Leptaxis

Abstract: Many species of land snails have one or more sharp, calcareous “love darts” that are used to stab the partner during mating. These darts are produced and stored in specialized organs called stylophores. Because their number and position varies among species, stylophores are often used for identification and classification, especially in the family Hygromiidae. Having several stylophores, and thus several darts, is presumably the ancestral state from which species with one stylophore evolved. Species with small accessory sacs or rudimentary stylophores located above the functional stylophore are therefore thought to represent intermediate forms between species with double and single stylophores. We investigated the stylophores, darts, and associated reproductive organs of three species of the hygromiid genus Leptaxis – L. erubescens, L. nivosa and L. undata. In all the specimens of the investigated species, a small sac located just above the stylophore was found to be present. We conclude that this previously overlooked organ represents a rudiment of a stylophore, leading us to conclude that Leptaxis should be considered as an intermediate form in the evolution towards a single stylophore in the Hygromiidae.