INSECTA. 



925 



They afford strength and lightness to the wings, 

 with which they are in direct communication 

 like the bones in the wings of birds, although 

 the organs themselves in these different classes 

 are not analogous. Dr. Leach formerly desig- 

 nated these solidified tracheae in the wings of 

 insects Pterigostia, or wing bones, a name 

 that seems appropriate, both on account of 

 its convenience and as being indicatory of their 

 principal function, although it has sometimes 

 been objected to as incorrect on account of 

 their forming part of the respiratory system. 

 But it may be remarked that the true bones in 

 the wings of birds also communicate with the 

 respiratory organs, and perform functions simi- 

 lar to these in insects, while the interesting fact 

 noticed by Odier, that in their solidified con- 

 dition they are composed of the same kind of 

 earthy matter as that which enters into the 

 composition of other parts of the skeleton, is 

 sufficient to warrant us in retaining the designa- 

 tion. There appears to be no part of the body 

 in vertebrata analogous to the wings of insects, 

 except, perhaps, in the single instance of one 

 of the Saurian reptiles, Draco volans, in which 

 a pair of supernumerary organs to assist in 

 locomotion are developed from the sides of the 

 body, and which are formed by the ribs, 

 directed horizontally outwards and covered only 

 by the skin. We have thus in appearance the 

 remains in one class of the vertebrata of a con- 

 dition which is permanent in another class in 

 the invertebrata, which resemble them in their 

 general form and metamorphoses. In every 

 instance, then, the wings of an insect, like 

 these appendages of the thorax in the reptile, 

 are perfectly distinct in their origin from the 

 proper organs of locomotion ; they have their 

 normal condition in the lower invertebrata in 

 the superior branchial tufts of the Annelides, 

 and, consequently, are not more analogous to 

 the wing of the Bat, as they have recently been 

 supposed,* than to that of the Bird. We have 

 already seen that the full developement of the 

 wings takes place at the last change of the 

 insect, but it is commenced in the earlier 

 periods of the larva state, in which, with Oken 

 and Carus, we have detected these organs in 

 their most rudimentary condition. They are 

 distinctly seen on the second or third day after 

 the insect has assumed its last larva covering, 

 before changing to the pupa. They are then 

 scarcely so large as the head of a moderate 

 sized pin, and appear like newly-formed folded 

 portions of delicate tegument, extensively sup- 

 plied with ramifications of minute air-vessels, 

 derived directly from the principal tracheae. 

 They are at that time situated immediately 

 beneath the external covering, at the inferior 

 part of the sides of the meso- and meta-thoracic 

 segments, and continue to increase in size 

 during the growth of the larva. When the 

 insect has discontinued to feed, about a day 

 before changing into the pupa state, and the 

 new skin of the future pupa is nearly completed 

 beneath that of the larva, these rudiments of 

 the wings have become so much enlarged that 



Mod. Clas. Ins. vol. i. p. 11. 



their existence is distinctly indicated by the 

 swollen appearance of the segments. It is at 

 this period of the larva state that they were 

 formerly discovered by Swammerdam.* At 

 the moment of fissuring the skin of the larva, 

 they are suddenly somewhat enlarged, and 

 when the skin has been cast off, and the delicate 

 parts of the newly exposed naked pupa are 

 beginning to be agglutinated together and folded 

 upon each other previously to becoming solidi- 

 fied to form the strong pupa case, they again 

 acquire a considerable increase of size, owing 

 to the extension and enlargement of the tracheal 

 vessels within them, together with a corres- 

 ponding increase in the quantity of the fluids 

 in the circulatory canals, by which they are 

 every where accompanied. The wings are then 

 expanded so as to cover the whole under-sur- 

 face of the thorax and limbs, and when the 

 insect subsequently bursts from the pupa case 

 and is assuming the perfect state, they are 

 again suddenly enlarged, and acquire their full 

 expansion through the recurrence of similar 

 phenomena. 



It is thus evident that the wings are formed 

 from extensive ramifications of vessels inclosed 

 between two membranes, which are continuous 

 with, and are expanded portions of, the com- 

 mon tegument. In many instances, as in 

 Neuroptera, they are perfectly naked, or are 

 covered only with a few scattered hairs, as in 

 Hymenoptera. But in others they are densely 

 covered with peculiar cuticular developments in 

 the form of flattened scales, closely imbricated 

 upon each other, and inserted each by a little 

 footstalk or quill into little spaces in the exter- 

 nal membrane.f In other instances, as in the 

 Coleoptera, the anterior pair become solidified 

 and adapted to a new function, but are then 

 entirely useless as organs of flight. They serve 

 as covers to protect the posterior pair, which, 

 in a state of rest, are carefully folded beneath 

 them ; and when these are entirely absent, as 

 in some of the Tenebrionida, the anterior pair 

 become united together and form a strong 

 covering for the abdomen. Now we have seen 

 that the solidification of the tracheae alone 

 affords sufficient strength to the membranous 

 wings, which are always employed as organs 

 of flight, and that the earthy matter by which 

 they are consolidated is similar to that which is 

 the means of consolidating other parts of the 

 dermo-skeleton. It is by the deposition of a 

 greater quantity of the same kind of earthy 

 matter, not alone in the tracheae, but throughout 

 the whole substance of the wings, that the 

 anterior pair in Coleoptera are rendered entirely 

 useless, by their rigidity, as organs of flight, 

 and at the same time are made to assume a 

 new form and office, and become the means of 

 protecting the posterior pair, in those insects 

 whose habits might otherwise expose these 

 necessarily light and delicate organs to occa- 

 sional injury. This modification of structure, 

 then, in the form of elytra, consists simply in 

 the solidification, or, if we may venture so to 



* Biblia Natura, Tab. xxxv. fig. II. e. 



t Dr. Roget's Bridgewater Treatise, vol. i. p. 354. 



