Qi Mr Carpenter on Unit?/ of Function 



formed me, however, about two years ago, that he had wit- 

 nessed it in many perfect insects, especially the common house- 

 fly, if examined sufficiently soon after its emersion ; and I have 

 since had several opportunities of confirming his observations, 

 which have, I believe, been presented to the Royal Society. 

 The truth appears to me to be, that the circulation goes on as 

 long as the wing continues to grow, but ceases when it has ar- 

 rived at its full size ; and this view coincides with the fact that 

 slight injuries of the wing are not repaired in adult insects. 

 The question has been much agitated, whether the circulation 

 takes place in distinct vessels, or whether the fluid passes along 

 the interstices of the membranes forming the wings. Analogies 

 would certainly lead to the belief that distinct vessels exist ; 

 and it is easy to explain the difficulty of detecting them in the 

 wing after it has become dry, from the fact that when no longer 

 distended by fluid, they collapse and become shrivelled, so that 

 a transverse section of a rib shews only one canal, that of the 

 trachea, which is kept open by its elastic spiral filament. Mr 

 A. Pritchard of London, pointed out to me a few months ago, 

 however, a wing in which three tubes were distinctly visible in 

 each rib. This structure is exactly analogous to that which 

 exists in the gills of aquatic insects, and hence Oken, followed 

 by Blainville, termed the wings aerial gills, an idea which, 

 however ridiculed by succeeding writers on entomology, will, 

 I think, ultimately appear to be supported by the strictest ana- 

 logy in structure, situation, and development. 



The branchias of water insects plainly resemble the A\-ings 

 in being composed of expansions of the tegumentary membrane 

 spread out upon nervures formed by tracheae and vessels. 

 Sometimes the membrane is continuous, so that the gills assume 

 a foliaceous appearance, like that of the wings, but in other 

 cases it is divided, so that the branchiae more resemble the fila- 

 mentous tufts of the nereis. The elementary structure is the 

 same, however, in both cases. The position of the branchiae is 

 constantly varyii.g; sometimes they are attached to the thorax, 

 sometimes to the abdomen, but in every case they have an im- 

 portant relation with the movements of the animal, and are 

 frequently the sole organs of progression with which it is fur- 

 nished. 



