A foldable respiratory siphon powers near-surface locomotion through asymmetric rowing in Helophilus larvae

Extensible body structures have evolved repeatedly across animals, yet the mechanisms underlying the deployment of extensible organs often remain unknown. Eristalinae hoverfly larvae (rat-tailed maggots) possess exceptionally elongated posterior respiratory siphons, but the mechanism underlying their extension has not been experimentally investigated. Here, using wild-collected Helophilus virgatus larvae, we show that posterior siphon extension is achieved through a folding– unfolding mechanism revealed by fluorescence labeling. Phalloidin staining further demonstrated that, unlike Episyrphus sp. and Drosophila melanogaster, H. virgatus possesses a dense array of transversely oriented muscle fibers in the posterior siphon. Behavioral analyses further revealed that the posterior siphon functions not only in respiration but also as a propulsive organ for near-surface locomotion through asymmetric rowing. Together, our findings identify the structural and kinematic basis of posterior siphon deployment and demonstrate how a specialized respiratory organ can evolve into a multifunctional appendage that supports both respiration and locomotion.