Dec. 5, 1884.] 



♦ KNOWLEDGE 



459 



in their early days, and we left them airing their newly- 

 acquired wings just after exclusion from the pupa or 

 nymph-case. In general aj)pearance a dragon-fly is pro- 

 bably as well known and as easily recognised as any insect. 

 The glistening aspect of the four ample wings, the length 

 and slenderness of body, the brilliancy of colour, the vigour 

 of flight, are all characteristics which even careless observers 

 must have noticed again and again, and which, once seen, 

 are not easily forgotten. 



We have about fifty British species of these insects. 

 Some of them are popularly called Horse-stingers, a 

 sobriquet which, suggested no doubt by the long, narrow 

 body and bold and ferocious aspect of the insects, and 

 apparently confirmed by their writhing contortions when 

 seized, and their attempts to reach with their tails the 

 hand that seizes them, is nevertheless an instance of most 

 absurd misnaming. They have no power whatever of 

 stinging, anj- more than any other of the Neuroptera, the 

 members of which order are, so far as any such power is 

 concerned, the most harmless set of insects imaginable. 

 The body is very generally narrow and cylindrical, but not 

 always so ; there are a few instances in whicli it is broad 

 aud somewhat flattened, and at the same time propoi-- 

 tionately shorter than usual. It is at its broadest in the 

 genus Plateirum, the single species of which — P. depressum 

 — with its pale blue (male) or yellow (female) body, is one 

 of the most familiar of the whole group. Though so long, 

 the body does not consist of a larger number of segments 

 than usually compose this part of an insect ; tut, just as 

 there is the same number of vertebras in the long neck of 

 the giraflfe as in the short one of the elephant, so the 

 linear extension of the body of the dragon-fly is produced, 

 not by a multiplication of the number of the joints, but by 

 their individual elongation. To so great an extent is this 

 carried that in one exotic species we find a body of 6 in. 

 long, though the wings are only of ordinary size. 



No doubt these long bodies are serviceable in guiding the 

 creatures in their flight, and they also render possible the 

 deposition of the eggs in suitable positions at some distance 

 below the surface, without the disagreeable expedient of a 

 dive on the part of the mother, though, in some cases, 

 maternal solicitude does not hesitate even to riin this risk, 

 if the supreme end cannot otherwise be attained. West- 

 wood says that he has also seen the females beat their 

 tails upon the surface of the water with rapid succession, 

 till the eggs form a ma«s like a bunch of grapes ; and another 

 author records having seen them beating the sand close by 

 the edge of the water, an operation which, it has been sug- 

 gested, ma}^ have for its object rather the covering of eggs 

 already laid, than the actual laying of them. 



The thorax has a great vertical diameter, and thereby 

 acquires a hunchbacked appearance ; this is, of course, 

 necessary for the proper lodgment of the powerful muscles 

 that move the wings. The wings themselves are remark- 

 able for the enormous number of reticulations with which 

 they are covered. There are a few main nervures branch- 

 ing out from the base of the wing, and running more or 

 less longitudinally, and the spaces between these are divided 

 into a vast number of minute four-, five-, or six-sided 

 spaces, which are smallest at the tip and hinder edge. The 

 number of meshes is smallest, and the individual meshes 

 largest, in the smallest and most delicate and slender 

 species, the Agiionidre, while the Calopterygidae — i.e., those 

 whose wings are dark metallic blue or green — have the 

 greatest number of reticulations, extending to some l,-500 

 or more on each wing. 



The head is difiereutly shaped in different families, but 

 tl e huge masses of compound eyes almost always occupy a 

 very large proportion of its surface. These insects are the 



keenest possible of hunters, relentlessly pursuing to the 

 death any creature on whose tender body they have set 

 their gastronomic affections, and hence their remarkably 

 good eyesight, which, by giving them a wide range of 

 vision, enables them easily to descry their prey in the first 

 instance, and also to follow it in all the intricacies of its 

 movements, dodging it from place to place till it is hunted 

 down. There are few more beautiful objects in the whole 

 range of the insect world than the eye of a liviiig dragon- 

 fly, such as that which is often seen hawking about over 

 ponds or down the neighbouring woody glades, the 

 great ^Echna cyanea, one of the largest and commonest 

 of our native species, with clear wings, and body 

 prettily S[jotted with green and blue on a black ground. 

 Apart from the mere glistening of the surface, which 

 is in itself no slight adornment, there is a marvellous 

 play of colours in the interior as the light falls at 

 diflerent angles that altogether baffles description, and 

 must be seen to be appreciated ; unfortunately, no trace of 

 these beauties can be preserved after death, and the eyes 

 become little more than opaque brown masses, retaining 

 only the most shadowy relics of their former brilliancy. 

 The nature of the exterior, however, can be seen better in 

 the dead than in the living insect Very little magnification 

 is necessary to show that the surface is not uniform, but is 

 broken up into an enormous number of extremely minute 

 spaces, which greater magnification shows to be of hexagonal 

 form. The chitinous skin that covers the whole body of 

 the insect is continued over the eye as a transparent layer, 

 or — to use the terminology of vertebrate anatomy — cornea, 

 and it is this that is divided into hexagonal facets, each 

 one of which is generally somewhat convex on both its 

 external and internal surfaces, and contains in and beneath 

 itself all that is necessary to produce complete vision, and 

 this is what is meant when it is said that an insect's eye is 

 compound. But it no more follows from this arrangement 

 that an insect is gifted with multiple vision than that we 

 ourselves with our two eyes see double. The eyes in 

 our present insects form two rounded surfaces, which 

 occupy the whole of the sides of the head, and often 

 even meet above on the middle of the crown, thus 

 extending all round the head from one angle of 

 the mouth to the other. From that part of the 

 nervous system which, in an insect, to some extent does 

 duty for a brain, passes the optic nerve on each side into 

 the centre of the rounded space thus enclosed, and there 

 expands into a broad knob, fi-om which delicate threads, 

 surrounded with dark colouring matter, ai^d ending in 

 inverted cones of a highly-refracting medium, pass to the 

 inner edge of the facetted parts of the cornea. The latter 

 act as lenses, and produce the necessary convergence of the 

 rays of light, and thus at every point of this vast surface 

 light can be received and utilised for the production of 

 vision. What the exact nature of the image produced, 

 what the degree of accuracy with which it is perceived, who 

 shall attempt authoritatively to decide'? though it can 

 scarcely be doubted that vision is of a high order, else the 

 insect would hardly be able so readily and speedily to adapt 

 its own movements to those of the quarry it pursues. 



In the centre of the head, at the inner edge of the 

 eye-masses, are two apparently bristle-like, but really 

 jointed, appendages, which are all the insect possesses in 

 the way of antennw, and they are so inconspicuous as easily 

 to escape notice altogether. Just below these, the c^ ntre 

 of the head bulges out into a rounded protuberance, below 

 which again are placed the organs of the mouth. These 

 are very complicated ; two flaps open above and below, and 

 then the jaws proper are seen working laterally. They are 

 armed with sharp-pointed and powerful teeth, which easily 



