374 On the Uses of the Dorsal Vessel. [Mat, 



quantity of air, and which have very extensive vesicular tracheae, 

 \vc observe a particular apparatus destined to supply the elasticity 

 which they want. 



This apparatus is cooiposed of hemispherical cartilaginous hoops, 

 furnished with particular muscles, and which, in consequence of 

 this disposition, may be considered as a kind of ribs. In fact, 

 these ribs elevate at every inspiration the vesicular tracljeie and in- 

 crease their size, allowing tliem to receive a greater quantity of air; 

 on expiration they depress the trachees, and thus serve to drive out 

 the air. These ribs, tixed by tlieir base to the coriaceous envelope, 

 arc only moveable by their upper parts. They exist only in those 

 insects that have vesicular tracheae of a certain extent. We do 

 not observe them in the lepidopterae, the caleopterae lamellicornes, 

 and the dipteres, in which the vesicular tracheae are scarcely a half 

 millimetre (0 019(J inch) in extent. In certain orthopteres, on the 

 contrary, as the gryllus, truxcdis, acri/dhnn, in which the trachese 

 are large, several millimetres in extent, the cartilaginous hoops, or 

 ribs, always exist, and are in these animals very necessary. 



Such are the organs vvhicli serve in insects as reservoirs of air. 

 Very different from the lungs and bronchias, they are not placed in 

 any particular part; we see them, on the contrary, spread every 

 where, with a sort of profusion : so t!)at there is no part of these 

 animals that does not breatlie and receive the action of air. 



The general disposition of the tracheae, and the difi'erent parti- 

 cularities of the organization of insects, have occasioned ihe man- 

 ner in which tliese organs communicate with the external air, a 

 mode quite diflierent from what wc observe in other animals. The 

 organ of respiration in insects being very much extended, and very 

 much ramified, a single opening would not have been sufficient to 

 distribute the air in all parts with that regularity and abundance 

 that the circulation of that fluid required. Accordingly, the open- 

 ings by wiiich the tracheae receive air are always more than one, 

 having never icwcr than two. Most commonly that number varies 

 from eight to twelve, and sometimes amounts to 20. These open- 

 ings have been in general called stigmata. But, as in certain 

 species there exist some which open and shut, by means of an 

 apparatus of wliich the true stigmata are destitute, we shall divide 

 them into two different orders. We shall call the first simple stig' 

 mata, and the second tremaer * stigmata. 



'I'he simple stigmata are most frequently placed on the sides of 

 the body beiween the folds of the membrane of the back and ab- 

 domen. They are always disposed in pairs, presenting, in general, 

 a round opening like a button-hole, with a cartilaginous border. 

 Sometimes, however, this border is totally wanting, and the stig- 

 mata are then surrounded by a cartilaginous scale of a different 

 colour, from the coriacous envelope of the body. In the cater- 



* The word is derived frnn tch/^c, apcningi and ix^, air. 



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