole-cricket (Gryllotalpa vulgaris) (fig. 151.) In this creature, the anterior segment of the thorax, whereunto the fore-legs are appended, is wonderfully enlarged and of great strength, while the legs themselves are equally remarkable for their enormous bulk and muscularity. The tibia is excessively dilated, and terminates obliquely in four sharp and strong-spines. The whole of the tarsus would, at a first glance, appear to be wanting; but on inspection it is found to consist of three joints placed upon the inner side of the tibia, the first two being broad and tooth-shaped, while the last piece is very small and armed with two hooks. The direction and motion of these hands is outwards, thus enabling the animal most effectually to remove the earth when it burrows; and, by the help of such powerful instruments, it is astonishing how rapidly it buries itself*.
Fig. 151. Gryllotalpa vulgaris.
(791). Similar examples of adaptation in the mechanical structure of the legs of insects might be multiplied indefinitely; we shall, however, select but one other illustration before leaving this part of our subject, namely the conversion of these organs into instruments for swimming, whereby, in aquatic insects, they become adapted to act as oars. Nothing is, perhaps, better calculated to excite the admiration of the student of animated nature than the amazing results obtained by the slightest deviations from a common type of organization; and in examining the changes required in order to metamorphose an organ which we have already seen performing such a variety of offices into fins adapted to an aquatic life, this circumstance must strike the mind of the most heedless observer. The limbs used in swimming exhibit the same parts, the same number of joints, and almost the same shape, as those employed for creeping, climbing, leaping, and numerous other purposes; yet how different is the function assigned to them! In a common Water-beetle already referred to, the Dytiscus marginalis (fig. 152, c), the two anterior pairs of legs, that could be of small service as instruments of propulsion, are so small as to appear quite disproportionate to the size of the insect, while the hinder pair are of great size and strength; the last-mentioned limbs are, moreover, removed as far backwards as possible, by the development of the hinder segment of the thorax, in order to approximate their origins to the centre of the body; and the individual segments composing them are broad and compressed, so as to present to the water an extensive surface, which is still further enlarged by the presence of flat spines appended to the end of the tibia, as well as of a broad fringe of stiff hairs inserted all around the tarsus. The powerful oars thus formed can open until they form right angles with the axis of the body, and from the strength of their stroke are well adapted to the piratical habits of their possessors, who wage successful war, not only with other aquatic insects and worms, but even with small fishes, the co-inhabitants of the ponds wherein they live.