THE BASAL SEGMENTS OF THE HEXAPOD LEG* 



At the base of each leg in the Hexapoda are a series of 

 segments and sclerites which enter into the composition 

 and serve as a support of the appendage. These are the 

 trochanter, coxa genuina, meron^ trochantin, and ante- 

 coxal piece. The difficulties in the way of accounting 

 for the origin of these segments and homologizing them 

 in the various orders have caused morphologists more or 

 less trouble. 



By reason of the fusion which has taken place between 

 the trochanter and femur in the Myriopoda and Hexa- 

 poda, many writers on insect anatomy hold that the 

 trochanter is merely a portion of the femur which has in 

 some manner become constricted so as to form an ap- 

 parent but not an actual segment. The fusion, however, 

 between the two parts is a specialization acquired during 

 the later embryonic stages of development." A similar 

 case of ankylosis is often noticeable between certain seg- 

 ments of the appendages in Crustacea. Bordage^ has 

 advanced the theory, from observations based on certain 

 Phasmidas, that the two segments have become coalesced 

 in the Arthropoda as a result of ecdysis. Since the same 

 fusion, however, is very pronounced in the Myriopoda, 

 particularly among the Diplopoda, where Verhoeff"* be- 

 lieves the trochanter is in many cases entirely absent, it is 



■*■ In order to distinguish between these two parts, which have been confused under 

 the name "coxa," I have called the piece articulating with the trochanter coxa 

 genuina, and the posterior lateral part articulating with the epimeron, meron (from 

 fitjpos = thigh), since its lateral margin is always found in articulation with the 

 epimeron. 



^ The "two-jointed trochanter" of Hymenoptera [Terebrantia] appears to be a 

 secondary modification, the lower part {"apophysis," Ratzeburg] being derived from 

 the femur. This is the view held by Sharp, Camb. Nat. Hist., vol. v, p. 520. 



^ On the Probable Mode of Formation of the Fusion between the Femur and 

 Trochanter in Arthropoda. Comptes Kendus de la Societe de Biologic, tome v. No. 

 28, pp. 839-842, 1898. Also Ann. and Mag. Nat. Hist., vol. iii, pp. 1J9-I62, 1899. 



* Ein Beitrag zur Kenntniss der Glomeriden. 189 J. 



^Reprinted from The American Naturalist, vol. 34 (1900), pp. 267-274. 



