126 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



sors of the trochanter in Diplura (Maki, 1938), which seem not to 

 occur in any pterygote insect. They are omitted from this discussion 

 mainly because of the relative scarcity of information about their dis- 

 tribution and exact structural relationships. 



a. Transverse spinal muscles: 



1. isps-iils 2sps-2Us ssps-sils 



2. JSpS-epSz 2SpS-CpS3 



1. Two kinds of transverse muscles are recognized here. The first 

 is inserted laterally on the Us in each intersegment, and is either at- 

 tached medially on the corresponding spina, or passes directly across 

 the body to the Us of the opposite side. Examples of the latter con- 

 dition are found in the first thoracic intersegment in adult Neuroptera 

 (Czihak, 1956), in the trichopteran StcnopsycJie (Maki, 1938, muscle 

 No. ^2) , and perhaps by the same author as muscle No. 20 in Ctena- 

 croscelis (Tipulidae). It is also possible that the muscle recorded as 

 number 56 by Maki (1938) in the mecopteran Neopanorpa is of this 

 type. A similar arrangement exists in some Odonata, but in others 

 the muscle has been replaced by a sclerotized bridge that runs across 

 the body, as is also the case in the second thoracic intersegment of 

 these insects (Maloeuf, 1935; Maki, 1938; Clark, 1940). Even when 

 the first type of transverse muscle is not replaced by sclerotization, 

 it is frequently ligamentous. 



The first type of transverse muscle has also been noted, as icv-icv, 

 in the cervical intersegment of Myrmelcon (Czihak, 1956; M. inter- 

 cervicalis), and is probably represented as part of the "tentorium col- 

 laire" of lepismatids (Barlet, 1951) ; in neither case is there a median 

 attachment. The medial connection to the integument is present in 

 the third thoracic endosternum of Thysanura (Barlet, 1951), but is 

 of course ordinarily absent here in Pterygota, where postmetathoracic 

 transverse muscles or ligaments occur in some coleopterous larvae 

 (Speyer, 1922; Chadwick, unpublished), Mallophaga (Mayer, 1954), 

 Mecoptera and Phasmatodea (Maki, 1936, 1938). In Collembola the 

 third spina is preserved, so that the muscle actually appears as 3sps- 

 pls, (Maki, 1938, No. 8y in Neanura, No, 8^ in Folsomia). In larval 

 Corydalus, the ligament that corresponds to jsps-pls extends inward 

 from ^ils and supports some of the longitudinal ventral muscles, but 

 fails to reach the median junction identified as "ssps" by its recep- 

 tion of other muscles of the third spina. As already noted, transverse 

 muscles of the abdominal intersegments have been found in many in- 

 sects ; see Ford (1923) ; Maki (1938) ; Chadwick (1957). 



2. The second type of transverse muscle listed occurs only in the 



