Relationship between proboscis length and dilator muscle volume in Japanese hawkmoths (Lepidoptera: Sphingidae)

Abstract In Lepidoptera, extremely long adult proboscises have evolved in several lineages, enabling access to nectar deep within long‐tubed flowers. However, such elongation likely incurs biomechanical costs during nectar uptake due to increased fluid resistance. To extract nectar, adult lepidopterans are thought to use a suction pump within the head capsule, which generates negative pressure through the contraction of dilator muscles attached to the pump chamber. The development of these muscles may help compensate for the functional costs associated with proboscis elongation, but the relationship between dilator muscle volume and other morphological traits remains poorly understood. In this study, we examined 35 species of hawkmoths (Sphingidae: Macroglossinae, 19 species; Smerinthinae, 8 species; Sphinginae, 7 species; and Langiinae, 1 species) collected in Japan, encompassing a wide range of proboscis lengths (3.2–75.9 mm). Using X‐ray micro‐computed tomography (micro‐CT), we quantified the volume of the dilator muscles associated with the suction pump. We then investigated how proboscis length and other morphological traits (head width, body length, and forewing length) influence dilator muscle volume using phylogenetic generalized least squares (PGLS) models. Our results showed that the dilator muscle volume increased with both proboscis length and head width even after accounting for phylogeny. These findings suggest that the evolution of long proboscises in hawkmoths has been accompanied by the development of enlarged dilator muscles, likely as a compensatory adaptation to overcome increased resistance during nectar uptake. Moreover, the observed association with head width indicates that cranial dimensions may constrain and/or facilitate the evolution of suction musculature.