THE INTERNAL ORGANS OF RKSPIRATION. 175 



insects we find them in conjunction with pulmonary and vesicular 

 vessels, but the terminal ramifications, as well as the secondary ones, 

 are of the arterial description. 



129. 



Tubular air vessels are chiefly peculiar to such larvee as are provided 

 either only at one end or at both ends of the body with spiracles ; 

 besides which the communicating tubes of the stems of the spiracles 

 are tubular. Under the name of TUBULAR we understand such air- 

 vessels which proceed uninterruptedly from one end of the body to the 

 other, and which only send forth here and there small accessory 

 branches ; or else the simple communicating vessels between two ap- 

 proximate spiracles, and which are without any accessory ramifications. 

 Both have this in common, that they preeminently extend according 

 to the longitudinal axis of the body, whereas the arterial air-vessels 

 take their course in an opposite direction to this longitudinal course. 

 Whence it becomes apparent that the tubular air-vessels are never insu- 

 lated, but can only exist in conjunction with the arterial; the former 

 are, as it were, the main stems and the latter their twigs. 



We will now describe in greater detail some of the chief tubular air- 

 vessels. 



With respect to their first form we may assume that all larvae 

 which live in water possess more or less developed tubular main 

 stems. Among the Coleoptera this is the case in the larvae of 

 Dylicus and HydropTiilus. The yellowish green larvae, figured 

 by Roesel * of the large water-beetles (JDyticus marginalis, d'nni- 

 diatus, &c.), have two large spiracles at the apex of the last abdo- 

 minal segment, exteriorly contiguous to the short, plumose, anal apex. 

 Two large, broad, black tracheae originate from them, which ascend 

 undivided as far as the first thoracic segment, the future prothorax. 

 There each furcates, and then both branches run to the head, 

 one spreading over the muscle of the mandible and the other beneath 

 it. Two small accessory branches of these two main stems spring from 

 it at the commencement of each abdominal segment, but the inner one 

 of these two is considerably the largest in the fourth, tenth, and 

 eleventh segments, for these three pass to the intestinal canal, the 

 anterior one to the stomach, the posterior ones to the ilium and thick 



* Insectenbelustiguugen, torn. ii. Wasseriiisekten der Ersten Klasse, p. 8. PI. 1 

 f. 2-7. 



