Larva of Ephemera marginata, magnified. The central shaded line represents the great dorsal channel of the blood. The lateral branched lines the trachea. CHAPTER III. ON THE INTERNAL ANATOMY OF INSECTS. OF THE Muscles of Insects.-Muscle is a substance which yields to the touch: it consists of two parts; the body of the muscle, which is fleshy; and the extremities and coating of the muscle, which are tough, strong, and elastic, and are designated as tendon. The surface of muscle is a beautiful microscopic object: it presents a series of exceedingly fine transverse lines, which, in contraction, assume an undulated form. The fibre of muscle is much the same in every animal, from man to the most minute animalcule. The attachment of muscle is solely to the osseous plates or bones, which constitute the external covering of an insect; in these they originate precisely in the same manner as the muscles in the human frame are attached to, and originate in, the bones. The bulk and form of muscles in insects are beautifully apportioned to the offices they are required to perform; and unusual bulk in any part of an insect generally implies the presence of unusually developed muscle, and the object for which it is developed may frequently be ascertained. We have before seen, that the fore wings arise from the mesothorax, and the hind wings from the metathorax: these segments vary greatly in size, and this variation depends so precisely on the powers of flight possessed by each pair of wings, that an insect anatomist, on regarding these two segments alone, would at once decide on the relative power of the wing which they had borne. In flies, the fore wings alone are used in flight; the hind wings are rudimental; the whole bulk of muscle, therefore, required for flight, is placed in the mesothorax: in beetles the hind wings alone are used in flight, and the bulk of muscle is consequently transferred to the metathorax. It happens in some moths, that one sex flies and the other does not; and in these the different size of the wing-bearing segments proves the provision of muscle to be for the purpose of flight. In the common ant, the little worker never leaves the ground; wings, therefore, would be an incumbrance to it. We find that its pro- meso- and metathorax are very small and insignificant segments, while the mesothorax of the productive female is the largest segment in her body, because it is one of her duties to perform a long flight, and to use the wings which that segment bears. The muscles in those wing-bearing segments which do not employ their wings for flight, are probably not absent, but repose in a rudimental state; while the muscles, whose active uses are required in the neighbouring segments, are increased at their expense. The form of muscles in insects is as various as their size, and depends on the motion required of them: every muscle is precisely of the shape, as well as size, best adapted for the office it has to perform. The muscles which serve to raise a leg and lift it forwards, and all similar simple movements, are linear or cylindrical when at rest, but in contraction become fusiform, the extremities attenuating, the centre incrassating, and the whole being abbreviated. The muscles which serve to perform the wriggling undulating motion, so common in larvæ, especially those which are without legs, are triangular, and in motion contract alternately each side of the triangle, so that each angle becomes acute and obtuse in turn. Muscles in insects more frequently cross and intersect each other than in vertebrated animals: in some instances, where connected with the organs of respiration and circulation, they are excessively minute, and appear actually interwoven, crossing in every direction, like multitudes of slender threads. It is in such situations that dominant motion exists; motion altogether independent of, and uninfluenced by, the will. The principal muscles in insects are generally in pairs; that is, each principal muscle has an antagonist muscle, which, after any given movement, has the power to restore the original position. The muscles destined for the government of the limbs are all of the linear or cylindrical form, and all in pairs, a flexor and extensor muscle being found in each joint of the legs, &c. Nerves. In vertebrated animals the brain is situated in the head. Brain is supposed to be the seat of mind: all the nerves originate in the brain: the nerves are the organs of sensation. We find that in insects there is no part positively ascertained to be the brain: we discover nerves throughout the body, and we trace these to large masses or knots, situated at intervals throughout the whole length of the insect. Experience has shown us that, on the brain of vertebrated animals being separated from the body, or even greatly injured, both sensation and active vitality at once cease; but in insects the separating of the head or of the parts containing either of these masses of nerves, produces no immediate or ascertainable effect on sensation or vitality. This shows us, first, that mind or volition is, in vertebrated animals, situate in the brain; secondly, that in insects it is not exclusively confined to any part. These conclusions lead to the probability of a third, that brain and nerve are but different states of the same system of organs. The vitality, therefore, concentrated in a brain, may be diffused through the nerves when there is no brain, and each mass of nerves may be the seat of that small power of mind which insects possess. The nervous cord extending, as described, from one end. of the insect to the other, is compared to, and has been called, the spinal marrow; its first knot or incrassated portion, the brain; the following knots, ganglions: this nomenclature appears arbitrary, as we have seen that no particular portion exercises exclusively the functions of a brain. It appears better to consider each of the little knots a separate and independent centre of volition, as it undoubtedly possesses the characters of such, giving out its various nerves in the same manner as the human brain. All anatomical operations succeed best with the larva of insects, because the constant and obvious division of the body of larvæ into thirteen segments, affords us more ready means of describing the result of observation; whereas, in perfect insects, several segments are liable to become united, and the exact boundaries of each are thus rendered subjects of doubt. It must also be observed, that the internal systems of organs following and adapting themselves to the wonderful changes on the surface, require the utmost caution in the anatomist who attempts to characterize them. Let us then examine the caterpillar of a common butterfly, and trace its system of brains and nerves: in this we have the advantage of being able to examine it in all its stages, and to compare and ascertain, with tolerable certainty, both the mode and extent of change. In the larva of a butterfly the spinal cord has thirteen knots, one in each of its segments; these knots, from their similarity in office to brain, may be called cerebroids or cerebroidæ ; and each one may be distinguished by the name of the segment in which it is situated. The first cerebroid or capitis cerebroida, is composed of two hemispheres; from these arise, in perfect insects, several pairs of nerves; there are two optic nerves or nervi oculorum ; two antennary nerves or nervi antennarum; two mandibulary nerves or nervi mandibularum; two maxillary nerves or nervi maxillarum; and two labial nerves or nervi labii. To return, however, to the caterpillar, the first cerebroid is situated above the cavity of the mouth; from each hemisphere of the cerebroid a cord proceeds; these unite on leaving the head, just above the opening of the throat, forming the second cerebroid or prothoracis cerebroida; they then again divide, and, passing one on each side of the gullet, reunite below it, and enclose it in a ring. At the union they form the third cerebroid or mesothoracis cerebroida; and from this arise four principal nerves, two of which, the mesopedum nervi, descend to the feet, and two others, the prothoracis nervi dorsales, ascend to the |