Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/406

386 D R A G O N-F L Y with respect to the composition of the mouth parts, and by some authors the &quot; palpi &quot; have been termed the side pieces of the lower lip. la a dead dragon-fly the parts are closed on each other, and, for a just appreciation of their structure and power, it is necessary to take a living example in the fingers by the thorax, slight lateral pressure on which causes the insect to display the formidable arrangement. The prothorax is extremely small, consisting of only a narrow ring, the upper portion of which is often elevated into lobes. The rest of the thorax is very large, and con solidated into a single piece, with oblique sutures on the sides beneath the wings ; the portion in front of the wings is extremely robust, and offers a median carina or suture above, and a broad transverse sinus posteriorly- The inter- alar portion is somewhat excavated, and on each side of it above are nodosities forming the attachments of the power ful muscles that work the wings ; on each side is a large and distinct spiracle. The abdomen varies excessively in form, the two extremes being the filiform structure observ able in most Ayrionidce, and the very broad and depressed formation seen in our familiar Libellula depressa. It con sists of ten distinct segments, whereof the basal two and those at the apex are short, the others elongate, the first being excessively short. In a slit on the under side of the second in the male, accompanied by external protuberances, are concealed the genital organs : on the under side of the eighth in the female is a scale-like formation, indicating the entrance to the oviduct. The tenth is always provided in both sexes with prominent appendages, differing greatly in form, and often furnishing the best specific (and even generic) characters ; by some authors these appendages are considered as representing a modified eleventh segment. The basal segments often have additional transverse sutures, and in the common triquetrous abdomen there is a fine longitudinal dorsal carina, and prominent lateral angles ; invariably the ventral surface has a longitudinal membranous space connecting the here divided chitinous portion of the external skeleton. The legs vary in length and stoutness, but may, as a rule, be termed long and slender, and in a measure that appears disproportionate to the necessities of the insect ; for a dragon-fly can scarcely be said to walk after the short promenade it takes on emerging from its puparium. The anterior pair probably assist in capturing and holding its insect prey, but the greatest service all the legs render is possibly in enabling the creature to rest lightly, so that it can quit a position of repose in chase of passing prey in the quickest possible manner, in which the majority of the species are aided also by the horizontally extended wings. The coxa is short and stout, followed by a still shorter trochanter ; the femora and tibise long and slender, almost invariably furnished on their under surface with two series of strong spines, as also are the tarsi, which consist of three slender joints, the last having two long and slender claws, usually (but not invariably) with a small tooth internally below the tips ; the palms are absent or nearly so, and naturally are not necessary in a non-ambulatory insect. The wings are always elongate, and furnished with strong longitudinal neuration and dense transverse nervules strengthening the already strong (although typically transparent) membrane. In the Ayrionidce both pairs are nearly equal, and are carried vertically and longitudinally in repose, and the neuration and membrane are less strong ; hence the species of this family are not so powerful on the wing as are those of the other groups in which the wings are horizontally extended in a position ready for instant service. The neuration is peculiar, and in many respects without precise analogy in other groups of insects, but it is not necessary here to enter into more than some special points. On the coatal margin (excepting in some Calopterygina] there is a small dark space limited by nervules,termed the pterostigma; and between this and the base of the wing is a point termed the &quot; nodus,&quot; at which the sub-costal nervure is suddenly arrested. The arrangement of the nervures at the base of the wing is very singular, and slight differences in it form useful aids to classification. In the jEscknidce and Libellulidce this arrangement results in the formation of a triangular space (known as the &quot; triangle &quot;), which is either open or traversed by nervules ; but in many Agrionidoe this space, instead of being triangular, is oblong or elongately quadrate, or with its upper edge partly straight and partly oblique. This fixitude of type in neuration is not one of the least important of the many peculiarities exhibited in these insects. The internal structure is comparatively simple. The salivary glands appear to be absent, and the whole digestive apparatus consists of an elongate canal extending from mouth to anus, comprising the oesophagus, stomach, and intestine, with certain dilatations and constrictions; the characteristic Malpighian vessels are stated to number about forty, placed round the posterior extremity of the stomach. Dragon-flies eat their prey completely, and do not content themselves by merely sucking its juices ; the harder portions are rejected as elongate, nearh r dry, pellets of excrement. But the most extraordinary feature in the economy, one which has attracted the attention of naturalists from remote times, is the position of the genital organs, and the corresponding anomalous manner in which the pairing of the sexes and impregnation is effected. In the male the intromittent organ is (as stated above) situated in a slit on the under surface of the second abdominal segment ; it is usually very crooked or sinuous in form, and is accompanied by sheaths, and by external hooks or secondary appendages, and also by seminal vessels. But the ducts of the vessels connected with the testes unite and open on the under surface of the ninth segment ; hence, before copulation can take place, it is necessary that the vessels in the second segment be charged from this opening, and in the majority of cases this is done by the male previously to seeking the female. In the latter sex the entrance to the oviduct and genital organs is on the under surface of the eighth abdominal segment. The act of pair ing may be briefly stated as follows. The male, when flying, seizes the prothorax of the female with the strong appendages at the extremity of the abdomen, and the abdomen of this latter sex is then curved upward so as to bring the under side of the eighth segment into contact with the organs of the second segment of the male. This act must have been observed by all, though but few non- entomologists are acquainted with the reasons for this most extraordinary position. In the more powerful Libellulid(jc, &c., the act is of short duration, and it is probable that polygamy and polyandry exist, for it possibly requires more than one almost momentary act to fertilize all the eggs in the ovaries of a female. But in many Ayrionidte, and in some others, the male keeps his hold of the prothorax of the female for a lengthened period, retain ing himself in flight in an almost perpendicular manner, and it may be that the deposition of eggs and pairing goes on alternately. There is, however, much yet to be learned on these points. The gravid female usually lays her eggs in masses (but perhaps sometimes singly), and the operation may be witnessed by any one in localities frequented by these insects. She hovers for a considerable time over nearly the same spot, rapidly dipping the apex of her abdomen into the water, or at any rate touching it, and often in places where there are no water-weeds, so that in all probability the eggs fall at once to the bottom. But in sume of the A jriuiud^ the female has been often noticed