l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



The writer has made a special attempt to locate muscle fibers pass- 

 ing to these clavate structures (pi. 4, fig. 9) but has failed in every 

 attempt although the muscular tissues in the segment from which 

 they spring were clearly revealed. In some instances a few minute 

 strands (sensory nerve?) were found passing into the coxa from the 

 clavate structure, but in no instance did these reveal the striations 

 characteristic of muscles. Probably the more plausible explanation 

 of these clavate structures is that they are modified setae. Somewhat 

 similar pectinate setae are found on the body in P aura pus. But these 

 specialized body setae all possess a seta pit, or alveolus ; such a struc- 

 ture is wanting at the base of the clavate structures. 



THE PAUROPUS TYPE OF LEG 



The question may be asked : How does the leg-segmentation and 

 musculature of Paiiropiis compare with that of primitive insects? 

 Compared with that of Protura the following differences are noted : 

 The tarsus is three-segmented instead of one-segmented, two tro- 

 chanters may be regarded as present instead of one, and the subcoxa 

 exists either as a ringlike, functional, movable segment, instead of 

 being represented by two motionless plates, or according to the other 

 interpretation shows no vestige. In Pauropus, also, more of the 

 muscles pass through two or more segments. Compared with the legs 

 of Thysanura and Collembola, and of insect larvae a general cor- 

 respondence is indicated. In one respect the pauropods appear to be 

 more primitive in their leg characters than most insects, i. e., in the 

 great length of the flexor muscles, which pass through two or more 

 segments. 



LEG-SEGMENTATION AND MUSCLE ATTACHMENTS LN 

 SYMPHYLA 



In Symphyla the generalized legs, z. c, the ambulatory appendages 

 except for the first pair, show five complete and movable segments 

 and in addition an immovable basal sclerite with a hinge for the 

 first complete segment. The muscle attachments for these segments 

 are shown in pi. 4, fig. 10. According to tlie musculature the long 

 penultimate segment should be regarded as the tibia since it gives 

 rise to the flexor muscles of the claws. The last, claw-bearing seg- 

 ment is the tarsus. The antepenultimate segment, the one proximal 

 to the tibia, is vei*y short and is at or beyond the bend of the leg. It 

 has the size and function of a i)atella in the Arachnida. Furthermore, 

 this is the segment that is eliminated in the four-segmented front 



