XI 



Muscles, Fat Body and Circulatory System 



In the young larva each muscle is composed of a varying num- 

 ber of parallel fibres, which constitute the contractile portion of 

 the muscle. These fibres range from round to flat in tranverse 

 section, stain deeply and are themselves composed of fine fibrillae 

 (Figs. 73 A and B). Each fibre is continuous from its origin to 

 its insertion and remains of nearly the same diameter throughout 

 its length, tapering only slightly toward the two ends. In the 

 longitudinal trunk muscles, however, which are attached to the 

 intersegmental hypodermis, instead of terminating at these points, 

 the fibres appear to be continuous through at least two segments, 

 and probably more (Fig. 73A). A layer of undifferentiated 

 protoplasm, the sarcoplasm, enwraps all of the fibres which con- 

 stitute a single muscle. At the middle of a muscle the sarcoplasm 

 is common to all of the fibres of the muscle, binding them together, 

 but towards the two ends of the muscle the sarcoplasm divides, a 

 small portion, constantly diminishing, following each fibre to its 

 attachment (Fig. 73 A). Embedded in the sarcoplasm are the 

 muscle nuclei, arranged in rows along the fibres. Usually they 

 lie all on one side of the fibre — in the case of the longitudinal 

 trunk muscles the inner side — but this rule is subject to many 

 exceptions. In young larvae none of the muscle fibres are trans- 

 versely striated. In the imago, on the other hand, the greater part 

 of the muscles are striated, while the relation of the sarcoplasm 

 and the muscle fibres is the reverse of that existing in the larva, 

 the sarcoplasm and its nuclei in the imago constituting, so to 

 speak, the core or central portion of the muscle, about which are 

 arranged the muscle fibres. It is of interest to note that the con- 

 dition existing in the muscles of the bee larva is quite similar to 

 that found in the muscles of the vertebrates. 



The muscles may for convenience be divided into three groups : 

 the muscles of the head, the muscles of the trunk, and the muscles 

 of the alimentary canal. 



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