connected with Respiration in Insects generally, etc. 67 
These gills are circular in trans. sect. but taper longitudinally. 
During life they are curved upwards and backwards over the 
segments of the abdomen. 
The Larvae of Gyrinus and Trichoptera, which we mentioned 
above as being so like that of Svalvs in general form, will bear the 
comparison being carried as far as the microscopical structure of 
their gills. There are minor differences, for instance in Gyrinus 
they are fringed, and in some species of Trichoptera the main 
tracheal tube is double, but the essential structure is identical. 
The Ephemerid larvae have gills which agree with those just 
described in being segmentally arranged, but differ from them on 
two points. 
(i) Though the gills may be much subdivided they are 
fundamentally lamellate and not filiform. There are many grades 
of subdivision ; in some species there is simply the double blade, 
quite lobose in character; in Chloéén the lamella tapers distally to 
a filament; while in Caenis the lamella has a fringe on its margins, 
(ii) They contain a brown-black pigment. If a gill of Chloéén, 
which has been mounted unstained, be examined under a hand 
lens, the whole of the tracheae are visible as black lines spreading 
over the lamella and a single one can be seen passing up the 
filament mentioned above. On microscopical examination this 
coloration is found to be due to a brown-black pigment which 
is present in a finely granular state in the cells of the tracheal 
epithelium. This, however, is not always its position. In Caenis 
it is situated in the hypodermis. Hence it could be found all over 
the body surface. Examination of serial sections of the abdomen 
shows that its distribution is restricted (i) to the gill surfaces, 
(ii) to the lower surface of the anterior pair of gills, which act 
chiefly as shields to protect the others, and (ii) to that part of 
the general body surface which is also protected by the anterior 
gills; in fact it is found in practically the whole of the hypodermis 
of the “ Branchial chamber” and nowhere else. This distribution 
in the individual and its occurrence in other aquatic insects in 
analogous, but certainly not homologous, organs, suggest the 
hypothesis that this pigment, which we will call. “Spadicin” 
[Lat. spadix, dark brown], is a respiratory pigment. There are 
a number of objections to this. First of all, its state; the solid 
condition not being a very active one usually. It may be urged, 
however, that some substances, Platinum for instance, are very 
powerful chemical agents in a finely divided state. 
Again, its absence in many forms where we should most 
certainly have expected it, in Sialis, Gyrinus, etc. This objection 
is difficult to cope with owing to our lack of knowledge of all these 
larvae, but in Sialis, perhaps, the blood takes its share of the oxygen, 
and so the rapid interchange which necessitates the pigment in 
