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 seems 

 probable, however, there is no minute series of blood vessels in the 

 wings, but that the blood which passes into these peri-tracheal vessels, 

 simply 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 



