17f> ANATOMY. 



gut, whereas all the rest are branches which run off to the muscles. 

 But, on the contrary, the two exterior branches in the second segment 

 exceed the inner ones in size, turn upwards to the back of the seg- 

 ment, and here anastomose, whereby is formed one transverse commu- 

 nicating passage between the two main stems. All the transverse 

 accessory branches are here arterial, but the large main canal which 

 runs longitudinally in the insect is tubular. We find a similar dispo- 

 sition and structure, in all the essential portions, in the tracheal system 

 of the larva of Hydrophilus piceus, as is evident from Suckow's figures*. 



Tubular air-vessels are very general among the Orthoptera, where 

 likewise, as is always the case, they are connected with arterial 

 branches, or even with vesicular vessels. The tracheal system of Mantis 

 oratorio described and figured by Marcel de Serres may serve us for an 

 example f. Two narrow vessels originate from each of the seven 

 abdominal segments, the shorter exterior ones of which unite in a 

 direct tubular vessel, which runs beneath the margin of the abdomen, 

 and passes on to the third spiracle of the thorax. The inner somewhat 

 longer vessels unite in arches, forming a second longitudinal tube, 

 which proceeds in an undulating line close to the superior wall of the 

 intestinal canal, and also passes into the thorax. A third tubular 

 vessel comes out of the thorax, running very closely to the intestinal 

 canal : it also takes an undulating course, but beneath that organ, and 

 sends forth branches laterally, Avhich again unite in a fourth direct 

 tubular vessel, and which is connected at its anterior and posterior 

 extremities with the first named one, which runs at the edge of the 

 abdomen. All these tubular vessels give off but few branches, and it 

 is only from the central lower longitudinal tube that some delicate 

 branches are given off to the intestine, and it is from the central inner 

 small vessel, originating at the spiracle, that the air tubes come for the 

 sexual organs. 



The air-vessels of the larvae of the Libellulce are also tubular, and 

 are very uniform in their distribution with those of the larvae of the 

 beetles which live in water. Two large main stems, serpentine at the 

 dorsal portion of the intestinal canal, which, after being bound by the 



' In Heusinger Zeitschr. vol. ii. No. i. PL IV. f. 26. See a detailed description in 

 H. M. Gaede Dissert. Sistens. Observation, qucsd. dc Insector. Vcrmuniqiie Structure. 

 Chilon, 1817. 4to. 



t M<;m. du Museum, torn. iv. PL XVI. f. 1. 



