ON PSYCHOPTERA PALUDOSA. 75 



The filament (see Fig. 13) is composed of an exterior integu- 

 mental wall enclosing a trachea, the latter being a continuation of 

 one of the main tracheal trunks. The external wall is marked by 

 a thickened spiral ridge projecting internally, and having a corres- 

 ponding external spiral depression. This wall thins out towards 

 the extremity, where it becomes a mere film of membrane surround- 

 ing the trachea, see Fig. 15, m. At intervals between the coils there 

 are found elevations (see Figs. 13, 14, and 15), consisting of a 

 horny ring, over which the integument extends in a thin inflated 

 bladder. The wall, too, of the enclosed trachea appears here to 

 lose its spiral form, and to pass as a delicate internal lining 

 into the cavity of the bladder, to which it is closely applied. The 

 total thickness of both linings in these organs is so small as pro- 

 bably to readily admit the passage of air through them into the 

 trachea, either by direct contact, or through the medium of the sur- 

 face water in which the filament floats. They thus form closed 

 stigmata, admitting the air through their tissues, but excluding the 

 water ; a modification of the closed tracheal system peculiar to all 

 exclusively aquatic insects, another form of w^hich exists in the 

 tracheal gills of the EphenieridcB. It may be asked why these organs 

 are found in the respiratory filament of the pupa, while they are 

 absent from that of the larva. The question, I think, admits of 

 the reply, that the integument of the former is much denser, at 

 least in its basal portion, than that of the latter, hence the neces- 

 sity for specialised portions of it devoted to the fulfilment of its 

 function ; hence, also, we find that as we approach its extremity, 

 where the external wall thins out, these organs almost disappear, 

 the respiratory process being carried on over the whole surface of 

 this portion of the filament. The filament is terminated by a pretty 

 crown of incurved horny teeth, like the peristome of a moss, see 

 Fig. 16. I do not think this is an open stigma any more than the 

 bladder-like organs just described, but that it is also closed in by a 

 delicate film of membrane. 



The spirals of the external wall appear to be broken at opposite 

 points of their course, a fine of these interruptions thus occurring 

 on either side of the filament. The abortive corresponding fila- 

 ment to which allusion has been made is so small as easily to 

 escape detection ; it differs from its fellow, not only in point of 



