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what I have found in Nasonia; what he took for live degenerating larval muscles 

 are in reality the five syncytial columns to which I have referred above. 



But in spite of the close convergence, small differences do occur between 

 these wing muscles and other striated fibres — Hensen's line is much more 

 prominent than in skeletal muscle (compare figs. 5 and 7, pis. xi. and xii.) ; 

 the muscles also contract much less in extent than others. It should be observed 

 that, in spite of the name given them, they are not usually inserted on the wings- 

 directly; their function is simply to alter the shape of the thorax, movement of 

 the wings taking place by a "ratchet and pinion" mechanism between the wings 

 and the thorax. While many skeletal muscles can contract to nearly one-quarter 

 their length, contracted wing muscle fibres are not very much shorter than when 

 extended ; if this were otherwise they would immediately rupture the whole 

 thorax. It should also be observed that the rate of contraction of these structures 

 is probably considerably less than the rate of vibration of the wings ; a single 

 contraction of a fibre may conceivably bring about quite a number of vibrations. 



It was mainly in the sarcostyles of the wing muscles that Schafer (1891) 

 studied the process of contraction, and by comparing the appearance of relaxed 

 sarcostyles with what he regarded as contracted ones, he came to the conclusion 

 that contraction consisted in an absorption, by the sarcous element, of part of 

 the fluid from the clear region of the sarcomere. There is, however, it seems 

 to me, a fatal objection to be raised against Schafer's conclusion. His evidence 

 for a contracted state of the sarcostyle lies evidently in a comparison of the 

 thickness of these structures ; thus, the sarcostyle shown in fig. 3 of Schafer's 

 paper is evidently regarded as a contracted state of that shown in fig. 2, because 

 of its much greater thickness. I am convinced, however, that, even though the 

 statement may seem absurd, both sarcostyles shown in figs. 2 and 3 of his 

 paper are to be regarded as equally relaxed. I find that a single sarcostyle 

 may vary greatly in thiclaiess in various regions along its length, being narrowest 

 at the middle of the muscle, and gradually broadening out towards its termina- 

 tion, where it becomes inserted upon the thoracic walls. Thus, figs. 2 and 3 of 

 Schafer's paper may be compared with figs. 7a and 7c (pi. xii.) of the present 

 paper, which show that the sarcostyles may be actually three times as thick at 

 their insertions as in other regions. The criterion of a contracted state should 

 not be the thickness of the sarcostyles, but the distance between successive 

 Krause's membranes. Krause's membrane is only to be recognized when we 

 can observe it stretching from one sarcostyle to another across the interfibrillar 

 space, and no such structures are shown in Schafer's figures. Any dark lines 

 that one sees within the sarcostyles between the dark discs are to be regarded 

 probably as false optical effects, and an examination of Schafer's fig. 3a shows 

 that this line is not present in the five clear spaces in the middle of the sarcostyle. 

 Krause's membrane, in fact, is a most difficult structure to recognize. It should 

 be noticed, that if the distance between two clear areas in fig. 3a is measured 

 (which, according to my vicAv, is the distance between the successive Krause's 

 membranes, even if these are not visible), then we find that this distance is 

 approximately equal to that between the successive Krause's membranes shown 

 in fig. 2a. (The dark line within the clear area of this figure could scarcely 

 be an optical illusion, because the sarcostyle is so much thinner, and the distance 

 betweeen the dark discs ever so much greater.) It should also be noted, in 

 support of what I have said above, tJiat no Hensen's line is visible in Schafer's 

 fig. 3a, if his interpretation be correct. In reality, however, Hensen's line is 

 quite as prominent in this figure as elsewhere, but it is to be looked for, not within 

 each of the dark areas, but between every second pair. The force of this is 

 more clearly seen when we examine the well-known figure to be found in most 

 histology books of the three sarcostyles from the wing muscles of a wasp (see 



