Coevolution of spermatozoa and spermathecae in Lonchopteridae (Diptera)

Across the species of spear-winged flies (Diptera: Lonchopteridae) there is a remarkable variation in size of the female reproductive tract, especially of the spermathecae. In this family there are two tubular spermathecae, which are divided into four morphologically and histologically distinct sections of different lengths and functions. The dimensions of the spermathecae and their individual sections were examined across 11 Lonchoptera species and related to the dimensions of the respective spermatozoa. 3D reconstructions from serial sectioning made it possible to include the volume in these considerations, which is a new approach in this context. Results show that the spermathecae are always longer than the respective spermatozoa. There is a highly significant positive linear correlation between the length of the spermatozoa and the length of the spermathecae in total as well as some of the individual spermathecal sections, suggesting a coevolution of these characters. Moreover, the volume of the spermathecae is much larger in those species with longer and more voluminous spermatozoa, but the volume increase is not sufficient to keep constant the number of spermatozoa that fit within. The observed patterns are discussed with respect to their functional and evolutionary implications, including a new hypothesis on the possible selective advantage of increased spermatozoon length.