connected with Respiration in Insects generally, etc. 69 
trunks running the length of the body. A large volume of water 
is sucked into the rectum at pleasure, and from this the tracheae 
draw a fresh supply of oxygen. The vitiated water can be expelled 
_ either gently, or, when the larva requires to propel its body forward, 
with considerable force. The above account is taken from Miall 
_ who unfortunately does not give Oustalet’s reference. He goes on 
to say that these larvae, under strictly natural conditions, may 
increase the effectiveness of their respiratory organs by coming to 
the surface and taking in air directly. This would be made use 
of particularly if the water became foul. 
The pigment, as far as can be made out from the sections 
already examined, is not situated in the tracheal epithelium, but 
in the hypodermis. If this is the case, we have in the Odonata 
just the same variation in the position of Spadicin as there is in 
the Ephemeridae. Whether there is any significance in this fact, 
that though it may be found in one of two layers of epithelium, it 
is never, as far as I know, found in both in the same species, we 
have not the slightest hint, but it is one of the many points which 
a physico-chemical explanation of the action of these gills will 
have to elucidate. 
Of all the methods of respiration found amongst these true 
aquatic insects, the larvae of Chironomus, and perhaps others, 
‘exhibit the one most divergent from that of a typical insect. 
Whether they live at the surface or down in the depths of the 
pond they breathe by blood gills on the last two segments, two 
pairs of finger-like gills on the penultimate segment, and a rosette 
of four on the last. The only tracheae to be found in the fullgrown 
larvae are a few twigs in the thorax. The blood of these larvae 
performs, therefore, all the functions it is called upon to do im all 
groups of animals possessing a vascular system, with the exception 
of the Tracheates. In those species of Chironomus which live in 
holes in the mud their blood is highly pigmented with haemoglobin. 
This acts most likely as a storehouse of oxygen between two 
excursions to the upper layers of water. 
Before bringing this paper to an end I must again refer to 
Spadicin because it has been suggested that its function is ex- 
eretory. This brings us face to face with the following problem. 
In a larva living in the air with its numerous stigmata giving free 
communication between the tracheal system and the exterior, the 
lining of most of the tracheae is pulled out at each ecdysis, and 
a new chitinous tube of rather larger size is laid down, Thus the 
growth of the main tubes is allowed for and the useless lining got 
rid of. But what happens when such a larva as Chloéén under- 
goes ecdysis? If the lining is pulled out at this stage then the 
stigmata must open for a short time. This requires that (i) the 
openings be closed immediately after ecdysis or else the creature 
