connected with Respiration in Insects generally, etc. 65 
for aérial flight has not been lost yet m any form, and that although 
such important changes of an adaptive character have taken place 
in the larvae the adults have so far been unaffected. The larval 
stage is the one of growth and therefore more dependent on its 
surroundings than any other. Consequently it is all the more 
likely to be modified by any alteration in those surroundings. 
This modification, it is important to remember, is very 
different in closely allied forms. Let us take the case of Culex 
and Chironomus. The imagines of these two Diptera are almost 
identical and are distinguished from one another in the field by 
the way they hold their legs when at rest, yet the young stages 
of Chironomus contrasted with those of Culew would place it almost 
as far away as another Order; for these reasons: the larva of 
Culex comes to the surface and breathes by a pair of spiracles 
which are situated at the end of a respiratory syphon on the 
eighth abdominal segment. It is an active larva, living at the 
surface of the pool with colourless blood and a well-developed 
tracheal system ; in fact a typical false aquatic larva. ‘The larva 
of Chironomus, on the other hand, lives nearly the whole of its 
life in a tube in the mud at the bottom of a pool, has its blood 
deeply pigmented with haemoglobin, and breathes by blood gills 
which are tubular outgrowths on the last two segments, the 
tracheal system being almost if not entirely absent. 
When we examine the pupae of these two we find that 
respiration is effected in Culex by means of a pair of trumpets 
which come off from the thorax and open to the atmospheric air, 
whereas in Chironomus it is effected by a pair of respiratory tufts 
arising from the same part of the pupa as in Culex, which are true 
aquatic respiratory organs, i.e. they make use of the oxygen dissolved 
in the water. 
This divergence which has occurred between closely allied 
larvae has resulted in convergence between larvae of insects 
very widely separated phylogenetically. A good example is found 
in the comparison of the larvae of Gyrinus, Sialis and some of the 
Trichoptera. These in general shape and structure of respiratory 
organs are extraordinarily similar, but no one would say that the 
Coleoptera, Neuroptera and Trichoptera are as closely related as 
this semblance suggests. 
When we consider the changes in the physical state of the 
oxygen as it passes from the surrounding medium to the cells, 
the difference between the respiratory methods mentioned above 
is emphasised. In the larva of Chironomus it remains in solution 
all the time; in normal insects and false aquatics it remains in 
the gaseous condition until it reaches the cells needing it; but 
in the true aquatics with tracheal respiration it is taken out of 
solution in the gills and carried through the body as a gas. 
5—2 
