28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



coxae with the lateral margins of the sternum in generalized insects 

 (figs. 12 C, i6B, d) is further evidence that the sternum in such cases 

 includes the infracoxal parts of the subcoxae, especially since it is 

 found that, where distinct subcoxal laterosternal sclerites exist (figs. 

 1 6 A, 17, Ls) , the coxae articulate with these sclerites (d) . 



It is difficult to find in the insects a good example of a simple pri- 

 mary sternal plate, comparable with the sterna of the Chilopoda (fig. 

 15), that does not contain either the following intersternite or sub- 

 coxal laterosternal elements, or both. In the mesothorax and meta- 

 thorax of the ephemerid nymph shown in figure i6 A, the sterna 

 {S2, S3) may contain the intersternites, but the two small sclerites 

 in each segment (Ls, Ls) that articulate between the sternum and the 

 coxa on each side appear to be the only remnants of subcoxal lateroster- 

 nites. In the large embiid Cylindrachaeta (fig. 17) laterosternal plates 

 {Lsi, Lsz) likewise are distinct, though the intersternites are clearly 

 united with the primary stemites. In the prothorax of the aeschnid 

 larva shown in figure 16 B the intersternite {2lst) is independent of 

 the sternum, but the laterosternites {Ls) are fused into the lateral 

 sternal margins. 



These several forms make it clear that the definitive thoracic ster- 

 num of insects is typically a compound plate. It consists of a primary 

 sternite (fig. 18 A, B, Stn), to which may be added the succeeding 

 intersternite {1st), which becomes the spinasternum (C, D, Ss), and 

 a pair of laterosternites (D, Ls, Ls) derived from the ventral arcs of 

 the adjoining subcoxae (B, C, Sex). 



The possession of paired apophyses, or furcal arms, is character- 

 istic of the thoracic sterna of all pterygote insects. The apophyses 

 arise from the sternal plates between the bases of the legs, and their 

 outer ends are usually closely attached, either by fusion or by short 

 muscle fibers, to the inner ends of the corresponding pleural apophyses. 



Weber (1928, 1928a) advances the view that the sternal apophyses 

 are primarily invaginations formed on the line of union between the 

 primary sternites and the subcoxal laterosternites. In some insects, 

 however, in which there are laterosternal plates not united with the 

 sterna (figs. 16 A, 17), the origins of the sternal apophyses {sa) are 

 still well within the sternal margins; and in an aeschnid nymph (fig. 

 16 B) the apodemal invaginations {sa, sa) are removed from the 

 apparent margins of the laterosternite sections {Ls) of the definitive 

 sternum. From evidence of this nature the writer would regard the 

 sternal apophyses as invaginations in the primary sternal plate itself 

 (fig. 18 B, Stn), though there is much in favor of Weber's view. The 

 mesostemum of wingless females of the black aphis, Aphis fabae, 



