102 
THE entomologist’s RECORD. 
which the wing is composed are, in the unexpanded state, corrugated 
into a vast number of folds, transverse as well as longitudinal, and that 
the nervures are folded. Are any of our readers microscopists ? If 
so, will they try and verify this statement. Nicholson, in his Manual 
of Zoology, p. 318, states that “ each nervure consists of a central trachea 
or air-tube running in the centre of a larger blood-tube.” It seeiTis 
probable, however, there is no minute series of blood vessels in the 
wings, but that the blood which passes into these peri-tracheal vessels, 
simj^ly finds its way anywhere between the two membranes. Curiously 
enough, when I specially wanted this year to get specimens of Biston 
hirtaria with saccular dilatation, I have only met with one. Examining 
that with an indifferent microscope, it was evident that the fluid was 
lying free in the general cavity between the two membranes. I tried to 
see whether it was possible to press the fluid into other parts of the 
wing ; there was great resistance to any such proceeding, and it seemed 
as if the two membranes had become joined together, as is affirmed to 
be the case by Kirby and Spence in the extract quoted by Newman, I 
thought, however, that I did see some slight tendency for the fluid to 
make its way \ but, on this point I am in doubt, as the sac burst just as 
I observed this. Mr. Fenn’s report of Mr. Waterhouse’s demonstration 
seems opposed to the theory that the two membranes become united, 
but how did Mr. Waterhouse conduct his experiment? The specimen 
used was one in which the membranes would certainly be thicker than 
in the smaller moths, and it may be that some soaking process was 
used in order to get the membranes apart. Expansion must be the 
result of some vis a tergo, i.e . , the insect must unroll the corrugations 
by forcing between them either blood or air. I incline to the belief 
that the former is the agent, i. The wing, when expanded, is for a 
time limp. This would easily be accounted for if the expanding agent 
were fluid, it would be difficult to understand if it were air. 2. The 
attitude taken up by the insect is such that aid is given by gravitation, 
especially when it is remembered that the blood leaves the heart at its 
end nearest the head. Many insects bred in our cages go up to the 
roof of the cage to expand. I do not know whether in nature any 
take up a corresponding position on the underside of a branch. 3. I 
do not find any evidence that the tracheae forming the central tubes of 
the nervures discharge at their distal end into the general inter- 
membranous cavity, but the blood vessels surrounding them un- 
doubtedly do. Mr. Fenn seems to suggest that the blood is only forced 
into the blood vessels in the nervures. At present I incline to the idea 
that the phenomenon of saccular dilatation points to a more general 
presence of the blood, but on this, observations are too few to dogmatise. 
(2) Stiffening. After a certain time the limp expanded wing becomes 
firm and stiff. How is this brought about ? I am not sure that I quite 
understand Mr. Fenn’s suggestion. What I suppose him to mean is 
that the fluid which has been forced into the blood vessels, causing 
them to unroll and so expand the membranes, is again withdrawn into 
the body of the insect and exuded. My difficulty with regard to this 
is that I know of no organ which could thus suck the blood up from 
the wings. There must, of course, be muscular structure connected 
with the wings, but I should expect the muscles to be short and 
attached to the wing near its base, and therefore not in a position to 
