322 bulletin: museum of comparative zoology. 



formed in the pupa, it was impossible to say without a systematic 

 search. To settle this question, and also to find out precisely which 

 muscles remain unchanged, which metamorphose and which degenerate, 

 a detailed study of the musculature of the metathorax was made. This 

 is for Coleoptera, the most important somite as far as the muscular 

 system is concerned. After completing the study of the metathorax, it 

 was found to be unnecessary to investigate the anatomical changes of 

 the muscles of the other somites except in a general way. 



In connection with this study of Thymalus, a dissection of Colymbetes 

 sculptilis Harr., one of the Dytiscidae, was made in order to permit a closer 

 comparison with the dissection of Dytiscus marginalis by Luks. The anat- 

 omy of the imaginal musculature of Synchroa punctata Newm. (Melan- 

 dryidae) and of Bruchus obtectus Say (Bruchidae) has also been studied. 

 The two latter species have been studied from serial sections, both being 

 too small to be dissected successfully. This gives five beetles, of as many 

 different families, for comparison, to which may be added the dissection 

 of Melolontha by Straus-Durckheim. Several points of difference in 

 various muscles were found among these beetles, which are noted at the 

 end of the description of the muscle in question. Where nothing is 

 stated to the contrary, it may be understood that the conditions in the 

 other forms agree essentially with those in Thymalus. 



a. Metathorax. 



The muscles of the larval metathorax, or of any larval somite, may be 

 naturally separated into three groups; the dorsal antero-posterior, the 

 ventral antero-posterior, and the lateral dorso-ventral. 



The function of most of the muscles of the larval metathorax is to aid 

 in locomotion. Some of the lateral dorso-ventral muscles are attached 

 to the legs and serve as flexors or extensors. The antero-posterior 

 muscles of both groups serve to bend the body in one direction or an- 

 other. All of the muscles are employed in a not very successful creeping 

 movement, similar to the creeping movements of certain Annelids, such 

 as the earthworm. That is, the longitudinal muscles oppose the dorso- 

 ventral muscles through the medium of th6 body fluid. 



In the imago the muscles may, or may not, retain their larval func- 

 tion. Most of the leg muscles retain their former function, but many of 

 the others, including all of those which form tlie imaginal wing muscles, 

 change their function during pupal life. From this, it is readily seen 

 that many of the names of these muscles, given from their function in 



