would drown, and (11) that there is an attachment between tracheal. 
system and exoskeleton at base of gills, the only place where the 
tubes could be extracted. These two requirements have not been 
observed to be present and in cast skins of Agrionid nymphs no 
signs of tracheal system is found. Spadicin (then) cannot be ex-) 
cretory, in the sense that it will be cast out with the tracheal lining, 
but it may be excretory in the sense that it is a break-down product’ 
of the old chitinous tube, which has been absorbed. 
Another suggestion is that it is the enzyme or proenzyme for) 
the absorption of the chitin. 
Up to now no work has been done expressly to test these! 
suggestions, but they seem unlikely because of the distribution of | 
the substance. It would be found on all the tracheae in all} 
apneustic larva [not in the gills and main trunks of some only], | 
and with regard to its having anything to do with the absorption |) 
of chitin the case of Caenis, and perhaps of Aeschna also, puts that | 
out of court. But although Spadicin may have nothing to do with | 
the problem, the question of ecdysis and growth of the tracheal | 
system of apneustic larvae is one of the most important which the | 
study of these aquatic forms raises. 
This is the position up to the present. By careful examination 
of as many new forms as possible, and from the experiments which 
Mr Balfour Browne, to whom I am very grateful for much advice © 
and valuable criticism, is conducting on Agrionid nymphs, I hope 
soon to have sufficient new facts and observations to decide, to my 
own satisfaction at least, some of the numerous questions which | 
the examination of the respiratory organization of true aquatic 
insects has raised. 
70 Mr Purser, Preliminary notes on some Problems, etc. 
