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.
(750). The word Insect has at different times been made use of in a very vague and indeterminate manner, and applied indiscriminately to various articulated animals 1. In the restricted sense in which we now use it, we include under this title only such of the Homo-gangliata as in their perfect or mature state are recognizable by the following characters, by which they are distinguished from all other creatures.
(751). The body, owing to the coalescence of several of the segments which compose their external skeleton, is divided into three principal portions - the Head, the Thorax, and the Abdomen. The Head contains the oral apparatus and the instruments of the senses, including the antennae or feelers, which are articulated organs presenting great variety of shape, but invariably only two in number. The Thorax, formed by the union of three segments of the skeleton, supports six articulated legs, and sometimes four or two wings; these last, however, are frequently wanting. The Abdomen is destitute of legs, and contains the viscera connected with nutrition and reproduction.
(752). But insects, before arriving at that perfect condition in which they exhibit the above-mentioned characters, undergo a series of changes, both in their outward form and internal structure, which constitute what is generally termed their metamorphosis. When this is complete, as for example in the Butterfly, the insect, after leaving the egg, passes through two distinct states of existence before it arrives at maturity and assumes its perfect form. The female butterfly lays eggs which, when hatched, produce, not butterflies, but caterpillars - animals with elongated wormlike bodies divided into numerous segments, and covered with a soft coriaceous integument (fig. 143, a.) The head of the caterpillar is provided with horny jaws and several minute eyes: the legs are very short, - six of them, which are attached to the anterior rings, being horny and pointed, while the rest, of variable number, appended to the posterior part of the body, are soft and membranous. The caterpillars, or larvae*, live for some time in this condition, and frequently change their skin as they increase in size, until at length, the last skin of the larva being thrown off, the animal presents itself in quite a different form, enveloped in an oblong case, without any external limbs, and almost incapable of the slightest motion - resembling rather a dead substance than a living creature; it is then called a chrysalis, nymph, or poupa1 (fig. 148, b).
* Vide Cyclop, of Anat. and Phys., art. "Generation, Organs of" (Comp.Anat.). 1 The word Insect, derived from the Latin word Insecta, simply means divided into segments.
(753). On examining attentively the external surface of this pupa, we may discern, in relief, indications of the parts of the Butterfly concealed beneath it, but in a rudimentary condition. After some time the skin of the pupa bursts, and the imago, or perfect insect, issues forth, moist and soft, with its wings wet and crumpled; but in a few minutes the body dries, the wings expand and become stiff, and, from being a crawler upon the ground, the creature is converted into a gay and active denizen of the air (fig. 148, c).
(754). Such is the progress of the metamorphosis when complete; but all insects do not exhibit the same phenomena. Those genera which, in their mature condition, have no wings, escape from the egg under nearly the same form as they will keep through life; these form the Insecta Ametabola2 of authors: and even among those tribes which, when perfect, possess instruments of flight, the larva frequently differs from the complete insect only from its wanting wings, and the pupa is recognizable by being possessed of these organs in an undeveloped or rudimentary state: an example of this is seen in the House-cricket (fig. 145), in which a represents the imago; b, the pupa; c, the full-grown larva; d, the young just hatched; and e, the eggs.
(755). The extensive class of Insects has been variously arranged by different entomologists, and distributed into numerous orders §. Among § the classification of Insects here given is that of Burmeister, which we select the different systems which have been given, we select the following as best calculated to render the reader acquainted with the transformations, as well as the principal forms, to which allusion will be made in subsequent pages.
* So called, by Linnseus, because in this condition the perfect form of the insect is concealed as it were under a mask. Larva, Lat., a mask.
1 The first two of these names are purely fanciful: the last is derived from pupa, a baby wrapped up in swaddling bands.
2 A, without:
The larva resembles the perfect insect, but is without wings. The pupae of such species as have wings in their imago state possess rudiments of those organs. The pupa runs about and eats.
A. With sucking mouths composed of four fine setae lying in a sheath.
In such insects of this order as possess wings, which when present are always four in number, the anterior or upper pair are generally coriaceous in their texture for one-half of their extent, while the posterior portion is thin and membranous, - a circumstance from which the name of the order is derived. The Notonecta, or Water-boatman (fig. 144), is a familiar example: c and d represent immature, and f mature, larvae. The pupa (g, h) differs little in outward form from the perfect insect (e), but possesses only the rudiments of wings.
Fig. 144. Notonecta.
β. Having mouths furnished with jaws, or distinct mandibles and maxillae.
In this order the perfect insect possesses four wings, the posterior pair being the largest; and when at without giving any opinion as to its relative merits compared with others adopted by different entomologists, but simply as being most convenient for our present purpose. (Manual of Entomology, translated from the German of Dr. Hermann Burmeister by W. E. Shuckard. 8vo. 1836).