Data from: Eversion and withdrawal of an intromittent organ before sexual maturation prepares male beetles for copulation

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.部分负泥虫亚科（Criocerine beetles）甲虫的交配器官（intromittent organ）具有一段极度延长的结构，称为鞭突（flagellum）。静息状态下，该鞭突收纳于交配器官内特化的内部囊（internal sac）中。这种鞭突与内部囊的特化状态，在交配过程中对于鞭突插入雌性受精囊管（spermathecal duct）以完成精子传输而言不可或缺。然而，鞭突的形态发生过程并不会直接生成具有活性的鞭突，而是先形成处于非活性且完全卷曲状态的结构。羽化成虫后，冠饰负泥虫（Lema coronata）的雄虫在性成熟前会多次外翻并回缩内部囊，期间并不会攀附雌虫。该行为可借助内部囊表面的结构将鞭突解开卷曲，并引导其进入活性状态。近缘物种双突负泥虫（Lema dilecta）同样具有较长的鞭突，且会通过相同行为使鞭突处于活性位置。但另有部分负泥虫亚科甲虫的鞭突较短，无需展现此类行为即可进入活性状态。基于此前已提出的系统发育树（phylogenetic tree），本文探讨了鞭突极度延长现象及其伴随行为的演化历程。