NO. II THORACIC MUSCLES OF COCKROACHES — CHADWICK I5 



transparent. They are more easily seen, though not immediately 

 recognizable as muscles, in the nymph, where they were first dis- 

 covered by Scharrer (1948) as the bearers of the prothoracic glands. 

 The glandular tissue, which encases the tenuous contractile filament 

 and thus renders it more visible (fig. 7:3), degenerates soon after 

 metamorphosis, but the muscular core persists throughout life. The 

 origin is near the anterior end of the first cervical sclerite, lev, which 

 for this and other reasons is to be regarded as incorporating the cervi- 

 cal Us; and the insertion is on the proximal margin of the contralateral 

 coxa vera, just laterad of the coxotrochantinal articulation. Cor- 

 responding cruciate promotors of the second and third coxae have not 

 been identified in any pterygote insect, but there is a possibility that 

 they are represented in the usual spinal promotors isps-cxg, 2sps-cxs. 



c. Lateral jiireal intersegmental muscles. — In cockroaches furcal 

 muscles whose origins are on the Us or on their present equivalents in- 

 clude only the cruciate muscles epSg-ftiiX of the first intersegment ; 

 and the three postcoxal ligaments lUs-fiii, etc. The cruciate muscle 

 has been discussed in the preceding sections. 



The postcoxal ligaments are often frail and transparent, and there- 

 fore easily overlooked in dissection ; and they dissolve rapidly in alkali. 

 These characteristics no doubt explain why the ligaments have not 

 received more attention from morphologists, for they are quite fre- 

 quently present in primitive insects. (See fig. 6: 13, 24, 31, and fig. 7 : 

 24, as well as the figures of cockroaches.) 



By a process that has many analogies in the evolution of the ptery- 

 gote thorax, the postcoxal ligaments have often been replaced, in 

 phylogeny, by apodemal growths, a course of development that cul- 

 minates in a firm skeletal union between the furcal arm and the suc- 

 ceeding Us. Such unions constitute, or at least contribute to, the post- 

 coxal bridges, whose interpretation has interested several previous 

 students of insect morphology. 



Cockroaches, however, show little or no indication of the trend to- 

 ward formation of a postcoxal bridge by sclerotization along the line 

 of this former muscle. Only in Blaberus, of the blattids I have seen, 

 is the distal end of the ligament 2US-JU2 converted into a stiff, well- 

 sclerotized apodemc ; whereas the usual course of evolution of a post- 

 coxal bridge in other Ptergygota seems, contrariwise, to have been 

 via sclerotization from the furcal attachment outward. 



In fact, the general impression left by the blattids is that their 

 tendency is toward obliteration of these ligaments, and this tendency 

 is increasingly manifest as one passes from the prothorax to the 

 metathorax. All the cockroaches studied possess a fairly strong and 



