The hexapod appendage: Basic structure, development and origin

The present paper, based on comparative morphological and paleontological d ata and on some conclusions drawn from developmental biology, deals with th e origin of the uniramous appendages, their segmentation, and several probl ems concerning the constitution of their proximal and distal parts, protopo dite and telopodite respectively. Particular attention is focussed on extan t and fossil apterygotes. It is concluded that all arthropod limbs, despite their considerable diversity, present common morphological features and si milar genetic mechanisms in their development, so are probably derived from a common ancestor appendage. The uniramous legs of hexapods and myriapods probably arose from biramous appendages of certain crustacean groups. From the structure of their legs land many other characteristics), two basal cla des, the Protura and Collembola, appear to occupy phylogenetically isolated positions within the hexapods.