450 SANGUIFEROUS SYSTEM. 



observed in all their forms, to pass by one or more wide ves- 

 sels, outwards from the ventral veins, along the thick anterior 

 or upper margin, and to return to the same great venous 

 currents, along the thin posterior edge of the wings, as repre- 

 sented in the four dissimilar wings placed in the annexed 

 diagram (133. D. g. g.} In the first soft rudimentary state 

 of the wing in the larva, the excurrent and recurrent streams 

 form a single undivided loop around the margin, as in the 

 shooting branchial laminae of the tadpoles of amphibia, and 

 the wings of adult insects have been compared indeed to 

 dried branchiae. 



In the mature wings, the wide alar bloodvessels describe 

 various forms of curves, loops, and meshes, peculiar to 

 each species, and accompanying and perforated by the 

 ramifications of the alar tracheae; but these currents, 

 which are rendered visible only by the moving large ovoidal 

 blood-globules conveyed in the colourless serum, become 

 gradually obliterated, and restricted to the larger vessels, as 

 the wings dry and harden by use, as the respiration is in- 

 creased over the system by the extension. of the tracheal 

 ramifications, and as the necessity for nutrition and develop- 

 ment diminish over the system. There are sometimes hun- 

 dreds of blood-canals meandering through a single membra- 

 nous wing, and most of these canals embrace each one or 

 more tracheal branches which follow along their interior, 

 and which appear to oxygenate, while they are surrounded 

 by, the circulating blood. The circulating system of insects 

 is thus perforated and permeated by the respiratory, as the 

 alimentary canal of many mollusca is permeated by the great 

 trunks of the sanguiferous system. There are more excurrent 

 than recurrent streams along the margins of the wings; the 

 vascular parietes which bound these wide currents are not 

 perceptible ; and the number or size of the accompanying 

 atrcheae are not proportioned to the width of the blood-canals, 

 which embrace them. There are of course no valves in these 

 alar canals, and the blood is observed, on the slightest inter- 

 ruption to its free course, to stop, or retrograde, or ocillate 

 backwards and forwards in the same vessels, and by the con- 

 stant anastomoses of these canals a reticulate structure is 

 produced, like that of the adult capillaries and the first formed 

 embryo-vessels in the highest vertebrata. 



