I20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



M. lateralis primus (il). — A tergosternal dilator of the abdo- 

 men. Origin on ventral margin of tergum IV; insertion dorsally 

 on lateral apodeme (c) of sternum IV. 



M. lateralis secundus (2I). — Origin dorsally on anterior lateral 

 part of tergum IV external to dil; extends posteriorly and ventrally 

 to insertion on posterior lateral part of sternum IV. 



M. lateralis tertius (5/). — Origin dorsally on lateral part of ter- 

 gum IV external to dil; extends ventrally and anteriorly, external 

 to 2I, to lateral margin of sternum IV. The second and third laterals 

 are tergosternal compressors of the abdomen. 



M. occlusor spiracidi. — A very small muscle between the ends of 

 the closing valve of the spiracle. (Spiracle lies external to lateral 

 muscles.) 



M. dilator spiraculi (dls). — Origin on lateral arm of sternum; 

 insertion on lower end of closing valve of spiracle. 



The musculature of the first postabdominal segment (fig. 39, //) 

 is somewhat reduced. The internal dorsals (dim, dil) are both pres- 

 ent, though they are shorter than segmental length. The external 

 dorsal (de) arises dorsally on the posterior part of tergum // and is 

 inserted ventrally on the antecosta of tergum ///. Of the sternal 

 muscles the lateral internal ventral is absent; and there is but one 

 lateral muscle present (2I), which corresponds with the second 

 of the other segments. 



The last exposed segment of the postabdomen, or segment VII, 

 is conical in shape. Its basal part is largely retracted within the 

 sixth segment ; its apex forms the tip of the abdomen. The tergum 

 and sternum are separated posteriorly on each side by a deep cleft 

 (fig. 43 B), and the narrow posterior end of the sternum (A) is 

 cut by a median notch through which the sting glides when it is pro- 

 truded. The tergum (B, VIIT) is a large bonnet-shaped plate with a 

 prominent apodeme (a) projecting anteriorly from each side of the 

 dorsal part of its anterior margin, on which is inserted a strong 

 protractor muscle from the posterior margin of the sixth tergum. 

 The seventh sternum (A, VIIS) is a broad heart-shaped plate with 

 its expanded basal lobes concealed above the sixth sternum. The 

 body cavity in the seventh segment is greatly reduced by the in- 

 vagination cavity containing the genital segments, the sting, and the 

 proctiger (fig. 40). This cavity, formed by inflection of the in- 

 tegumental membrane (&zyi) from the posterior margins of the 

 tergum and sternum of the seventh segment, is rather more than an 

 ordinary vestibular invagination, and is generally called the sting 



