NO. 6 INSECT ABDOMEN SNODGRASS 35 



the individual muscles of the median internal dorsals may be 

 specifically indicated idiiii, 2diiit, ^d'un, etc., the external laterals ilc, 

 2le, ^le, etc.. and the muscles of the other groups in like manner (fig. 

 15 A). If it is desired to show that a muscle belongs to a particular 

 segment, this may be expressed by adding to its symbol a Roman 

 numeral designating the number of the segment, thus idhnll , ^vimlV , 

 2leVI , etc. In describing the complete musculature of a species, how- 

 ever, the writer has found it more practical to number the muscles 

 with Arabic numerals, rather than to attempt to follow any system 

 of lettering that ])retends to identify homologous muscles in con- 

 secutive segments. 



The dorsal inusdcs. — The muscles of the dorsum are primarily 

 composed of longitudinal fibers of segmental length attached on the 

 intersegmental folds ; in many larvae the principal dorsal fibers retain 

 this primitive condition. Wherever the dorsum, however, contains 

 fully-developed sclerotic terga, a secondary segmentation is estab- 

 lished, and the folds on which the dorsal muscles are attached become 

 the antecostae of the definitive tergal plates (fig. 14 C, Ac). The 

 longitudinal, primitively intrasegmental muscles thus become func- 

 tionally intersegmental, and serve to contract the abdomen in a length- 

 wise direction by retracting each tergum into the posterior end of 

 the segment preceding, as far as the intersegmental membrane will 

 allow. The anterior end of a longitudinal abdominal muscle, there- 

 fore, may be termed the origin, and the posterior end its i)iscrtion. 



The dififerentiation of the dorsal fibers into internal and external 

 muscles is the rule in both adult and larval stages of pterygote in- 

 sects. The internal dorsals commonly retain their longitudinal posi- 

 tions, their segmental lengths, and their attachments on the antecostae ; 

 but there are many departures from this generalized condition. Fre- 

 quently the fibers take an oblique position, and sometimes they become 

 shorter than segmental length by a migration of their origins to the 

 postcostal area of the tergum, or of their insertions to the precostal 

 area. The external dorsals seldom retain a segmental length ; typically 

 they are short muscles lying in the posterior parts of the segments 

 (fig. 14 C, (/('), and often they become strongly oblique, sometimes 

 actually transverse, giving a movement of torsion between the twcj 

 segments they connect. Finally, the external dorsals mav become 

 completely reversed in position (D, dc), their origins being so far 

 back on each tergum that they lie ]:)Osterior to the points of insertion 

 on the anterior edge of the precostal rim of the following tergum. 

 In such cases, the external dorsals liecome antagonistic to the internal 



