PROC. ENT. SOC. WASH., VOL. 20, NO. 3, MAR., 1918 41 



ps, which is situated behind the anterior margin of the pronotum, 

 instead of extending forward into the neck region as is the case 

 with the plate ps of figure 7, and on this account, the two may 

 not be strictly homologous. Merely for the sake of convenience, 

 however, the designation "presternum" has been applied to all 

 of the sclerites labeled ps (in all figures) which are situated im- 

 mediately in front of the basisternum bs, regardless of whether 

 they are exactly homologous in every instance. 



The third sternal subdivision is the basisternum, or basisternite 

 bs of figure 14. In Gryllus the coxo-basisternal muscles extend- 

 ing from the coxa to the basisternum, and in Periplaneta, the 

 basisterno-prescutal muscles, extending from the basisternum 

 to the prescutal region, are of some value in determing the iden- 

 tity of the basisternal region. When the basisternal region is 

 not chitinized, it is represented by the region somewhat in front 

 of, and between the coxae, as in figure 3, bs. In some larvae, as 

 in the Hydrophilid larva (fig. 5) and that of Corydalis (fig. 4) 

 the basisternum bs and pleural region are connected by a continu- 

 ous chitinous region extending in front of the coxae. 



In the prothorax of Capnia (fig. 14) the lateral portion of the 

 basisternum, Is, is still connected with the sternal region, but in 

 the mesothorax of the nymphal Plecopteron Perla shown in 

 figure 15, the lateral region of the sternum, called the latero- 

 sternite, Is, has become demarked from the remainder of the 

 sternum, as in the earwig Arixenia, shown in figure 13, Is. Voss, 

 1904, terms the region Is the "coxosternum," but this designation 

 had already been used by Verhoeff to denote the fusion product 

 of the coxa with the sternum in Apterygotan insects. In Gryllus, 

 the laterosternite (Is of figs. 13 and 15) of the mesothorax is con- 

 nected with the episternal region by the laterosterno-episternal 

 muscles, and with the scutum, by the laterosterno-scutal muscles; 

 while in the prothorax, the laterosternite is connected with the 

 cervical plates by the laterosterno-cervical muscles, and with 

 the occipital region, by the laterosterno-occipital muscles; and, 

 when they are retained, these muscles aid in identifying the re- 

 gions homologous with the laterosternite. 



The fourth sternal subdivision, fs of figure 14, is called the 

 furcasternite or furcasternum, because it bears the furca, or paired 

 internal apophyses, for muscle attachment. Since the furca is 

 composed of the two invaginations of the body wall forming the 

 apophyses, it is usually hollow, and the position of the apophyses 

 which form it is indicated externally by the furcal pits, f, of fig- 

 ures 3, 4, 14, etc. When the furca, or the paired apophyses 

 homologous with it, are present, they serve to identity (lie fur- 

 casternal region, although their position in this region is not- al- 

 ways precisely the same with regard to the coxae and other usual 

 landmarks. 



