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.
Without wings. Wingless insects.
Parasita (Latreille.) Lice (Pediculus).
Bushy-tailed. Spring-tails (Lepismenae).
(770). Insects, examined generally, differ from all other articulated beings in one remarkable circumstance: they are capable of flight, and can maintain themselves in the air by means of wings. It is true, indeed, that some species are met with, in all the orders described above, which are apterous, being destitute of such organs; but these form exceptions, to be noticed hereafter. Such a mode of progression, through so rare a medium as that of the atmosphere, necessarily demands an exercise of muscular power of the most vigorous and active description, and a correspondent strength and firmness in the skeleton upon which the muscles act. It is sufficient to cast a glance at the external construction of any of the Annelidans or Hyriapoda which have come under our notice, to be convinced that in such animals flight would be impossible under any circumstances. Their long and flexible bodies present no point to which efficient wings could be appended; neither is any part of their divided skeleton possessed of sufficient strength to support the action of muscles so forcible and energetic as would be indispensable to wield the instruments used in flying, or raise the body above the surface of the ground.
Fig. 148. Metamorphoses of Lepidoptera.
Fig. 149. Metamorphoses of Melolontha.
(771). Similar changes, therefore, to those which we found requisite in order to convert the aquatic Annelid into the terrestrial Myriapod, must be still further carried out before the animals last mentioned could be adapted to become inhabitants of the air. The number of segments composing their elongated bodies must be materially reduced; certain parts of the skeleton must be strengthened in order to sustain the efforts of muscles sufficiently strong to raise the weight of the animal; and, in the last place, the nervous ganglia, by a like concentration of hitherto separated parts, must be gathered into masses of increased power, sufficient to animate the more vigorous muscles with which they are in relation.
(772). Such changes are precisely those which are most remarkable when we compare the external appearance of a centipede with that of a winged insect: the entire number of segments, and consequently the proportionate length, of the latter is obviously reduced. The head is seen to be more distinct from the rest of the body, to which it is connected by a moveable joint. The three anterior segments of the trunk become largely developed, and, from the density of their substance, form by far the strongest part of the skeleton, constituting what is called the thorax of the insect; they are, moreover, generally united together, especially the two posterior, so as to be consolidated, as it were, into one piece; and to these rings only, the organs of locomotion are appended. The remaining segments of the body are much less firm in their texture, especially in insects with hard or horny wing-covers, in which indeed they are almost of a membranous consistence, so as to increase, as far as possible, the lightness of the animal in parts where strength is not required.
Here, then, is an annulose skeleton adapted to flight; dense and unyielding where support is required for the attachment of the locomotive organs, but thin and flexible elsewhere.
(773). The above conditions being required in the arrangement of the pieces which compose the outward framework of the body in insects, we may easily conceive that the mode of union between the various segments above described is by no means a matter of indifference, inasmuch as very different degrees of motion are required between the individual rings. In the Annelida and Myriapoda a very simple kind of junction was sufficient; for in them the segments were all united by the mere interposition of a thinner coriaceous membrane extending between their contiguous margins; but in insects several kinds of articulation are met with in the construction of the trunk, adapted to the mobility of different regions.
(774). The first mode of connexion is effected by suture, or rather by a species of "harmony," as it is technically termed by anatomists; two plates of the skeleton being accurately and immoveably fitted to each. other, but without being decidedly fastened together by serrated edges. This kind of junction is met with in the thorax, and serves an important purpose; for at the point of union both plates are bent inwards, and prolonged internally, so as to form numerous partitions and processes, from which the muscles moving the wings and legs derive extensive origins.
(775). A second means whereby the pieces of the thorax are fastened together is by symphysis, in which a somewhat soft membrane is interposed between two plates, so as to admit of a slight degree of motion.
(776). More extensive movement is required between the pieces which compose the abdomen; for in this region, that rigidity and firmness which are essential in the construction of the thorax would be highly disadvantageous, inasmuch as the abdominal viscera must be subject to constant variations in bulk, caused either by food taken into the intestines, or, in the case of the female, by the development of the eggs after impregnation. The rings of the abdomen are therefore united by a membrane passing from one to another, - but so loosely, that the edges of the individual plates wrap over each other to some extent, and thus may be separated by the slightest pressure from within.