Abstract
The modification of male pedipalps into secondary sexual intromittent organs is one of the hallmark characteristics of spiders, yet understanding the development and evolution of male genitalia across the order remains a challenging prospect. The embolus – the sclerite bearing the efferent spermatic duct or spermophor, and used to deliver sperm directly to the female genitalia during copulation – has always been considered the single unambiguously homologous palpal sclerite shared by all spider species, fundamental to the bauplan of the order and to the evolution and functional morphology of spider reproductive systems. Indeed, after two centuries of comparative research on spider reproduction, the presence of a single spermophor and embolus on each of a male spider’s two pedipalps remains a central tenet of evolutionary arachnology. Our findings challenge this premise, and reveal a remarkable twin intromittent organ sperm transfer system in a lineage of Australian palpimanoid spiders, characterized by a bifurcate spermophor and the presence of two efferent ducts leading to a pair of embolic sclerites on each pedipalp. This is the first time such a remarkable conformation has been observed in any group of arachnids with direct sperm transfer, complicating our understanding of palpal sclerite homologies, and challenging ideas about the evolution of spider genitalia.
Highlights
Sperm transfer in Arachnida is achieved either indirectly with spermatophores, or directly using a penis, the chelicerae or modified appendages
Even after two centuries of taxonomic, ontological and comparative morphological research, no species has ever been found with multiple “emboli,” and by extension, no spermophor has ever been recorded as having more than one efferent duct
Our findings challenge this premise, as in this study we report a lineage of palpimanoid spiders with a remarkable twin intromittent organ, whereby males possess a uniquely bifurcate spermophor with two efferent distal ducts leading to a pair of prominent embolic sclerites on each palpal bulb (Figures 1, 2)
Summary
Sperm transfer in Arachnida is achieved either indirectly with spermatophores (e.g., in Amblypygi, Scorpiones, and Pseudoscorpiones), or directly using a penis (e.g., in Opiliones), the chelicerae (e.g., in Solifugae) or modified appendages (e.g., in Araneae and Ricinulei). In most spider taxa the palpal organ is more complex – spectacularly so in some lineages, e.g., Linyphiidae (Hormiga, 1994, 2000) – with divided bulb (i.e., tegular and subtegular) sclerites, inflatable membranous elements (haematodochae) that rely on hydraulic pressure, and multiple tegular apophyses in addition to the ubiquitous internal spermophor and sclerotized embolus bearing a terminal sperm pore (Griswold et al, 2005) These palpal sclerites function in concert during copulation, engaging and inter-locking with the female genitalia (e.g., Uhl et al, 2007; Mouginot et al, 2015; Poy et al, 2020), which may explain the diversity of species-specific morphologies that occur in these structures. Our secondary aims were to understand the morphology of the palpal organ with a focus on the sperm transferring parts in the Afrotropical archaeid genera and the remaining palpimanoid families, and to pose a number of hypotheses that could be used to understand possible evolutionary or ontogenetic drivers in the context of correlated female genitalic morphology and sexual selection
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