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

Abstract

Specialized animal structures often perform multiple functions, yet how their structural adaptations support distinct behaviors remains poorly understood. Eristaline hoverfly larvae, commonly known as rat-tailed maggots, possess exceptionally elongated and extensible posterior respiratory siphons that allow them to breathe while submerged. Here, we investigated the structural basis of siphon extension and its potential locomotor function in Helophilus virgatus larvae. Fluorescence labeling showed that siphon elongation involves the unfolding of previously concealed structures, while phalloidin staining revealed an extensive array of transversely oriented muscle fibers within the siphon. Behavioral observations further revealed a previously undescribed mode of horizontal locomotion just beneath the air water interface, involving asymmetric rowing movements of the posterior siphon. Kinematic analyses showed that most forward body displacements occurred during the backward stroke, which was associated with increased siphon straightness, whereas the forward stroke produced little net forward displacement. These findings reveal structural features underlying siphon extensibility and demonstrate that a specialized respiratory organ also contributes to locomotion, highlighting the multifunctionality of specialized body structures in aquatic environments.