INSECT SPINASTERNAL MUSCULATURE — CHADWICK I47 



with the orthopteroid groups, but their possession of jsps-pls (as 

 pls-jils) raises the same obstacle that we have just confronted in dis- 

 cussion of the biting lice. Thus, the position of the Mallophaga and 

 Phasmatodea remains uncertain. One may note in passing that sev- 

 eral of the structural peculiarities of these two orders are shared by 

 the Mecoptera. 



Direct derivation of the Embiodea from the Orthoptera or Psocop- 

 tera (p. 142) overlooks the fact that the Embiodea have been re- 

 ported (Maki, 1938, fig. 16, No. 9) to have an anomalous f ureal 

 muscle inserted on the prosternum between the nerve cords, anterior 

 to the normal isps-fui, which is present. Apparently there are also 

 other anomalies in the ventral body musculature of the Embiodea, and 

 their true affinities will not be clarified until the nature of these pe- 

 culiar muscles has been resolved. Meanwhile, it seems most probable 

 that the Embiodea are offshoots of the orthopteroid stem somewhere 

 in the vicinity indicated on page 143. 



The spinasternal musculature of the remaining orders that have 

 been suitably studied, namely Ephemeroptera, Thysanoptera, Ho- 

 moptera, Hemiptera, and Diptera, is not such as to permit further 

 inferences as to the relationships of these groups. 



CONCLUSION 

 The conclusions that we have been able to reach, on the basis of 

 the spinasternal musculature, are given for the orthopteroid and 

 ncuropteroid orders in table 3. In figure i, some groups of more 

 doubtful affinity are included, in positions that reflect what seem the 

 most likely among the various possibilities for their derivation. Since 

 our judgment still rests on rather scanty data for a number of orders, 

 we have been concerned primarily to suggest the sort of analysis that 

 a really thorough comparative study of the musculature of insects 

 would permit ; in this sense, we should like to regard our present con- 

 clusions as a beginning rather than an end. 



Table 3. — Probable patlnvays of evolution of the spinasternal muscles in certain 

 orthopteroid and ncuropteroid orders 



1. Prethysanura (28 muscles; see table 2) 



Less 2sps-epSs; convert to ligaments isps-fui, 2sps-fui, ssps-fuj, isps-iils, 

 2sps-2ils, ssps-sils, isps-cpsi; convert 2Sps-iils, 3sps-2ils, Isps-sils 

 to 2sps-pli, 3sps-pU, Isps-pli, respectively Thysanura 



Less Isps-sils Prepterygota (2) 



2. Prepterygota 



Less isps-2sps, 2sps-3sps, ssps-Isps Preneuropteroids ( 10) 



Less 2sps-iils, ssps-si, ssps-sils, 3sps-fui Preorthopteroids (3) 



(Continued) 



