This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(800). The above observations relate only to the general disposition and connexion of the different parts of the skeleton, and locomotive appendages connected with it; it remains for us now to speak more fully of the texture of the external integument, and those modifications which it presents, adapting it to various purposes.
(801). The hard covering of an insect, like the skin of vertebrate animals, consists of three distinct layers. The outer stratum, or epidermis, is smooth, horny, and generally colourless, so that it forms a dense inorganic film spread over the whole surface of the body. Immediately beneath the epidermis is a soft and delicate film, the rete mu-cosum, which is frequently painted with the most lively hues, and gives the characteristic colouring to the species. The third and principal layer is the true skin, or cutis, which is generally of a leathery texture, and, especially in the elytra of Beetles, of considerable thickness: this layer is abundantly supplied with nutritive juices; and in its substance the bulbs of hairs, scales, and similar appendages, to be described hereafter, are imbedded and nourished.
(802). The wings are mere derivations from this common covering, and are composed of two delicate films of the epidermis, stretched upon a strong and netlike framework. Every membranous wing is, in fact, a delicate bag formed by the epidermic layer of the integument, and in the recently-developed insect can be distinctly proved to be such by simply immersing the newly-escaped imago in spirit of wine, which gradually insinuates itself between the still fresh and soft membranes, and, filling the cavity enclosed between them, distends the organ until it represents a transparent sacculus, in which the ribs or nervnres of the wing are enclosed*. This structure, however, is only to be displayed while the wings, after being withdrawn from the pupa-case, are still soft and moist; for they soon become so intimately united with the horny framework upon which they are extended, that they seem to form a single membranous expansion.
(803). The ribs, or nervures, whereby the two plates of the wing are thus supported, are slender hollow tubes, filled with a soft parenchyma: in the interior of some, Burmeister detected an air-vessel (recognizable by the texture of its walls) and a minute nervous filament.
(804). We have still, in order to complete our description of the external anatomy of an insect, to describe certain appendages which not unfrequently clothe the exterior of the skeleton, and exhibit great diversity of appearance in different tribes. These may be divided into spines, hairs, and scales; and, however much they may appear to be distinct structures, all these are essentially very nearly related to each other.
(805). The spines are horny processes developed from the epidermis; and sometimes, especially in the Coleopterous order, as in some lamel-licorn Beetles, exhibit considerable dimensions. These spines are sometimes bifurcated or branched; but, whatever their shape or size, they never grow from bulbs implanted in the cutis, but are mere prolongations of the exterior layer of the integument.
(806). The hairs appear to resemble those of quadrupeds in their mode of growth, inasmuch as they are secreted from roots imbedded in the substance of the cutis or true skin: they are fine horny cylinders, and frequently are found to be branched and divided like the feathers of birds; but the manner of their formation will be more conveniently discussed hereafter.
(807). The wings of the Lepidoptera are covered with minute flat scales of various shapes, and not unfrequently tinted with the most beautiful colours; such scales, nevertheless, are in reality only flattened hairs,into which, indeed, they frequently degenerate by insensible transitions; and, moreover, they grow from bulbs of precisely similar construction. The variety of colours exhibited by the scales of a Butterfly depends upon a film of pigment interposed between the two plates of transparent epidermic matter forming each; but the gorgeous hues derived from this source must not be confounded with the iridescent tints for which they are not unfrequently remarkable, as these have a very different origin: the surface of every scale, that with the changing light reflects evanescent prismatic colours, is seen, when examined under a microscope, to be marked with regular parallel striae of exquisite minuteness; and such a surface, even when grossly imitated by human art, has been found to give rise to the brilliant appearances exhibited by polarized light.
* Heusinger, System der Histologie, 2. Heft. Burmeister, op. cit. p. 224.
(808). The muscular system of insects has always excited the wonder and astonishment of the naturalist, in whatever point of view he examines this part of their economy - whether he considers the perfection of their movements, the inconceivable minuteness of the parts moved, or the strength, persistence, or velocity of their contractions. Insects are proverbially of small comparative dimensions - "minims of nature," ....."that wave their limber fans.
For wings, and smallest lineaments exact.
In all the liveries deck'd of summer's pride;" their presence, indeed, around us is only remarked as conferring additional life and gaiety to the landscape; and except when, by some inordinate increase in their numbers, they make up by their multitude for their diminutive size, the ravages committed by them are trifling and insignificant. Far otherwise, however, would it be if they attained to larger growth, and still possessed the extraordinary power with which they are now so conspicuously gifted; they would then, indeed, become truly the tyrants of the creation - monsters such "as fables never feigned or fear conceived " - fully adequate to destroy and exterminate from the surface of the earth all that it contains of vegetable or of animal life.