Untangling the coxa-trochanter joint appendotomy in spiders (Arachnida: Araneae)

The ability to discard body parts at a preferential locus of weakness is widespread across the Metazoans' Tree of Life and has received attention from ecological and behavioral perspectives. Still, in many cases, a clear understanding of its genetic, morphological, and anatomical pathways is lacking. Distinct terms (autotomy, autotilly, autospasy) have been used in the literature to describe the process of limb detachment, usually carrying in their meaning information regarding the mechanics involved in it, and appendotomy is the most general word to encompass all of them. Appendotomy is found early in the evolution of the Arthropoda lineage and varies among taxa. Spiders, for example, have four distinct pre-determined fracture lines on their exoskeleton, and little is known about the co-occurrence of these lines within spider species. In this study, we examine the appendotomy at the trochanter of spiders using light and scanning electron microscopy to reconstruct the evolution of this character across the spider's tree of life. We describe trochanter structures and correct previous misunderstandings on how the appendotomy at the trochanter occurs in spiders. Our data show a clear, preferred breakage plane on the spider trochanter and although we focused on this appendotomy, we investigated its concomitant occurrence with other fracture areas of the leg. We show that the proximal rim of the trochanter breaks during appendotomy, leaving fragments on the distal end of the coxal arthrodial membrane. We also present several new records of appendotomy among 127 spider families examined and reconstruct the ancestral state of this character. Due to these discoveries, we conclude that the term "coxa-trochanter" appendotomy is misleading, and we propose replacement by the term "trochanter appendotomy".