NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK 3 



writer's dissections for representatives of a number of other orders. 

 All obsei-\'ations cited in this paper without a statement as to source 

 are from my own work. 



Each morphologically distinct muscle was given a designation 

 formed by hyphenating the accepted abbreviations for the skeletal 

 parts between which the muscle is stretched. Under this system, if 

 an attachment is segmental, the segment is identified by an arabic 

 or roman postsubscript for the thorax or abdomen, respectively, while 

 the designations of intersegmental structures are preceded by the 

 appropriate arabic numeral, beginning with o for the cervical inter- 

 segment. Exception: the customary abbreviations icv, 2cv ... for 

 the cervical sclerites, and lax, 2ax ... for the axillary sclerites of 

 the wing, the latter with segmental subscripts, are retained. Cruciate 

 muscles, with origin and insertion on opposite sides of the longitudinal 

 body axis, are distinguished by adding X to the usual designation. The 

 skeletal abbreviations used are for the most part those given currency 

 by Snodgrass (1929, and numerous other publications). 



Examples: 2sps-epSs, a muscle stretched between the second (post- 

 mesothoracic) spina (sternite) and the metepisternum ; fus-snA, the 

 longitudinal ventral muscle from the metafurca to the second abdomi- 

 nal sternum ; epSo-cxiX, a cruciate muscle of the procoxa, with origin 

 on the contralateral mesepisternum. 



A glossary of abbreviations is given at the end of the text. 



OBSERVATIONS AND INTERPRETATION 



The ventral intersegmental muscular system of the blattid thorax 

 includes elements with primary attachments on the spinae {sps) or 

 on the intersegmental laterosternites {Us), as well as f ureal (/w) 

 muscles that run between successive segments. This report is divided 

 accordingly into three main sections. 



I. THE SPINAE 



Cockroaches have two authentic spinae {isps, 2sps), and in addi- 

 tion possess in the third thoracic intersegment a common junction of 

 serial homologs of the more anterior spinasternal muscles that lacks 

 the median connection with the integument but obviously represents 

 a postmetathoracic spina. This junction {"ssps") is attached by 

 fibrous ligaments (figs. 4, 6, 7, 9, 10, 1%: 27) to the bases of the 

 metathoracic furcal arms {jus), between which it floats above the 

 nerve cord. Comparative evidence leaves little doubt that these non- 

 contractile ligaments, which now usually appear as fug-fus, have been 



