14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



similar to those from which the Crustacea descended. If the insects 

 have descended from Crustacea-Hke ancestors, a primitive condition 

 in Insecta should simulate some condition evolved in the Crustacea ; 

 but in the Arachnida it should simulate a condition established among 

 tJie ancestors of the Crustacea. 



LEG SEGMENTATION AND MUSCULATURE IN PAUROPODA 



Because of their relation to the Crustacea on the one hand and 

 to the Protura on the other (see Berlese, 1910) the Pauropoda should 

 be considered as a likely group in which to increase our knowledge 

 as to the typical form of an insectan leg. According to Kenyon ( 1895) 

 all the legs in pauropods are composed of six segments except the 

 first and last pairs which have only five segments. Kenyon called 

 the basal segment the coxa and believed that the fifth and sixth seg- 

 ments in six-segmented legs equalled together the last segment in the 

 five-segmented legs. After studying specimens prepared in various 

 ways the writer has concluded that two additional rings should be 

 recognized in all legs of Pauroptis. Near the base of the segment 

 regarded as the third by Kenyon there is a small rudiment of a seg- 

 ment which in some of the legs has considerable size (pi. 4, fig. 8, 

 frll), and his sixth segment shows an incomplete division near its 

 middle (pi. 4, fig. 8). Thus the writer would recognize eight rings to 

 the generalized Paurdpus leg ; however, as we shall see when the muscle 

 attachments are studied, not all of these are to be considered as true 

 segments. 



In well-mounted specimens of Pauropus all the legs except the first 

 and last show the following segmentation (pi. 4, fig. 8) : 



The basal segment is barely a completed ring. It is more or less 

 triangular in shape and ends in a long apodeme which extends upward 

 in the lateral body wall. The next segment is short and stout and 

 sets on the first by means of a rocking hinge. The third segment, 

 never a completed ring, is fused with the fourth, which is short. The 

 fifth is the longest leg segment. It is singly hinged dorsally to the 

 fourth and bears the sixth by means of a similar hinge. The seventh 

 and eighth segments are not completely separated by a suture, which 

 is always evident along the ventral margin of the leg. The claws set 

 directly on the end of the last segment without any interpolated 

 chitinous piece. 



When the muscles are studied it is found that there are no fibers 

 originating in any of the last three segments. Segment six is a com- 

 plete ring and is movably fastened to seven by membranous tissue 

 yet gives rise to no muscle fibers. All three, of these segments 



