Fig. 262.—A, Agrion pulchellum, natural size; B, Aeschna cyanea, profile; C, same from front to show position of legs. ⅔ natural size.
The meso- and meta-thorax are very intimately combined, and their relations are such that the former is placed much above the latter. This peculiarity is carried to its greatest extent in some of the Agrioninae (Fig. 262, A), where not only are the wings placed at a considerable distance behind the three pairs of legs, but also the front pair of wings is placed almost directly above the hind pair. In the Anisopterides these peculiarities are much less marked (Fig. 262, B), nevertheless even in them the three pairs of legs are placed quite in front of the wings. This peculiar structure of the wing-bearing segments is accompanied by an unusual development of the pleura, which, indeed, actually form the larger part, if not nearly the whole, of the front region of the dorsal aspect of these two segments. We shall not enter into more minute particulars as to the structure of the thorax, for difference of opinion prevails as to the interpretation of the parts.[[332]] The abdomen is remarkable for its elongation; it is never broad, and in some genera—Mecistogaster, e.g.—it attains a length and slenderness which are not reached by any other Insects. It consists of ten segments and a pair of terminal calliper-like or flap-like processes of very various sizes and forms.
The wings of the dragon-flies are usually transparent and provided with a multitude of small meshes. The hind wings are about as large as the front pair, or even a little larger; the main nervures have a sub-parallel course, and are placed in greater part on the anterior region of each wing. The relations of the more constant nervures and the cells of which they are parts form a complex subject, and are amongst the most important of the characters used in classifying these Insects. The wings are always elongate in comparison to their breadth and have no folds; they are held partially extended, or are placed so as to project backwards, or backwards and outwards. They exhibit another peculiarity, inasmuch as the front or costal margin is slightly uneven before or near the middle, giving rise to an appearance such as might result from the breaking and subsequent mending of the marginal rib at the spot in question, which is called the nodus. In some forms a peculiar character exists in the shape of a small opaque space called the membranule, lying close to the body of the Insect in the anal area of the wing, as shown in Fig. 260.
The legs are slender and are chiefly remarkable for the beautiful series of hair-like spines with which they are armed, and which in some forms (e.g. Platycnemis, Fig. 264) are of considerable length. We believe that the legs are of great importance in capturing the prey, they being held somewhat in the position shown in Fig. 262, C. The tarsi are three-jointed. In the male of Libellago caligata the legs exhibit a remarkable condition, the tibiae being dilated, and on the upper side of a vivid red colour, while below they are white. This coloration and form are each unusual in the family. The male of Platycnemis pennipes, a British species (Fig. 264), shows a similar dilatation of the tibiae, but to a less extent and without any great difference in the colour of the two faces of the dilatation. This dilatation reaches its maximum in Psilocnemis dilatipes M‘Lach. The position of the legs in relation to the other parts of the body is peculiar to the dragon-flies; the legs seem to be unfit for walking, the Insects never using them for that purpose.
Several peculiarities in the internal anatomy deserve notice. The alimentary canal in Libellula is about as long as the body, the oesophagus and chylific stomach being elongate, while the intestine is short and divided into only two parts; there is no definite proventriculus. The Malpighian tubules are shorter than usual; they are about forty in number. The male has no vesiculae seminales; the vasa deferentia are elongate, and the ejaculatory duct is very short, being in fact merely a common sinus formed by the terminations of the vasa deferentia. The opening of this duct is situated on the penultimate ventral plate; the organs of intromission are, however, placed much anterior to this, on the under side of the second segment. The mode in which the fertilising fluid is transferred from the ninth to the second segment is not well understood, but it is known that the abdomen is flexed by the Insect so as to bring the ninth ventral plate into contact with the second. The three thoracic ganglia of the nervous chain are all contiguous, though not completely amalgamated; the abdominal ganglia are seven in number, and are all separated, the terminal one being larger than the others. Dufour, after repeated dissections, was unable to find any salivary glands, but Olga Poletajewa[[333]] states that they exist.
The Odonata must be ranked among the most highly-organised Insects so far as external structure and powers of locomotion are concerned; the peculiar modifications of the thoracic segments and the relative positions of the wings and legs mark a great departure from the normal type of Insect structure. Their prey consists of living Insects, which they capture on the wing by their own superior powers of flight. They destroy a great many Insects, their appetite for food being, as in the cases of the Mantidae and of the tiger-beetles, apparently almost insatiable. They are admirably constructed for the purposes of their predatory lives; they fly with great swiftness and change the direction of their flight with admirable facility. They are, however, dependent on sunshine, and conceal themselves in dull and cloudy weather. The larger Insects of the family belong to the division Anisopterides (Fig. 260, Anax formosus) and some of these may, in our own country, usually be seen, in the bright sunshine of the summer and autumn, engaged in hawking in their favourite haunts. Places where other Insects abound are naturally those most frequented; the glades of woods, country lanes and hedge-sides, the borders of streams and the margins of sheets of water are the places they most affect. They inspire the rustics with some feeling of fear, and hence have received the name of "horse-stingers," and in North America are called "devil's darning-needles." The aversion to dragon-flies may perhaps be due to their appearance, which is certainly, in the case of some of our species of Aeschna, Cordulegaster, and Gomphus, very remarkable, consisting of a dark ground-colour with bars and spots of vivid green or yellow, giving, it must be admitted, a peculiar, even savage appearance to the Insects. Whatever the reason may be, they are, it is certain, held in much fear, and it is difficult to induce a country lad to touch one even when it is captured and held by another person. The idea of dragon-flies being dangerous to anything but their Insect victims is, however, entirely erroneous; they may be captured and handled without their inflicting any injury. It is probable that the life of the imago may endure for several weeks if not months. It is known that Sympycna fusca—a common European though not British dragon-fly—hibernates in the imago state.
In the case of the large dragon-flies we have mentioned, each individual appears to have a domain, as it were, of its own. Westwood tells us that he has seen what he believed to be the same individual hawking daily for several weeks together over a small pond. The writer observed a specimen of Cordulegaster annulatus to frequent a particular bush, to which it returned—frequently to the same leaf—after an excursion in search of food. The way in which these Insects actually seize their prey has not yet been made clear; it is certain that they capture flying Insects, and it seems most probable, as we have already said, that this is done by means of the legs. These, as we have said, are inserted so as to be very near to the mouth; they are directed forwards, and are held bent at right angles so as to form a sort of net, and are armed with a beautiful system of fine spines; it is probable that if the dragon-fly pursue an Insect on the wing and strike it with the trap, formed by its six legs (Fig. 262, C), then these immediately come together under the mouth, so that the victim, directly it is captured by the leg-trap of its pursuer, finds itself in the jaws of its destroyer. It is perhaps impossible to verify this by actual observation, as the act of capture and transfer is so very brief and is performed in the midst of a rapid dash of flight, but it seems more probable that the prey is first struck by the legs than that the mouth is the primary instrument of capture. The excessive mobility of the head permits the victim to be instantly secured by the mouth, and the captured fly is turned about by this and the front pair of legs, and is nipped rapidly so that the wings and drier parts fall off; the more juicy parts of the prey are speedily squeezed into a little ball, which is then swallowed, or perhaps we should rather say that the mouth closes on it, and submits it to further pressure for the extraction of the juices. We have already noted that many of these large and active dragon-flies, particularly in the Libellulinae and Aeschninae, have their eyes distinctly divided into two parts, the facets in the lower part of the eye being different from those of the upper part. Exner considers[[334]] that the upper division is for the perception of movement, the lower for the perception of the form of resting objects. Plateau thinks[[335]] that the dragon-flies perceive only movement, not form.
Fig. 263.—Inner view of a portion of the left side of body of Libellula depressa, showing a part of the mechanism of flight, viz. some of the chitinous ridges at base of the upper wing, and some of the insertions of the tendons of muscles. A, line of section through base of upper wing, the wing being supposed to be directed backwards; C, upper portion of mechanism of the lower wing; b, lever extending between the pieces connected with the two wings. (After von Lendenfeld.)
The splendid acts of flight of the Anisopterid Odonata are accomplished by the aid of a complex arrangement of chitinous pieces at the bases of the wings (Fig. 263). In Insects with considerable powers of flight the hind wings are usually subordinate in functional importance to the anterior, to which they are attached by a series of hooks, or some other simple mechanism, on the wings. In the Odonata the two wings of each pair are quite free, but they are perhaps brought into correlative action by means of a lever of unusual length existing amongst the chitinous pieces in the body wall at the base of the wings (Fig. 263, b). The wing muscles are large; according to von Lendenfeld[[336]] there are three elevator, five depressor, and one adductor muscles to each wing: he describes the wing movements as the results of the correlative action of numerous muscles and ligaments, and of a great number of chitinous pieces connected in a jointed manner.