NO. I INSECT THORAX — SNODGRASS 97 



as a rudimentary median claw (/>). In Cam pod ca and Machilis the 

 median claw has disappeared and only the lateral claws remain. The 

 median arolium of many pterygote insects might be regarded as the 

 transformed body of the dactyloi)odite, though it appears more likely 

 that arolium, empodium, and pulvilli are all secondary formations. 



The origin of the two lateral claws of the insects pretarsus should 

 be determined by a study of the transformation of a single-clawed 

 larval pretarsus, such as that of a caterpillar, into the two-clawed 

 structure of the adult, but this has not been done. De Meijere believes 

 that the claws are merely outgrowths of the base of the dactylopodite, 

 though he admits no evidence of this has been found in insects, but 

 he points out that the possibility of claws arising thus is shown by 

 the presence of dorsal claw-like structures on the dactylopodite of 

 certain isopods (locra, Janira, Munna). The articulation of the claws 

 to the end of the tarsus might suggest that they belong to the tarsus, 

 as claimed by Jeannel (1925), but the fact that the cavities of the 

 claws open through their wide bases into the lumen of the pretarsus 

 shows that the claws lielong tf) the terminal segment. The tarsal 

 articulation of the claws, then, becomes of no more significance than 

 the articulation of any other leg segment to the segment proximal 

 to it. That the claws are not transformed setse is demonstrated by 

 their multicellular structure, and by the frequent occurrence of true 

 setae upon them. 



There still remains to be discussed the question as to what was the 

 nature of the first articulations between the segments of the insect 

 leg. Borner (1921) believes that the double, or dicondylic, hinge is 

 the primitive one, and that the single articular point, or monocon- 

 dylic hinge, has resulted from a dorsal migration and union of 

 anterior and posterior condyles. Prell (1912) argues the reverse, 

 basing his claim on the fact that the joints of the telopodite in the 

 Protura have each a single dorsal articulation, and are provided with 

 flexor muscles only. The coxo-trochanteral joint is so constantly 

 dicondylic in arthropods that there is no reason for supposing it ever 

 harl any other structure ; but the common occurrence of monocondylic 

 articulations at the other joints in the legs of larvae of holometabolous 

 insects, and their association with a dactylopodite-like end segment, 

 as in the tChilopoda (fig. 32), furnishes a basis for believing that the 

 primitive joints in the telopodite of the insect leg were of the mono- 

 condylic type. Further evidence of this is to be seen in the fact that 

 the segments of the maxillary and labial palpi are provided each with 

 only one muscle, a condition seldom found in connection with dicon- 

 dylic joints. 



