68 Mr Purser, Preliminary notes on some Problems 
connection with the tracheal system is to a large extent done away 
with. 
asphyxiation. Since both Haemoglobin and Haemocyanin exhibit 
a colour change when reduced it was hoped that Spadicin would — 
show one also. As far as experiments have gone no such result 
has been obtained. 
There is very little of the general chemistry of Spadicin known. 
It is apparently insoluble in water and alcohol, the latter up to 
boiling point, but it is decolorized (dissolved ?) by strong Potash. 
During the reaction there is a colour change from brown-black 
through reddish to almost if not complete colourlessness. Owing 
to the difficulty of obtaining sensible amounts of the pigment, tests 
are usually unconfirmed and the results must therefore be received 
with caution. 
The great point in favour of Spadicin being respiratory is its 
distribution. Jt is found in the respiratory organs, gills and tracheal 
tubes in Ephemeridae and Odonata. If these organs were homo- 
logous this would not be a very strong point but they are most 
certainly not. In Ephemerids they are tracheal gills arranged 
segmentally; in Agrionids they are also tracheal gills but compose 
a group of three round the anus; while in Aeschna the rectum is 
modified to form a respiratory organ. Why is it that Spadicin only — 
appears in the respiratory organs of true aquatic insects if it has 
no respiratory function ? 
Now let us examine the respiratory organs of the Odonata: 
The Agrionid larvae have three lamellae at the posterior 
end of the body with its tracheal system spreading over the 
surface like the veins of a leaf. The pigment, as far as I can tell, 
is the same as in the Ephemerids and the same remark applies to 
Aeschna. The Agrionid larval tracheal gills, however, cannot be 
considered very important or very useful in connection with a 
discussion of the question of respiration, owing to an extraordinary 
lack of definite knowledge about their function. It is thought by 
many that they are not respiratory at all, but purely locomotory, 
being analogous to the uropods and telson of the Decapod Macrura, 
and although I cannot go that far owing to their structure being 
so typically that of a gill, it is significant that they seem to be able 
to live without them just as well as with them. 
The rectal wall of Aeschna has six thick longitudinal bands 
separated by thin and flexible membranes. Each longitudinal band 
bears a double row of transverse folds which enormously increase 
the epithelial surface and at the same time lodge the tracheal 
branches. Oustalet estimates that there are no fewer than twenty- 
four thousand of the folds. The tracheal branches enter larger, 
regularly arranged air-tubes which in turn open into the main 
Some direct evidence was sought by subjecting larvae to 
