All Sensilla on the Body of the Smallest Beetle Scydosella musawasensis (Coleoptera: Ptiliidae) and Their Implications for the Evolution of the External Sensory Apparatus During Miniaturization

Extreme miniaturization may impose severe constraints on the organization of insect sensory systems, yet the extent to which sensory diversity can be reduced while retaining essential functions remains poorly understood. Here we characterize the sensilla distributed across the whole body surface of the adult minute featherwing beetle Scydosella musawasensis, the smallest known species of free-living insect. We identify four principal types of sensilla and four subtypes of trichoid sensilla, with 851–883 sensilla distributed across the body surface excluding the antennae, studied earlier. Despite this quantitative reduction compared to other Staphylinoidea, the beetle retains the sensory modalities required for interacting with its environment. Mechanosensory trichoid sensilla dominate the body surface, whereas campaniform and thorn-like trichoid sensilla are concentrated near appendage articulations and probably contribute to proprioception. Basiconic sensilla on the maxillary palps are consistent with contact chemoreception, whereas coeloconic sensilla on the head may perform thermo- and hygroreception. Thus, the distribution of sensilla types and modalities is similar to those of other insects. The sensillar complement and distribution are remarkably similar between males and females and stable between individuals. Together with observations of the antennae, these findings confirm that extreme miniaturization primarily reduces the number of sensory units while preserving the diversity and organization of the major sensory modalities.