130 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



in Psolodcsinus and Crocotheniis. Whether this also corresponds to 

 Clark's (1940) /// vim 2 is not sure. Walker's (1938) description of 

 Grylloblatta gives the attachments of his No. iiib as 2sps-pls (in our 

 notation), but it is probable that the second abdominal sternum has 

 been indicated as the first, in which case iiih is really 2sps-Iils. The 

 muscles / vim j and // vim 4 of Korn (1943) in larval Myrmeleon 

 (Neuroptera) may include portions equivalent to isps-2ils and 2-sps 

 ^ils, respectively, as well as parts identifiable as isps-fuo and 2sps-fnz. 

 The exact relationships are doubtful. In the adult of this species all 

 these muscles have disappeared (Korn, 1943; Czihak, 1956). 



5. In contrast to the narrow distribution of the oblique spinal mus- 

 cles of the Us, the absence of a muscle from the spina to the following 

 furca is quite unusual. Thus, for example, isps-fuo is unknown only 

 in the Odonata, Ephemeroptera, Phasmatodea, Mallophaga, Der- 

 maptera, Hymenoptera, Diptera, and possibly in Hemiptera. With 

 the exception of the Dermaptera which, as we have seen, appear to 

 have retained isps-2ils in place of the more usual isps-fu2, these 

 groups have all undergone extensive modification of the anterior ven- 

 tral region of the thorax, and the absence of isps-fu^ is probably 

 related to the skeletal changes that have occurred. In some of these 

 insects, the first spina has been lost, and elsewhere it has often seem- 

 ingly disappeared, through fusion with other exoskeletal or endo- 

 skeletal parts. 



This same list of orders, again excepting the Dermaptera, lacks the 

 corresponding metathoracic muscle, 2sps-fih. In addition, this muscle 

 has vanished, as far as is known, from the Thysanoptera, Homoptera, 

 and Hemiptera ; and from the series Mecoptera, Trichoptera, and 

 Lepidoptera. In Neuroptera, it is absent from adults, though present 

 in the larva of Mynnelcon (Korn, 1943). Possibly study of larval 

 Mecoptera would also disclose a larger number of spinasternal muscles 

 than is now known to occur in this order. Additional work on nema- 

 tocerous Diptera is also needed, for while they, like other flies, seem 

 to lack all true spinae, it is impossible to decide from Maki's (1938) 

 description of Ctenacroscclis just which spinasternal muscles may have 

 been preserved in what he refers to (pp. 244, 248) as "a. common net 

 of ventral transverse muscles." 



A homologue, ^sps-si, from the third spina to the first abdominal 

 sternum, which of course lacks a sternal arm, is known only from 

 Thysanura (Barlet, 1953, No. 52) and from larval Corydalus (Chad- 

 wick, unpublished). In the megalopteran, the muscle runs from the 

 common junction ("ssps") of spinasternal muscles of the third inter- 



