The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly,[[27]] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps;[[28]] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum.
With these facts before us we may speculate as to the probable origin of the wings of insects. The views held by some are those of Gegenbaur, also adopted by Lubbock, and originally by myself.[[29]] According to Gegenbaur:
“The wings must be regarded as homologous with the lamellar tracheal gills, for they do not only agree with them in origin, but also in their connection with the body, and in structure. In being limited to the second and third thoracic segments they point to a reduction in the number of the tracheal gills. It is quite clear that we must suppose that the wings did not arise as such, but were developed from organs which had another function, such as the tracheal gills; I mean to say that such a supposition is necessary, for we cannot imagine that the wings functioned as such in the lower stages of their development, and that they could have been developed by having such a function.”
Fig. 158.—Changes in external form of the young larva of Calotermes rugosus, showing, in A and B, the mode of origin of the wing-pads: A, newly hatched, with 9 antennal joints, × 8. B, older larva, with 10 joints, × 8. C, next stage, with 11 joints, × 8. D, larva, with twelve joints; the position of the parts of the alimentary canal are shown: v, crop; m, stomach; b, “paunch”; e, intestine; r, heart, × 16
3.—After Fritz Müller, from Sharp.
If we examine the tracheal gills of the smaller dragon-fly (Agrion), or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out;[[30]] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. The latter are only developed in the aquatic representatives of the Neuroptera and Pseudoneuroptera, and are evidently adaptive, secondary, temporary organs, and are in no sense ancestral, primitive structures from which the wings were developed. There is no good reason to suppose that the aquatic Odonata or Ephemerids or Neuroptera were not descendants of terrestrial forms.
To these results we had arrived by a review of the above-mentioned facts, before meeting with Fritz Müller’s opinions, derived from a study of the development of the wings of Calotermes (Fig. 158). Müller[[31]] states that “(1) The wings of insects have not originated from ‘tracheal gills.’ The wing-shaped continuations of the youngest larvæ are in fact the only parts in which air tubes are completely wanting, while tracheæ are richly developed in all other parts of the body.[[32]] (2) The wings of insects have arisen from lateral continuations of the dorsal plates of the body-segments with which they are connected.”
Now, speculating on the primary origin of wings, we need not suppose that they originated in any aquatic form, but in some ancestral land insect related to existing cockroaches and Termes. We may imagine that the tergites (or notum) of the two hinder segments of the thorax grew out laterally in some leaping and running insect; that the expansion became of use in aiding to support the body in its longer leaps, somewhat as the lateral expansions of the body aid the flying squirrel or certain lizards in supporting the body during their leaps. By natural selection these structures would be transmitted in an improved condition until they became flexible, i.e. attached by a rude hinge-joint to the tergal plates of the meso- and metathorax. Then by continued use and attempts at flight they would grow larger, until they would become permanent organs, though still rudimentary, as in many existing Orthoptera, such as certain Blattariæ and Pezotettix. By this time a fold or hinge having been established, small chitinous pieces enclosed in membrane would appear, until we should have a hinge flexible enough to allow the wing to be folded on the back, and also to have a flapping motion. A stray tracheal twig would naturally press or grow into the base of the new structure. After the trachea running towards the base of the wing had begun to send off branches into the rudimentary structure, the number and direction of the future veins would become determined on simple mechanical principles. The rudimentary structures beating the air would need to be strengthened on the front or costal edge. Here, then, would be developed the larger number of main veins, two or three close together, and parallel. These would be the costal, subcostal, and median veins. They would throw out branches to strengthen the costal edge, while the branches sent out to the outer and hinder edges of the wings might be less numerous and farther apart. The net-veined wings of Orthoptera and Pseudoneuroptera, as compared with the wings of Hymenoptera, show that the wings of net-veined insects were largely used for respiration as well as for flight, while in beetles and bees the leading function is flight, that of respiration being quite subordinate. The blood would then supply the parts, and thus respiration or aëration of the blood would be demanded. As soon as such expansions would be of even slight use to the insect as breathing organs, the question as to their permanency would be settled. Organs so useful both for flight and aëration of the blood would be still further developed, until they would become permanent structures, genuine wings. They would thus be readily transmitted, and being of more use in adult life during the season of reproduction, they would be still further developed, and thus those insects which could fly the best, i.e. which had the strongest wings, would be most successful in the struggle for existence. Thus also, not being so much needed in larval life before the reproductive organs are developed, they would not be transmitted except in a very rudimentary way, as perhaps masses of internal indifferent cells (imaginal discs), to the larva, being the rather destined to develop late in larval and in pupal life. Thus the development of the wings and of the generative organs would go hand in hand, and become organs of adult life.[[33]]
The development and structure of the tracheæ and veins of the wing.—The so-called veins (“nervures”) originate from fine tracheal twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing.
