Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/408

388 388 D R A G X-F L Y there are two different methods, lu the Agrionidas there are three elongate flattened plates, or false gills, full of trachual ramifications, which extract the air from the water, and convey it to the internal trachea? (in Calopteryx these plates are excessively long, nearly equalling the abdomen), the plates also serving as means of locomotion. But in the other groups these external false gills are absent, and in their place are five valves, which by their sudden opening and closing force in the water to the rectum, the walls of which are furnished with branchial lamellae. The alternate opening and closing of these valves enables the creature to make quick jerks or rushes (incorrectly termed &quot;leaps&quot;) through the water, 1 and, in conjunction with its mouth parts, to make sudden attacks upon prey from a considerable dis tance. The lateral angles of the terminal abdominal segments are sometimes produced into long curved spines. In colour these larvas are generally muddy, and they frequently have a coating of muddy particles, and hence are less likely to be observed by their victims. If among insects the perfect dragon-fly may be termed the tyrant of the air, so may its larva be styled that of the water. Aquatic insects and larvaa form the principal food, but there can be no doubt that worms, the fry of fish, and even younger larv.-e of their own species, form part of the bill of fare. The &quot; nymph &quot; when arrived at its full growth sallies forth from the water, and often crawls a considerable distance (frequently many feet up the trunks of trees) before it fixes itself for the final change, which is effected by the thorax splitting longitudinally down the back, through which fissure the perfect insect gradually drags itself. The figures on last page indicate this process as observed in JEschna cyanea. For a considerable time after its emergence a dragon-fly is without any of its characteristic colours, and is flaccid and weak, the wings (even in those groups in which they are afterwards horizontally extended) being held vertically in a line with the abdomen. By degrees the parts harden, and the insect essays its first flight, but even then the wings have little power and are semi-opaque in appearance, as if dipped in mucilage. In most species of Calopterygina, and in some others, the prevailing colour of the body is a brilliant bronzy green, blue, or black, but the colours in the other groups vary much, and often differ in the sexes. Thus in Libdlula depressa the abdomen of the fully adult male is covered with a bluish bloom, whereas that of the female is yellow ; but several days elapse before this pulverulent appearance is attained, and a comparatively young male is yellow like the female. The wings are typically hyaline, and colourless, but in many species (especially Caloplerygina and Libellulina) they may be wholly or in part opaque and often black, due apparently to gradual oxydization of a pigment between the two membranes of which the wings are composed ; the brilliant iridescence, or metallic lustre, so frequently found is no doubt due to interference the effect of minute irregularities of the surface. and not produced by a pigment. A beauti ful little genus (C helicopter yx] of Caloptcrygina from the Amazon is a gem in the world of insects, the posterior wings being of the most brilliant fiery metallic colour, whereas the anterior remain hyaline. These insects are pre-eminently lovers of the hottest sunshins (a few are somewhat crepuscular), and the most powerful and daring on the wing in fine weather become inert and comparatively lifeless when at rest in dull weather, allowing themselves to be captured by the fingers without making any effort to escape. Many of the larger species (jEschna, &amp;lt;tc.) have a habit of affecting a particular 1 A similar contrivance vras suggested and (if the vriter mistakes not) actually tried as a means of propelling steam -ships. twig or other resting place like a fly-catcher among birds, darting off after prey and making long excursions, but returning to the chosen spot. Mr Wallace, in his Malay Archipelago, states that the inhabitants of Lombock use the large species for food, and catch them by means of limed twigs. They are distributed over the whole world excepting the polar regions, but are especially insects of the tropics. At the present day about 1700 species are known, dispersed unequally among the several sub-families as follows : Agrionina, 490 species; Calopterygina, 170; Gomphina, 210; uEschnina, 150; Corduliina, 100; Libellulina, 580. In Europe proper only 100 species have been observed, and about 46 of these occur in the British islands. New Zealand is excessively poor, and can only number 8 species, whereas they are very numerous in Australia, Some species are often seen at sea, far from land, in colm weather, in troops which are no doubt migratory ; our common Lilellala quadrimaculafa, which inhabits the cold and temperate regions of the northern hemisphere, lias been frequently seen in immense migratory swarms. One species (Pantala flavesccns) has about the widest range of any insect, occurring in the Old World from Kamtchatka lu Australia, and in the New from the Southern States to Chili, also all over Africa and the Pacific islands, but is not found in Europe. The largest species occur in the JEsclmina and Agrionina; a member of the former sub family from Borneo expands to nearly G| inches, and with a moderately strong body and powerful form ; in the latter the Central American arid Brazilian Megaloprepus ccendatns and species of Mecistogaster are very large, the former expanding to nearly 7 inches, and the latter to nearly as much, but the abdomen is not thicker than an ordinary grass-stem and of extreme length (fully 5 inches in Mecistogaster]. Among living entomologists the dragon- flies have received, and are receiving, great attention, especially from the Baron de Selys-Longchamps of Liege, and from Dr H. A. Hagen, formerly of Konigsberg, now of Cambridge, Massachusetts. It is impossible to prepare dragon-flies for the cabinet so as to retain all the brilliant colours the bodies have in life. They are excessively brittle when dry, and in the smaller species it is advisable to run a bristle into the under side of the thorax, pushing it down till it reach the extremity of the abdomen, when the other end can be cut off close to the thorax. But the larger species should be disembowelled through a slit along the under surface of the abdomen, and then filled (but not too tightly) with clean white cotton wool. The colours stand a much better chance of not greatly altering if the insects be not killed until some hours after they are captured, so as to allow the contents of the intestinal canal to be naturally passed away, for it is the decomposition of the food that assists materially to alter or obliterate the colour and markings. Among fossil insects dragon-flies hold a conspicuous position. Not only do they belong to what appears to have been a very ancient type, but in addition, the large wings and strong dense reticulation are extremely favourable for preservation in a fossil condition, and in many cases all the intricate details can be as readily followed as in a recent example. In this country they have been found more especially in the Purbcck beds of Swanage, and the vales of Wardour and Aylesbury, in the Stonesfield Slate series, and in the Lias and Ilhaetic series of the west of England. But the richest strata appear to be those of the Upper Miocene at (Eningen, in the Rhine valley ; the Middle Miocene at lladaboj, in Croatia ; the Eocene of Aix, in Provence ; and more especially the celebrated Secondary rocks furnishing the lithographic stone of Solenhofen, in Bavaria. This latter deposit would appear to have been cf marine origin,