Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/407

DRAGON-FLY 387 by trustworthy observers to creep clown the stems of aquatic plants several inches below the surface, emerging after the act of oviposition has been effected ; and in the case of Lestcs sponsn, Von Siebold saw the male descend with (he female. The same exact observer noticed also in this species that the female makes slight incisions in the stems or leaves of water plants with the double serrated apparatus (vulva) forming a prolongation of the ninth seg ment beneath, depositing an egg in eacli excision. He has seen two pairs thus occupied beneath the surface on one and the same stem. FIG. 1. The anterior portion of the body of jEschna cyanca freed from the puparium. FIG. 2. The tail being extricated. The duration of the sub-aquatic life of a dragon-fly is no doubt variable, according to the species. In the smaller forms it is probably less than a year, but precise evidence is wanting as to the occurrence of two broods in one year. On the other hand, it is certain that often a longer period FIG. 3. The whole body extricated. is requisite to enable the creature to attain its full growth, and three years have been stated to be necessary for this in the large and powerful Anax formosus. Like all insects with incomplete metamorphoses, there is no quiescent pupal condition, no sharp line of demarcation between the larval and so-called " nymph" or penultimate stage. The creature goes on eating and increasing in size from the moment it emerges from the egg to the time when it leaves the water to be transformed into the aerial perfect insect. The number of moults is uncertain, but they are without doubt numerous. At probably about the antepenul timate of these operations, the rudimentary wings begin to appear as thoracic buddings, and in the full-grown nymph these wings overlap about one-half of the dorsal surface of the abdomen. In structure there is a certain amount of resemblance to the perfect insect, but the body is always much stouter and shorter, in some cases most dis proportionately so, and the eyes are always separated; even in those genera (e.g., jEschna) in which the eyes of the imago are absolutely contiguous, the most that can bo seen in the larva is a prolongation towards each other, and there are no ocelli. The legs are shorter and more fitted for crawling about water plants and on the bottom. In the mouth parts the mandibles and maxillae are similar in form to those of the adult, but there is an extraordinary and unique modification of the lower lip. This is attached to an elongate and slender mentum articulated to the Fia. 4. The perfect insect (the wings having acquired their full dimen sions) resting to dry itself, preparatory to the wings being horizon tally extended. posterior portion of the lower surface of the head, slightly widened at its extremity, to which is again articulated tho labium proper, which is very large, flattened, and gradually dilated to its extremity ; but its form differs according to group as in the perfect insect. Thus in the Ayrionidce it is deeply cleft, and with comparatively slender side- pieces (or palpi), and strongly developed articulated spines ; in the jEschnidce it is at the most notched, with narrow side-pieces and very strong spines ; in the Libet- lulidce it is entire, often triangular at its apex, and with enormously developed palpi without spines, but having the opposing inner edges furnished with interlocking serra tions. The whole of this apparatus is commonly termed the mask. In a state of repose it is applied closely against the face, the elongated mentum directed backward and lying between the anterior pair of legs ; but when an approaching victim is seen the whole apparatus is suddenly projected, and the prey caught by the raptorial palpi ; in some large species it is capable of being projected fully half an inch in front of the head. The prey, once caught and held by this apparatus, is devoured in the usual manner. There are two pairs of thoracic spiracles, but respiration is

mostly affected by a peculiar apparatus at the tail end, and