178 ANATOMY. 



ramose or arterial vessels, so that the tubular main stem is less insu- 

 lated *. Precisely the same structure is exhibited in the larvae of the 

 Lepidoptera, but the peculiar tubular structure is still more indistinct, 

 for in general the transverse connecting tubes are also wanting. 



130. 



The vesicular air vessels are properly only distended tubes, or the 

 distended ends of accessory branches, it is thence that they are never 

 found alone, but they are always in conjunction with arterial or tubular 

 air vessels. They also appear under two chief forms, for they are either 

 very large bladders, lying chiefly in the abdomen, whence arterial air 

 vessels originate, or they are the vesicular distensions of the branches 

 of arterial air vessels themselves. 



The first form of the vesicular air vessels is found in the Hymen- 

 optera, Diptera, Cicada, and in a somewhat altered figure in many 

 grasshoppers. 



In the Diptera, at least in the true flies ( Muscidte) the Syrphodea 

 and the (Estridce, two large air bladders have been observed at the base 

 of the abdomen, contiguous to the intestinal canal, which are tolerably 

 uniform in structure with the large tubular vessels, but the twistings 

 of the thickish spiral filament are wider apart, the filament itself 

 divides here and there, and is interrupted at other parts, whence the 

 entire surface does not appear so regularly transversely striated as in 

 the tubular vessels (PI. XXII. f. 12., membrane of the air bladder of 

 Musca vomitoria). Their form is regulated by that of the abdomen, 

 so that they are often ovate or very generally vertically compressed, 

 and are here and there angular, in consequence of constrictions. A 

 large trachea originates from their under surface ; it runs forward and 

 backward to the head and anus, and gives off lateral tracheae to the 

 spiracles of the thorax and abdomen. Other finer vessels run over the 

 superior surface of the bladder, and ramify to the internal organs. 

 Whether they originate from the bladder itself or from the connect- 

 ing vessels lying beneath it I could not perceive distinctly in flies, but 

 it is the case in Scolia and in Apis according to Leon Dufour. But 

 this whole air bladder is nothing else than the tubular vessel of the 

 larva, which during the pupa state has shortened and distended, and of 

 which we took notice in the preceding paragraph ; this air bladder must 



* Compare Swanimerdam Biblia Natune, PI. XXIV. f. 1. in Apis Mellijica. 



