198 LECTURE XVI. 



ing. Now what are these " limber fans," that give the little articulate 

 animals such command over aerial space ? I do not mean their 

 structure or composition ; the anatomical question has been already 

 answered. I do not ask for their analogy ; that is rightly expressed 

 by their common name ; they have the same relation to the insect as 

 instruments of motion, which the feathered wing bears to the bird. 

 But what is their essential nature, or with what are the wings of the 

 insect homologous ? Are they modified anterior limbs, like the wings 

 of bats and birds and flying-fishes ? Not so, for they co-exist with and 

 are superadded to the jointed anterior pair of legs. Are they such 

 expansions as form the parachute of the little dragon {Draco volans)? 

 These do, indeed, co-exist w ith arms and legs, but they consist of a 

 fold of integument stretched out upon elongated and straightened 

 ribs, which are appendages of a vertebral column. But an insect has 

 no vertebral column, no true internal skeleton. The strong and nu- 

 merous nervures which sustain the thin alar membranes of the Libel- 

 lula are articulated processes of the external chitinous tegument. 



A circulation can be traced through these membranes, at least in 

 their early and softer state ; air-vessels are abundantly spread over 

 the supporting frame-work: the wings of the Lepidoptera appear 

 after the third moult, as tegumentary flattened vesicles, soft, and per- 

 meated by tracheae, and when fully expanded in the imago, they must 

 still take their share in the business of respiration. Nay, it has been 

 found that the rudimental wings of the pupse of certain water insects 

 are the gills of such ; they perform the same function as the very 

 similar membranous and vascular tegumentary expansions in certain 

 Anellides (see ^^. 68. p. 130.) ; which expansions are developed, as 

 in the larvae of the Ephemera, from the tergal arch of the segment, 

 and co-exist with rudimental legs from the ventral arch of the same 

 segment. 



Well, therefore, has the deep-thinking Oken * called the wings of 

 insects " aerial gills ; " they are, in fact, the homologues of the tergal 

 branchiae of the vermiform Articulata, raised to a higher function in 

 corellation with a generally transmuted state of the rest of the 

 organisation, which is advanced to the utmost perfection of which 

 the Articulate type of structure is susceptible. And have we not 

 already seen the membranous aliform branchiae of the beetle pro- 

 tected, like the gills of the lobster, by an elytral carapace developed 

 from a more advanced segment ? Have we not likewise found the 

 metamorphosed branchial wings of the Pterophora subdivided length- 

 wise like the tufted tergal gills of the Nereis ? 



* Natur. Philosophic, 2d Ed. p. 418. 



