FIG. 288. e a the Cricket. The ligula is extremely developed in the Fly kind, in which it forms the chief part of what is commonly called the "proboscis" (Fig. 287); and it also forms the "tongue" of the Bee and its allies (Fig. 288). In the Diptera or two-winged flies generally, the labrum, maxillæ, mandibles, and the internal tongue (where it exists) are converted into delicate lancetshaped organs termed setæ, which, when closed together, are received into a hollow on the upper side of the labium (Fig. 287, b), but which are capable of being used to make punctures in the skin of animals or the epidermis of plants, whence the juices may be drawn forth by the proboscis. Frequently, however, two or more of these organs may be wanting, so that their number is reduced from six to four, three, or two. In the Hymenoptera (bee and wasp tribe), however, the labrum and the mandibles (Fig. 288, b), much resemble a, labial palpi; e, ligula, or prolonged labium, those of mandibulate insects, commonly termed the tongue :-B, portion of the and are used for corresponding purposes; the maxilla (c) are greatly elongated, and form, when closed, a tubular sheath for the ligula or "tongue," through which the honey is drawn up; the labial palpi (d) also are greatly developed, and fold together like the maxillæ, so as to form an inner sheath for the "tongue;" while the "ligula" itself (e) is a long tapering muscular organ, marked by an immense number of short annular divisions, and densely covered over its whole length with long hairs (B). It is not tubular, as some have stated, but is solid; when actively employed in taking food, it is extended to a great distance beyond the other parts of the mouth; but when at rest, it is closely packed up and concealed between the maxillæ. "The manner," says Mr. Newport, "in which the honey is obtained when the organ is plunged into it at the bottom of a flower, is by 'lapping,' or a constant succession of short and quick extensions and contractions of the organ, which occasion the fluid to accumulate upon it and to ascend along its upper surface, until it reaches the orifice of the tube formed by the approximation of the maxillæ above, and of the labial palpi and this part of the ligula below." A. Parts of the mouth of Apis mellifica (Honeybee):-a, mentum; b, mandibles; c, maxilla; surface of the ligula, more highly magnified. 387. By the plan of conformation just described, we are led to that which prevails among the Lepidoptera or butterfly tribe, and which, being pre-eminently adapted for suction, is termed the haustellate mouth. In these insects, the labrum and mandibles are reduced to three minute triangular plates; whilst the maxillæ are immensely elongated, and are united together along the median line, to form the haustellium or proboscis, which contains a tube formed by the junction of the two grooves that are channelled out along their mutually applied surfaces, and which serves to pump up the juices of deep cup-shaped flowers, into which the size of their wings prevents these insects from entering. The length of this haustellium varies greatly; thus in such Lepidoptera as take no food in their perfect state, it is a very insignificant organ; in some of the white Hawk-moths, which hover over blossoms without alighting, it is nearly two inches in length; and in most Butterflies and Moths it is about as long as the body itself. This haustellium, which, when not in use, is coiled up in a spiral beneath the mouth, is an extremely beautiful microscopic object, owing to the peculiar banded arrangement it exhibits (Fig. 289), which is probably due to the disposition of FIG. 289. its muscles. In many instances, the two halves may be seen to be locked together by a set of hooked teeth, which are inserted into little depressions between the teeth of the opposite side. Each half, moreover, may be ascertained to contain a trachea or air-tube (§ 391); and it is probable, from the observations of Mr. Newport,' that the sucking up of the juices of a flower through the haustellium (which is accomplished with great rapidity) is effected by the agency of the respiratory apparatus. The proboscis of many Butterflies is furnished, for some distance from its extremity, with a double row of small projecting barrel-shaped bodies (shown in Fig. 289), which are surmised by Mr. Newport to be organs of taste. Numerous other modifications of the structure of the mouth, existing in the different tribes of Insects, are well worthy of the careful study of the Microscopist; but as detailed descriptions of most of these will be found in every systematic treatise on Entomology, the foregoing general account of the principal types must suffice. Haustellium (proboscis) of Vanessa. 388. Parts of the Body.-The conformation of the several divisions of the Alimentary Canal presents such a multitude of 1 "Cyclopædia of Anatomy and Physiology," vol. ii, p. 902. diversities, not only in different tribes of Insects, but in different states of the same individual, that it would be utterly vain to attempt here to give even a general idea of it; more especially as it is a subject of far less interest to the ordinary Microscopist, than it is to the professed Anatomist. Hence we shall only stop to mention, that the muscular gizzard in which the œsophagus very commonly terminates, is often lined by several rows of strong horny teeth for the reduction of the food, which furnish very beautiful microscopic objects. These are particularly developed among the Grasshoppers, Crickets, and Locusts, the nature of whose food causes them to require powerful instruments for its reduction. 389. The Circulation of Blood may be distinctly watched in many of the more transparent Larva, and may sometimes be observed in the perfect Insect. It is kept up, not by an ordinary heart, but by a "dorsal vessel," which really consists of a succession of muscular hearts or contractile cavities, one for each segment, opening one into another from behind forwards, so as to form a continuous trunk, divided by valvular partitions. In many larvæ, however, these partitions are very indistinct, and the walls of the "dorsal vessel" (so named from the position it always occupies along the middle of the back) are so thin and transparent that it can with difficulty be made out, a limitation of the light by the diaphragm being often necessary. The blood which moves through this trunk, and which is distributed by it to the body, is a transparent and nearly colorless fluid, carrying with it a number of "oat-shaped" corpuscles, by the motion of which its flow can be followed. The current enters the dorsal vessel at its posterior extremity, and is propelled by the contractions of the successive chambers towards the head, being prevented from moving in the opposite direction by the valves between the chambers, which only open forwards. Arrived at the anterior extremity of the dorsal vessel, the blood is distributed into three principal channels; a central one, namely, passing to the head, and a lateral one to either side, descending so as to approach the lower surface of the body. It is from the two lateral currents that the secondary streams diverge, which pass into the legs and wings, and then return back to the main stream; and it is from these also, that, in the larva of the Ephemera marginata (day-fly), the extreme transparency of which renders it one of the best of all subjects for the observation of Insect circulation, the smaller currents diverge into the gill-like appendages with which the body is furnished (§ 393). The blood-currents seem rather to pass through channels excavated among the tissues, than through vessels with distinct walls; but it is not improbable that in the perfect Insect the case may be different. In many aquatic larvae, especially those of the Culicidae (gnat tribe), the body is almost entirely occupied by the visceral cavity; and the blood may be seen to move backwards in the space that surrounds the alimentary canal, which here serves the purpose of the channels usually excavated through the solid tissues, and which freely communicates at each end with the dorsal vessel. This condition strongly resembles that found in many Annelida. In some larvæ, whose development is yet less advanced, even the dorsal vessel appears to be wanting, although the fluid of the visceral cavity (in which corpuscles abound) is in a state of continual oscillatory movement. 390. The Circulation may be easily seen in the wings of many insects in their Pupa state, especially in those of the Neuropterous order (such as dragon-flies and day-flies) which pass this part of their lives in water, in a condition of activity; the pupa of Agrion puella, one of the smaller dragon-flies, is a particularly favorable subject for such observations. Each of the "nerves" of the wings contains a "trachea" or air-tube (§ 391), which branches off from the tracheal system of the body; and it is in a space around the trachea that the blood may be seen to move, when the hard framework of the nerve itself is not too opaque. The same may be seen, however, in the wings of the pupa of Bees, Butterflies, &c., which remain shut up motionless in their cases; for this condition of apparent torpor is one of great activity of their nutritive system, those organs, especially, which are peculiar to the perfect insect, being then in a state of rapid growth, and having a vigorous circulation of blood through them. In certain Insects of nearly every order, a movement of fluid has been seen in the wings for some little time after their last_metamorphosis; but this movement soon ceases, and the wings dry up. The common Fly is as good a subject for this observation, as can be easily found; it must be caught within a few hours or days of its first appearance; and the circulation may be most conveniently brought into view, by enclosing it (without water) in the animalcule cage, and pressing down the cover sufficiently to keep the body at rest, without doing it any injury. 391. The Respiratory Apparatus of Insects affords a very interesting series of microscopic objects; for, with great uniformity in its general plan, there is almost infinite variety in its details. The aeration of the blood in this class is provided for, not by the transmission of the fluid to any special organ representing the lung of a Vertebrated animal (§ 438) or the gill of a Mollusk (§ 353), but by the introduction of air into every part of the body, through a system of minutely distributed trachea or air-tubes, which penetrate even the smallest and most delicate organs. Thus, as we have seen, they pass into the haustellium or “proboscis" of the Butterfly (§ 386), and they are minutely distributed in the elongated labium or "tongue" of the fly (Fig. 287). Their general distribution is shown in Fig. 290; where we see two long trunks (f) passing from one end of the body to the other, and connected with each other by a transverse canal in every segment; these trunks communicate, on the one hand, by b d e FIG. 290. a short wide passages, with the "stigmata," "spiracles," or breathing-pores (g), through which the air enters and is discharged; whilst they give off branches to the different segments, which divide again and again into ramifications of extreme minuteness. They usually communicate also with a pair of airsacs (h) which are situated in the thorax; but the size of these (which are only found in the perfect insect, no trace of them existing in the larvæ) varies greatly in different. tribes, being usually f greatest in those insects which (like the bee) can, sustain the longest and most powerful flight, and least in such as habitually live upon the ground or upon the surface of the water. The structure of the air-tubes reminds us of that of the "spiral vessels" of Plants, which seem destined (in part at least) to perform a similar office (§ 232); for within the membrane that forms their outer wall, an elastic fibre winds round and round, so as to form a spiral, closely resembling in its position and functions the spiral wire-spring of flexible gas-pipes; within this again, however, there is another membranous wall to the air-tubes, so that the spire winds between their inner and outer coats. The tongue of the Fly presents a curious modification of this structure, the purpose of which is not apparent; for instead of its trachea being kept pervious after the usual fashion, by the Tracheal system of Nepa (Water-scorpion):-a, head; b, first pair of legs; c, first segment of the thorax; d, second pair of wings; e, second pair of legs; f, tracheal trunk; g, one of the stigmata; h, air-sac. FIG. 291. Portion of a large Trachea of Dytiscus, with some of its principal branches. |