NO. I INSECT THORAX SNODGRASS 21 



bearing the ventral muscle attachments have thus come to form appar- 

 ent posterior elements of the sterna, or have been incorporated into 

 the posterior parts of the sterna, and in this respect, as pointed out 

 by Weber (1924), the sternal structure in the thorax is comparable 

 to the tergal structure in those segments where a postnotal plate and 

 phragma, originally intersegmental or a part of the succeeding tergum, 

 become a part of the preceding tergum. By analogy, the interseg- 

 mental or posterior part of the definitive sternum may be termed the 

 poststcrnuin (fig. 3 A, PS). That the poststernal part in each segment, 

 which sometimes consists of two parts (postfurcisternite and spinis- 

 ternite of Crampton) is a true intersegmental element has been effec- 

 tively stated by Weber (1924), who says : " One may conclude with 

 all appearance of truth that the postfurcasternite has arisen from the 

 membranous region between the true sterna, perhaps as a result of 

 the muscles attached to it, and that it is a structure in every way 

 similar to the postnotum of winged insects, though doubtless of older 

 origin." Where two postfurcal sclerites are present, the second, or 

 spinisternimi, Weber believes, is only a detached piece of the first 

 comparable to the phragma of a postnotum. Again, reviewing the 

 prothoracic structure of the Neuroptera, Trichoptera, and Lepidop- 

 tera, in his paper on the thoracic skeleton of the Lepidoptera, Weber 

 (1924 a) says : " The fourth section of the sternum, the postfurcaster- 

 nite, is usually clearly separated from the furcasternite and is obvi- 

 ously a secondary structure. In Sialis it is only suggested, in the 

 Trichoptera it is a distinct posterior appendage of the sternum, in 

 Hepialus it becomes extremely long, and here there begins the for- 

 mation of a posteriormost and likewise secondary piece of the ster- 

 nimi, which may be identified with the spinisternite of Crampton. 

 This sclerite borders so closely on the mesosternum that it becomes a 

 question whether it should be reckoned as a part of this sclerite or 

 of the prosternum." 



The poststernal sclerites, however, are not generally persistent 

 elements of the sterna, for in most insects they are either lost or 

 become indistinguishably fused into the posterior edges of the true 

 sternal plates. Some of the ventral muscles that remain in the adult 

 stage are attached to the spina, if this process is present ; the attach- 

 ments of the others become transferred to the posterior part of the 

 sternum, where, in the Pterygota, they are carried mostly by the 

 f ureal arms. The development of the f ureal arms, or lateral sternal 

 apophyses, lias differentiated the primitive sternal plate into basister- 

 nmn and furcistenium, but the presence of these processes is a char- 

 acter of the Pterygota, there being no trace of them or other homol- 



