* Kirby and Spence, Introd, to Ent. vol. ii. p.362.

Metamorphoses of Dytiscus.

Fig. 152. Metamorphoses of Dytiscus.

(792). The same principles are carried out even more perfectly in the construction of the swimming-legs of the Water-boatman (Notot-necta), a kind of water-bug. The resemblance of this creature (fig. 144, g, h) to a boat with its oars cannot escape the most inattentive examiner; and the similarity is still further increased by its manner of swimming; for, as it preys upon insects that have been accidentally drowned by falling into the water, it usually rows itself about upon its back, because in such a position it can best watch for its victims.

(793). The wings of insects, when present, are invariably attached to the two posterior segments of the thorax, which, as we have already seen, are strengthened in every possible manner, so as to afford a support of sufficient density and firmness to sustain the violent exertions of the muscles inserted into the organs of flight.

(794). In the most perfectly organized families the wings are four in number, as in the Neuroptera (fig. 146), the Hymenoptera (fig. 173), the Orthoptera (fig. 145), the Dictyoptera, the Hemiptera (fig. 144), the Lepidoptera (fig. 148), and the Coleoptera (fig. 149).

(795). In the Dipterous insects there are only two wings, which are fixed upon the central segment of the thorax; while, in the position usually occupied by the posterior pair, we find a pair of pedunculated globular bodies, generally named the halteres or poisers, as in the Gnat (fhdex) (fig. 177, r).

(796). But, in every one of the orders above enumerated, there are certain families which, throughout the whole period of their existence, are never provided with wings at all; and these by many entomologists have been formed into an order by themselves, under the name of Apterous insects. In the opinion of Burmeister*, whose classification we have adopted, such an arrangement is purely artificial, inasmuch as it must embrace insects of most dissimilar kinds. In proof of this, he adduces the fact that, in the same family, we not unfrequently meet with both winged and apterous species, nearly related to each other; and in many cases the males possess wings, while the females of the same insect are entirely destitute of such appendages. In such cases, the metamorphosis is necessarily what is called incomplete, inasmuch as the organs which characterize the perfect state are not developed. Thus, in the Flea (Pulex irritans) (fig. 153) the wings never become apparent, and in consequence the thorax, even in the imago state, does not exhibit that development and consolidation of its parts invariably met with in winged genera. The Flea, however, cannot on this account be looked upon as any other than the imago or complete insect, for it will be found to have undergone all the preparatory changes. The Flea, when it issues from the egg, is in fact a worm-like and footless larva, in which condition it lives about twelve days. When about to become a pupa, it spins for itself a little silky cocoon, wherein it conceals itself until, iiaving thrown off its last skin, it appears in its mature form, deprived indeed of wings, that, under the circumstances in which it lives, would be useless appendages, but still, with this exception, corresponding in every particular with other insects in their imago state.

* Manual of Entom. p. 623.

Pulex irritans.

Fig. 153. Pulex irritans.

(797). The wings of insects differ much in texture. In the Neuro-ptera, by far the most powerful fliers met with in the insect world, all four wings are of equal size, and consist of a thin membranous expansion of great delicacy and of a glassy appearance, supported at all points by a horny network (fig. 146.) Few things are met with in nature more admirable than these structures; they present, indeed, a combination of strength and lightness absolutely unequalled by anything of human invention; and as instruments of flight they far surpass the wings of birds, both in the power and precision of their movements, inasmuch as these insects can fly in all directions - backwards, or to the right or left, as well as forwards. Leeuwenhoek* narrates a remarkable instance in which he was an eye-witness of the comparative capabilities of the Dragon-fly and the Swallow, as relates to the perfection of their flight. The bird and the insect were both confined in a menagerie about a hundred feet long, and apparently their powers were fairly tested. The swallow was in full pursuit; but the little creature flew with such astonishing velocity, that this bird of rapid flight and ready evolution was unable to overtake and entrap it, - the insect eluding every attempt, and being generally six feet before it. "Indeed," say the authors from whom we quote the above anecdote 1, "such is the power of the long wings by which the Dragon-flies are distinguished, and such the force of the muscles which move them, that they seem never to be wearied with flying.

I have observed one of them (Anax imperator, Leach) sailing for hours over a piece of water - sometimes to and fro, and sometimes wheeling from side to side, and all the while chasing, capturing, and devouring the various insects that came athwart its course, or driving away its competitors - without ever seeming tired or inclined to alight".

* Epist. 6, Mart, 1717. 1 Kirby and Spence, op. cit. vol. ii. p. 351.

(798). In Hymenopterous insects (figs. 171 and 173) the wings are much more feebly organized, but their structure is similar. The nervures, or horny ribs supporting the membranous expansion, are comparatively few; and in the Diptera they are still less numerous.

(799). In several orders the anterior pair of wings are converted into shields for the protection of the posterior; such is the case in the Ortho-ptera, many of the Hemiptera, and more especially in the Coleopterous genera. In the latter, indeed, they are very dense and hard; and, being nearly unserviceable in flight, the hinder pair are necessarily developed to such a size as to present a very extensive surface (fig. 149, a), and when in repose are closely folded up beneath the elytra, and thus carefully preserved from injuries, to which they would be constantly exposed without such provision for their security.