Structure, function and evolution of the ‘glans’ of the anisopteran vesica spermalis (Odonata)

Abstract

Comparative investigations of the distal part of the vesica spermalis (‘glans’) of the anisopteran male secondary copulatory apparatus reveal three different ‘solutions’ of combining the emptying-mechanism of the sperm-reservoir with a ‘washing out’ of sperm of the male predecessor. The responsible apparatus of the glans—actually driven by pressure-changes inside the erectile organ, which is a part of the whole vesica spermalis—is shortly apostrophized as ‘two-way tap’ (Gomphaeschnidae, Aeshnidae), pressure-suction pump (Austropetaliidae, Gomphidae, Petaluridae, Chlorogomphidae, Neopetaliidae, Cordulegastridae) and suction-pressure pump (‘Corduliidae’, Cordulephyidae, Gomphomacromiidae, Synthemistidae, Libellulidae). The two types of sperm-pump are interpreted to effectuate an intensification of the sperm-jet and to serve as auxiliary devices in emptying the sperm-reservoir. On account of the opposite co-ordination of extension and compression, the two types of sperm-pump are interpreted as alternative; no possibility could be detected to form evolutionary transitions without total loss of functions. This indicates two monophyletic groups: Petaluroidea and Libelluloidea. The phylogenetic relationships between these groups and the Gomphaeschnidae and Aeshnidae remained questionable. The different stages of evolution of the glans, which reflect phylogenetic splittings, are reconstructed. It is assumed that at the beginning a pre-gomphaeschnoid glans (or a gomphaeschnoid ‘two-way tap’ with tongue?) used two functional pores of ejaculation and scattered sperm on account of the erectile organ-coupled movements of the glans. Advanced glans-types of the Petaluroidea execute a three-phased delivery of sperm portions, the sperm transfer and displacement falling into the compression phase. Instead, in the Libelluloidea the sperm-transmission is two-phased and sperm-transfer and displacement are performed in the decompression phase.

Keywords: Odonata, dragonfly, functional morphology, secondary copulatory apparatus, sperm-transfer, sperm displacement, evolution, phylogenetic systematics

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