OF THE TRACHEA IN INSECTS. 
31 
and an internal membrane, enclosing between them a spiral filament. Marcel de Berrcs 
and Straus-Diirckheim deny the existence of the spiral thread in the air- vesicles, which, 
however, is affirmed 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. 
Leydig* also appears to me to have given the best description of the struct nre of the 
tracheae. He considers that the outer "peritoneal tunic" is a " connective-tissue, trans- 
parent, and generally colourless 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 globules. In other words, the external membrane of the trachea 
is continuous with that of the fatty tissue ; the traehea 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. 
Burnieisterf describes this membrane as structureless; but it undoutedly consists of a 
union of cells, whose walls can generally be perceived without much difficulty. 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 Burmcister and Leydi ". Bnr- 
meister, however, like DeGeer, Lyonet, Bonnet, Straus-Durckheim, Newport, and others, 
describes a third, inner 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 
spires 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 structure, however, is shed at the same time. I have not yet 
been able to investigate the mode in which this 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 of Dasychiro piuUbiuida, and in a pupa-case of Deilephlla yalii, 
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. Histologic, p. 386. f Handbook of Entomology, Shuckard's Transl. p. 1/1. 
