Where the principal difficulty arises, however, is in attempting 
to explain the most. peculiar feature of the whole arrangement, 
64 Mr Purser, Preliminary notes on some Problems 
the extraordinarily fine ramifications of the tracheal system into | 
the deepest tissues, with the result that the blood has been | 
deprived of one of its most important functions, the carriage of 
oxygen from the respiratory organ to the cells requiring it and 
the excretion of CO,. The reason for the origin of the haemocoele 
would perhaps help us towards the solution of this difficulty but 
unfortunately no adequate explanation is forthcoming. On the 
supposition, however, that the Insects arose when the haemocoele 
had only just begun to enlarge, we can quite easily imagine the 
course of evolution. The respiratory organs at that time would 
be simple pockets of ectoderm and cuticle serially arranged along 
the body. As the haemocoele enlarged the rate of circulation 
would most likely decrease. This would stimulate an increase in 
the surface of the tracheal system, owing to the need of a higher 
oxygen concentration in the blood, and of a better excretion of 
CO,. Anastomosis and further ramification would follow until 
we get, as a final result, the oxygen being delivered straight to 
the cells requiring it in the gaseous state. All this, however, is 
pure speculation and depends upon the assumption, which is 
perhaps unwarrantable, that when the Insecta became differen- 
tiated from the rest of the Arthropoda the haemocoele was of 
quite small size, or, at any rate, that the circulation was very 
much faster than at present. 
When Insects took to aquatic life [for the consensus of opinion 
at the present day is that aérial forms gave rise to aquatic forms 
rather than vice versa] their method of respiration was one, if not 
the chief, of their physiological difficulties. This same difficulty 
has arisen in other groups, the Mammalia, for example, where the 
Cetacea have become marine. These, however, have shirked the 
difficulty, and take long breaths, having, therefore, to come to the 
surface at intervals to renew their supply. Similarly many aquatic 
insects have to come to the surface to breathe. ‘These, for want 
of a better term, I call False Aquatics. The methods by which 
they obtain their supply of oxygen and prevent the entrance of 
water into their tracheae are extraordinarily diverse and make a 
delightful branch of study, but since they do not differ physio- 
logically from aérial forms I will say no more about them. The 
other division into which I divide aquatic insects is, in contra- 
distinction from False Aquatics, True Aquatics. These have, in 
one way or another, become specialized to make use of the oxygen 
dissolved in the water in which they live. 
On examination of aquatic insects generally we find that, 
though many false ones are imagines, none of the true group are. 
This is an interesting point because it seems to show that the need 
