Abstract
Some species of criocerine beetles have a hyper-elongated part of the intromittent organ called a flagellum. In resting position, the flagellum is stored in a specialized internal sac in the intromittent organ. This specialized state of the flagellum and internal sac is indispensable during copulation for flagellar insertion into the female spermathecal duct for sperm transfer. However, the morphogenesis of the flagellum does not generate the active state of the flagellum; rather, the flagellum is generated in an inactive and completely coiled state. After eclosion, males of Lema coronata evert and withdraw the internal sac multiple times before sexual maturation, without mounting a female. This behaviour serves to uncoil the flagellum and guide it into the active state with the aid of surface structures on the internal sac. A closely related species, Lema dilecta, also has a long flagellum and undergoes the same behaviour to place the flagellum in the active position. However, some other species of criocerine beetles with much shorter flagella can attain the active state without exhibiting this behaviour. Based on a previously proposed phylogenetic tree, we discuss the evolutionary history of the hyper-elongation of the flagellum and associated behaviour.
Highlights
The hyper-elongation of part of an intromittent organ is widespread in animals, and in some cases intromittent organs can reach lengths several times longer than body length [1,2].2017 The Authors
To evaluate the uniqueness of the rearrangement of the flagellar position and the behaviour observed in L. coronata, we explored these features in three closely related species [24]: Lema dilecta Baly, 1873, L. diversa Baly, 1873, and L. scutellaris Kraatz, 1879
The flagellum was merely coiled in the middle of an inflated pocket in which folds were recognizable, but the flagellum was not slotted into the fold of the pocket
Summary
(h) evaginated membrane evaginated membrane pocket (i) sclerite slotted flagellum flagellum coiled flagellum evaginated membrane. The evolution of such hyper-elongation is widely accepted as the result of sexual selection [3,4,5,6,7] and is accompanied by the evolution of supportive structures and/or behaviours. Many organisms have acquired a hyper-elongated intromittent organ. We describe a case in which a beetle species has overcome the challenge of acquiring a hyper-elongated structure in a unique way
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