20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



moving segment and in it arise the levator and depressor muscles of 

 the femur; the tarsus is only two-segmented (false segments). The 

 other segments have the same relationships as in Machilis. 



REMARKS ON THE THYSANURAN TYPE OF LEG 



A comparison of the thysanuran type of leg with those of other 

 primitive groups indicates its closest affinity with the pauropod type. 

 In fact there appear to be a similarity and homology throughout the 

 segmentation series of both groups. If the first segment of the pauro- 

 pod leg be regarded as the coxa, however, instead of the subcoxa, the 

 small third segment of the pauropod leg should be regarded as a part 

 of the femur. 



THE NATURE OF THE COXAL APPENDAGE IN THE MACHILIDAE 



For years after Verhoefif (1903) claimed to have demonstrated a 

 muscle attached to the coxal appendage the belief grew that this 

 appendage should be regarded as a true branch of the limb. By some 

 it was considered as a homologue of the exopodite of crustaceans, and 

 exponents of the crustacean theory of the origin of insects seized 

 upon this bit of evidence in support of their contentions. The writer 

 has examined this structure with care and has noted the following : 



1. It arises at a break in the transverse apodeme of the coxa on 

 the outside of this segment (pi. 5, fig. 13). 



2. It has no muscles in it and no muscles passing to it and none 

 having any relation with it. 



3. Muscle fibers do pass from near the base of the appendage and 

 extend distally. These are probably the ones figured by Verhoeff 

 (1903), but they are clearly a part of the conspicuous levator muscle 

 of the trochanter. 



4. The appendage can in no way be considered as a modified seta, 

 since it possesses a lining. of hypodermis with many cells from which 

 several complete and typical setae themselves have been developed. 



5. The appendage is a development from the outer wall of the 

 coxa of which it is a direct continuation. 



THE LEG SEGMENT.S OE COLLEATBOLA 



In general form and position the collembolan legs have been modi- 

 fied somewhat by the fact that the body is compressed and locomotion 

 is greatly aided by a powerful springing organ, the furcula, near the 

 end of the abdomen. The compression of the body has frequently 

 brought the legs of a pair into a contiguous position at their bases. 



