124 



ANATOMY. 



distinguish the proper membrane between them as a separate layer. A 

 free space is naturally not found, as in the stomach. 



THE (ESOPHAGUS. 

 102. 



The oesophagus (PI. XVII. 22, A, A,) extends from the pharynx to 

 the stomach ; it is distinguished from the former by its smaller capacity, 

 and from the latter by a variation in structure. The most remarkable 

 form of the oesophagus is doubtlessly its very general furcate division 

 in the Lepidoptera, and that from each of the two spiral sucking 

 tubes it originates by a distinct branch, which branches then 

 unite into one channel. In general the branches of the fork are 

 very short, but in the swallow-tail butterfly (Pieris Machaon, O.) 

 their union into one tube commences only at the thorax *. In the 

 other orders of insects the oesophagus passes through the entire cavity 

 of the thorax as a simple tube, and either terminates where the cavity 

 of the abdomen commences, or before this, within the thorax itself; for 

 example, in its centre in those insects the cavity of whose thorax is 

 broad, and which consequently admits of a greater expansion of the 

 organs which traverse it. The length of the oesophagus therefore 

 depends upon the length and dimensions of the thorax. Insects with 

 a thin and narrow, and in particular with a petiolated abdomen, have 

 a long oesophagus, when the thorax also is long (Pimpla, Fcemts) ; and 

 it is the longer in proportion to the entire intestinal canal, the shorter, 

 narrower, and smaller we find the abdomen. The most remarkable 

 proportions must occur in this respect in the genus Evania, but which 

 has never yet been anatomically investigated. The longest oesophagus 

 yet observed consisted of more than half of the entire intestinal canal f ; 

 and among the shortest is that of the cockchafer, which occupies 

 scarcely one-sixtieth of the entire length of that canal J. 



We are already acquainted with the texture of the oesophagus ; its 

 central layer however is very slight, whence the two other membranes 

 lie closer together, which, as Ramdohr assures us, makes their separation 

 very difficult. The inner membrane is generally here quite uniform, much 

 more rarely thicker in parts, almost like parchment, or, as in Carabus, 



* See Treviranus, Vermischte Schriftcn, vol. ii. p. 200. 



f In Pimpla Enervator and Pompilus viaticus, Ramdohr, PI. III. f. 2 and 3. 



t Ramdohr, PI. III. f. 1. 



