400 



The Structure and Post -embryonic Development of 

 the Larval Muscles. 



The General Body Musculature. 



Tlie hitttology of insect muscle can be very clearly observed 

 in material derived from Nasonia larvae, and as a number 

 of structures, not hitherto observed, were revealed by the 

 Haidenhain haematoxylin method of staining employed here, 

 I shall briefly describe the structure of muscle fibres, as it 

 occurs in this insect. 



The longitudinal body muscles will be considered first. 

 The portion of a single muscle band situated in any one seg- 

 ment, and inserted at its anterior and posterior extremities, 

 is a single muscle fibre, containing three to five nuclei (and 

 developed, as will be seen below, from as many cells). The 

 inner, posterior portion of one muscle fibre, i.e., of one intra- 

 segmental muscle, is connected by a short process with a 

 similar process given off from the outer, anterior part of the 

 succeeding muscle (fibre) of the longitudinal band immedi- 

 ately internal to it (fig. 99), i.e., there is a "dovetailing" of 

 muscles (muscle fibres) not unlike what occurs in vertebrate 

 cardiac muscle, which results in a direct communication 

 between all the longitudinal muscle bands. This "dovetailing" 

 is particularly clearly seen in young larvae and in the hind 

 region of adult larvae. The connecting piece is always devoid 

 of a nucleus (fig. 100), and does not, therefore, represent a 

 distinct cell (see below). 



When a muscle fibre is examined in sections, the longi- 

 tudinal fibrillae are very clearly seen ; each consists of a 

 number of minute spindle-shaped sarcomeres (fig. 127), the 

 "spindle" shape being due to the concentration of the fluid 

 contents at its middle. At other times the liquid contents 

 pass to either end of the sarcomere, leaving a clear space in 

 the middle (Hensen's line) which may be quite wide. The 

 two dots often figured on the ends of each sarcomere are 

 optical representations of the extremities of the sarcomere 

 (fig. 127). Moreover, Krause's membrane is apparently not 

 a membrane so far as the muscle fibre is concerned; thous^h 

 the contrary view is sometimes held, it appears to consist 

 rather of closely concentrated minute "dots," each repre- 

 senting the point of junction of successive sarcomeres. Fig. 127 

 shows a muscle portion of a fibre in longitudinal section ; 

 the individual sarcomeres, each a spindle-shaped structure, 

 are clearly visible, and Krause's membrane is the effect 

 obtained by the junction of successive sarcomeres approxim- 

 ately along one line. It is only with respect to the fibrils 

 apparently that we can speak of a "Krause's membrane" as 



