132 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



reasonably certain component of the ancestral pattern, even though it 

 does not invariably occur in all species of the groups in which it is 

 found. In Thysanura, we identify Barlet's (1953) No. j/ vi^ith this 

 muscle, since No. 57 is attached on what seem to be appropriate por- 

 tions of the first and second thoracic endosterna and passes ventral 

 to the transverse ligaments n and n\ Further comment on the muscle 

 2sps-fui may be restricted to the statements that, although it is not 

 found in larval dytiscids (Speyer, 1922; Chadwick, unpublished), it 

 does occur, together with 2sps-iils, in larval Corydalus (Megaloptera) 

 (Kelsey, 1957; Chadwick, unpublished) ; and that in Neuroptera it is 

 known only from larval Myrmcleon (Korn, 1943, No. lisnt^). 



The serial homologue, ^sps-fuo, has been found among Pterygota 

 only in the larva of Cybister (Chadwick, unpublished). Apparently 

 this muscle is absent in the closely related larva of Dytiscus (Speyer, 

 1922). In Thysanura, Barlet (1953) has indicated that muscle No. ^5 

 is the serial homologue of No. 57; hence we interpret No. 4^ as 

 equivalent to ^sps-fu^. A still more posterior homologue (No. 55 of 

 Barlet, 1953), which would be hps-fih in our notation, has not been 

 identified in other insects. 



9. The muscles from the second and third spinae to the respective 

 preceding Us are seldom found. No apparent equivalent is seen in 

 Thysanura, but Maki (1938) has described in a dipluran, Lepido- 

 campa, two muscles (Nos. 42 and 67) that probably belong here. 

 Muscles similar to both of these occur in larval Cybister (Chadwick, 

 unpublished) ; in larval Dytiscus only the anterior is found (Speyer, 

 1922, No. 1140), as in larval Myrmeleon (Korn, 1943, No. lisnts) 

 and in larval Corydalus (Kelsey, 1957; Chadwick, unpublished). 



From the little that is known of these muscles we can infer only that 

 they probably represent a primitive part of the hexapod pattern that 

 began to disappear at a relatively early date, and that they are distinct 

 morphologically from isps-fu^ and 2sps-fiiz, as shown by their simul- 

 taneous occurrence with these muscles in certain primitive forms, as 

 well as by the fact that the f ureal muscles must pass beneath the trans- 

 verse muscles, and hence beyond the Us, in order to reach their an- 

 terior site of attachment. 



10. The pleuro-endosternal muscles of Thysanura (Barlet, 1953, 

 Nos. 5p, 60, 61,) are not included in table i but are considered here 

 because their posterior attachment is on a region of the endosternum 

 that evidently corresponds with the spina of Pterygota. They are 

 known only from Lcpisma. The spinal attachment of No. 61, Isps-pl^, 

 is abdominal. As Barlet (1953, p. 229) has suggested, these muscles 



