66 Mr Purser, Preliminary notes on some Problems j 
This last change of state is very remarkable owing to the | 
difficulty of providing a physical and physico-chemical explanation 
of it. For a discussion of this subject I will refer to Miall in his 
admirable book on the Nat. Hist. of Aquatic Insects, page 37. ui 
Now let us examine the microscopical structure of a typical _ 
simple tracheal gill, for instance, such an one as the larva of Sialis 
possesses. These gills, like in so many other forms, are arranged : 
one pair to each abdominal segment for at least the first few 
segments, seven in the case of Sialis. The last segment bears a 
single filament which resembles two ordinary gills fused, but differs 
from them in being unjointed. This jointing is a feature peculiar — 
to Stales and is absent in those simpler forms found among the 
Trichoptera. fi 
Diagrammatic transverse section of Gill of Sialis. 
In trans. sect. they show the following structures (see fig.). 
(1) Outer chitinous covering, the exoskeleton. 
(II) The epidermal layer of cells, which secrete the exo- 
skeleton. I shall refer to this as the hypodermis. 
(III) The Haemocoele, in which may be seen, if the section 
be taken near the base, muscles and portions of F at-body ; and 
wherever it may be taken 
(IV) The Tracheal Tubes of chitin covered by their layer of 
epithelium. One main tube, with small side branches running to 
the periphery. 
