STRUCTURE OF THE WING-MUSCLES OF INSECTS. 



291 



of the longitudinal view correspond to the more delicate network seen in other and 

 deeper planes of the disks. It is also clear from what has been said that these 

 rectilinear appearances do not denote the presence of a network of filaments, but 

 are the optical effect of septa separating the muscle-columns, which septa, at the 

 level of the apparent dots, are thickened by the accumulation of a larger amount of 

 earcoplasm. 



In muscular fibres which have been treated first with acid and afterwards with chloride of 

 gold, and which have been placed in formic anid for twenty -four hours, or until the gold has 

 become reduced in the tissue, the sarcoplasm becomes stained of a dark violet colour, while all 

 the rest of the muscular substance remains unstained. The reticular appearances are thereby 

 rendered very distinct, and they have led many histologists to believe that the muscle- 

 eubstance consists of a contractile reticulum composed of longitudinal filaments and transverse 

 networks, and enclosing in its meshes a non-contractile fluid substance (enchylema, Camoy), 

 which is continuous in every direction in the fibre. But it can be proved, as will be 

 seen immediately when the structure of the wing-muscles of insects is considered, that 

 the inter-reticular substance (i.c., the substance forming the muscle-columns) is undoubtedly 

 the contractile part of the muscle, and that the apparent reticulum or sarcoplasm is not con- 

 tractile. Moreover, careful observation with the highest powers distinctly shows that the 

 filaments of the reticulum are actually septa which subdivide the fibre into longitudinal 

 elements. 



Structure of the wing-muscles of insects. The wing-muscles of insects 

 may be looked upon as furnishing the key to the proper understanding of the 



Fig. 333. SARCOSTYLES OF THE WING-MUSCLES 



OF A WASP. HIGHLY MAGNIFIED. (E. A. S. ) 



A, A', sarcostyles showing degrees of retrac- 

 tion (? contraction). 



B, a sarcostyle extended, with the sarcous 

 .elements separated into two parts. 



C, three sarcostyles moderately extended. 

 The structure of the sarcous elements is shown 

 fiemidiagrammatically in these. 



A! 



13 



A 



structure of muscle. Although often 

 regarded as a contractile tissue sui 

 generis they in fact agree in all essen- 

 tial particulars of structure with the 

 -ordinary muscles. Moreover, in some 

 insects they are replaced by muscles of 

 the ordinary type, and in others mus- 

 cular fibres occur which may be regarded as transitional 

 forms between the wing-fibres and ordinary fibres. The 

 wing-fibres are large bundles of muscle-columns or sarco- 

 fityles (often spoken of as " wing-fibrils ") which are 

 imbedded in a considerable amount of sarcoplasm con- 

 taining peculiar granules, the whole being usually enclosed within a sarcolemma 

 (figs. 333, 334). The nuclei of the fibre are scattered here and there in the 

 sarcoplasm, and this has all the appearance and many of the chemical characters 

 of cell-protoplasm. Amongst these characters is that of staining darkly with 

 chloride of gold, in which respect the sarcoplasm of the wing-muscles exactly 

 resembles the apparent reticulum of ordinary muscles. The main difference is one 

 of amount, the quantity of sarcoplasm in the wing-muscles being relatively far 

 greater (fig. 334) than in the ordinary muscles. 



When a living wing-fibre is broken up with needles in a small drop of white of 

 egg, the sarcostyles become easily separated from the sarcoplasm which surrounds 



