OF THE TRACHE.E IN INSECTS. 31 



and an internal membrane, enclosing between tbem a spiral filament. Marcel de Serres 

 and Straus-Diirckheim deny the existence of the spiral thread in the air-vesicles, which, 

 however, is afiirmed by Suckow and Sprengel. The truth seems to be that it is certainly 

 present in some cases, while in others it is either imperfectly developed or altogether 

 absent. In some instances the chitinous thickening has a zigzag appearance, which 

 Leydig also has observed. 



Ley dig* also appears to me to have given the best description of the structure of the 

 tracheae. He considers that the outer " peritoneal tunic " is a " connective-tissue, trans- 

 parent, and generally colomdess membrane, formed by the union of the same cells which 

 form the fatty tissue, and with which also they remain in intimate connexion." 



This description seems to me quite correct ; and any one may test its accuracy by 

 examining the fatty tissue of a caterpillar, where he will see that the external membrane 

 here and there leaves the trachea and encloses around it a considerable space, which is 

 occupied by the fat globiiles. In other words, the external membrane of the trachea 

 is continuous with that of the fatty tissue ; the trachea possesses therefore an external 

 membrane only so long as its course is between the masses of fatty tissue, and loses it as 

 soon as it enters one of the masses. 



Burmeisterf describes this membrane as structureless; but it undoutedly consists of a 

 union of cells, whose walls can generally be perceived without much difficu.lty. The 

 nuclei also are generally visible. 



Among the older writers, Sprengel appears to have had the most correct idea of the 

 so-called spiral filament. He considers it merely as a local spiral thickening of the 

 inner membrane, — in which view he has been followed by Burmeister and Leydig. Bur- 

 meister, however, like DeGeer, Lyonet, Bonnet, Straus-Diirckheim, Newport, and others, 

 describes a third, mner membrane, which, like Sprengel and Leydig, I have been unable 

 to see. 



PI. II. fig. 13 represents a fragment, which shows clearly the membrane uniting the 

 spkes of the " thread." It is, however, possible that the spirally thickened portion of the 

 inner membrane may sometimes detach itself more or less completely from the membrane 

 by which it is formed. Some of those naturalists who have convinced themselves of the 

 presence of an inner membrane lining the spiral thread, appear to have arrived at this 

 conclusion mainly on account of the inner membrane of the tracheae being shed at each 

 change of skin;. the spiral structiu*e, however, is shed at the same time. I have not yet 

 been aljle to investigate the mode in which tliis exuviation takes place so carefully as I 

 could wish ; but the fact does not justify the conclusion which has been deduced from it. 

 In the cast skin of a larva oi Dasychira pudibimda, and in a pupa-case of Deilepliila galii, 

 I convinced myself that the spiral filament is cast at the time of moulting. 



Nervous System. — Ganglia. — (PI. I. figs. 1, 3, 4, 5 & 6.) 

 Throughout the Insecta the tracheae of the nervous system and of the muscles vary 

 less than those of almost any other part of the body, from which it may probably be 



* Lehb. d. Histologie, p. 386. f Handbook of Entomology, Shuckard's Transl. p. 171. 



