Appendage loss and regeneration in arthropods: a comparative view

Evidence for loss and regeneration of arthropod appendages is reviewed and discussed in terms of comparative developmental biology and arthropod phylogeny. The presence of a preferential breakage point is well documented for some, but not all, lineages within each of the four major groups - chelicerates, myriapods, crustaceans and hexapods. Undisputed evidence of true autotomy, however, is limited to isopods, decapods and some basal pterygotes, and claimed for other groups. Regeneration of lost appendages is widespread within arthropods, even if not present or documented in some groups. During regeneration, growth and differentiation of epidermis, nerves, muscles and tracheae are to some extent mutually independent, thus sometimes failing to reproduce their usual developmental interactions, with obvious consequences on the reconstruction of the lost part of the appendage. In the regeneration of appendages composed of ‘true segments’, all the segments the animal is able to regenerate are already present (with extremely rare exceptions) following the first post-operative molt, whereas the regeneration of flagellar structures is often accomplished in steps, e.g., the first regenerate may show a reduced number of flagellomeres. Lack of autotomy is likely to be the plesiomorphic condition in arthropods, a condition maintained in the Myriochelata (myriapods plus chelicerates). Autotomy evolved within the Pancrustacea, perhaps close to the origin of a Malacostraca-Hexapoda clade, and was subsequently lost by some lineages, e.g., the Hemipteroidea and the endopterygote insects. A diaphragm reducing the risk of hemorrhage at the preferred breakage point of the appendage is generally associated with autotomizing appendages, but this anatomical specialization has been lost in some groups, including one (the Dictyoptera) where autotomy is still present.