They are named the halteres, or poisers, from their supposed use in balancing the body, or adjusting with exactness the centre of gravity when the insect is flying. Whatever may be their real utility, they may still be regarded as rudiments of a second pair of wings; and they afford, therefore, when thus viewed, a striking instance of the operation of the tendency which prevails universally in the animal kingdom, and modifies the structure of each individual part so as to preserve its conformity to one general type.

The innumerable tribes of butterflies, sphinxes, and moths, are all comprehended in the order Lepidoptera, and are distinguished by having wings covered with minute plumes or scales. These scales are attached so slightly to the membrane of the wing as to come off when touched with the fingers, to which they adhere like fine dust. When examined with. the microscope, their construction and arrangement appear to be exceedingly beautiful, being marked with parallel and equidistant striæ, often crossed by still finer lines, the distinct visibility of which in many kinds of scales, as those of Pontia brassica, or cabbage butterfly, and the Morpho Menelaus of America, constitutes a good criterion of the excellence of the instrument. The beautiful colours which these scales possess may perhaps generally be owing to the presence of some colouring material: but the more delicate hues are probably the result of the optical effect of the striæ on the surface; and in some cases they result from the thinness of the transparent plate of which they consist; for I have observed in several detached scales that the colours they exhibit by transmitted light are the complementary colours to those which they display when seen by reflected light.

The forms of these scales are exceedingly diversified, not only in different species, but also in different parts of the wings and body of the same insect; for the surface of the body, generally, as well as the limbs, and even in some species the antennæ are more or less covered with these scales.*

• In the posthumous work of Lyonet, which has lately appeared, nearly the whole of six quarto plates are crowded with the delineations of the different forms of the scales found in the Bombyx Cossus.

Fig. 164 exhibits some of the more usual shapes as they appear when viewed with high magnifying powers.

Each scale is inserted into the membrane of the wing by a short pedicle, or root, and overlaps the adjoining scales: and the whole are disposed in rows with more or less regu

larity; one row covering the next, like tiles on the roof of a house. This imbricated arrangement, together with the marks that are left on the membrane of the wing where the scales have been rubbed off, are shown in Fig. 165, which is a faithful delineation of the appearance of the wing of the Hesperia Sloanus, seen through a powerful microscope. The membrane of the wing itself, when stripped of its scales, is as perfectly transparent as that of the bee, and is, in like manner, supported by diverging nervures. Many butterflies exhibit in some parts of the wing smooth pearly spots, called by entomologists, ocelli, or eyes, which arise from those parts being naturally destitute of scales. The number of these scales necessary to cover the surface of the wings must, from their minuteness, be exceedingly great. The moth of the silk worm (Bombyx mori, Fig. 148,)

The scales on the wing of the Lepisma are of two kinds; one set being arranged in rows, as usual, and the others, which are of a different shape, being inserted between and over the former, so as to fasten each firmly in its place.

which has but a small wing, contains, according to Lewenhoeck, more than two hundred thousand of these scales in each wing.

These scales doubtless contribute to the protection of the wing; but they at the same time add considerably to their weight, and impede the velocity of their action. This inconvenience appears to have been in a great measure compensated by the greater size of the wings, and by the extent of the surface with which they strike the air. Still, however, it is sufficiently obvious that insects of this order fly with less rapidity and steadiness than most others. But this unsteadiness, again, is turned to good account; for the butterfly, by its irregular and apparently capricious movements, alternately dipping and rising in the air, so as to describe a series of zigzag lines, more easily eludes capture when pursued, not only by naturalists, but also by birds that are eagerly seeking to secure them. It is astonishing to what a distance the silk worm moths will fly: some have been known to travel more than a hundred miles in a short time. The Papilio Iris often rises to so great a height in the air as to be quite invisible.

A mechanical contrivance is adopted in many of the Lepidoptera for keeping their wings steady during flight, consisting of a hook covered with hair and scales, attached to the under side of the upper wings near their base, and connected also by means of bristles to the base of the lower wing: by this attachment all the wings are locked together and brought into action at the same time. Insects of the Sphinx tribe are also provided with a kind of rudder formed by the expansion of the tail, enabling them to steer their course with more certainty. The Lepidoptera in general fly with the body nearly upright, contrary to the habits of most other winged insects, whose bodies, while flying, are nearly in a horizontal position.

The feats of agility and strength exhibited by insects have often been the theme of admiration with writers on natural history; and have been considered as affording incontrovertible proofs of the enormous power with which their muscles

must be endowed. We have already had occasion to notice a remarkable instance of the force and permanence of muscular contraction in those caterpillars which frequently remain for hours together in a fixed attitude, with their bodies extended from a twig, to which they cling with their hind feet alone.* Ants will carry loads which are forty or fifty times heavier than their own bodies: and the distance to which many species, such as the Elater, the Locust, the Lepisma, and above all the Pulex, are capable of leaping, compared with the size of the insects themselves, appear still more astonishing. Linnæus has computed that the Melolontha, or chaffer, is, in proportion to its bulk, more than six times stronger than the horse: and has asserted that if the same proportional strength as is possessed by the Lucanus, or stag-beetle, had been given to the elephant, that animal would have been capable of tearing up by the roots the largest trees, and of hurling huge rocks against his assailants, like the giants of ancient mythology.

But while we must admit that all these facts indicate a remarkable degree of energy in the contractile power of the muscular fibres of insects, we should at the same time recollect that the diminutive size of the beings which display those powers is itself the source of a mechanical advantage not possessed by larger animals. The efficacy of all mechanical arrangements must ultimately depend on a due proportion between the moving and the resisting forces: hence mechanism of every kind must be adjusted with reference. not merely to the relative, but to the absolute dimensions of the structures themselves. This will be evident when we consider that the forces which are called into action are resisted by the cohesion of the particles composing the solid parts of the machine: and this cohesion being not a variable, but a constant and definite force, must necessarily limit the dimensions of every mechanical structure, whether intended for stability or for action. An edifice raised beyond a certain magnitude, will not support itself, because the weight

* See Fig 148*, p. 224.

of the materials increases more rapidly than the strength. How often has it been found that a machine which works admirably in a small model, will totally fail in its performance when constructed on a larger scale? Any lever, of whatever form, may be increased in its dimensions until the force of gravity becomes superior to the cohesion of its own particles: and consequently any structure, like a vegetable or animal body, composed of a combination of levers, would, if its size were to exceed a certain limit, fall to pieces merely by its own weight. This can be prevented either by employing materials of greater cohesive strength, or by increasing, at the points where the strains are greatest, the thickness of the parts compared with their length: but the choice of materials is necessarily restricted within narrow limits, and the latter expedient would entirely alter the relative proportions of the parts, and would require a complete change in the plan of their construction. In passing from the smaller to the larger animals, we find, accordingly, that new models are adopted, a new order of architecture introduced, and new laws of development observed. We have next, then, to direct our attention to the procedure of nature in the execution of this more enlarged and comprehenhensive scheme of animal organization.