Abstract
Insect spermatozoa are often very long helical structures, propelled through the medium by a modified flagellum. Ultrastructural and hydrodynamic investigations on two model species (Aleochara bilineata, A. curtula, Coleoptera: Staphylinidae) allowed establishing a new model of insect sperm motility. However, locomotion of spermatozoa in fluids is not efficient. Furthermore, the dimensions of sperm and the female´s genital tract suggest that swimming in free media can never occur, the spermatozoa “crawl” in narrow ducts of the female´s genitalia. Here, long sperm move faster than short ones. The passage of sperm from the male into the spermatheca (the female´s storage organ), the performance within the spermatheca, and the pathway out to the site of fertilization is followed under natural conditions. The interface between sperm and female on the way “in” is the spermatophore, a secretory device injecting sperm, accomplished by osmotic forces. On the way “out”, spermatozoa have to overcome several mechanical obstacles within the spermatheca.
After having reduced the proximate factors to basic physical and physiological principles, the allocation of sperm is followed by counting sperm through the “ins and outs”, resulting in hypotheses on sperm competition and cryptic female choice. In A. curtula, these ultimate reasons correlate with genetic determination of paternity.
Comparative investigations on many species of the large genus Aleochara show a tremendous variety of spermathecal structures, e.g. elongations, valves, constrictions and compartments. Spermatophores also show different degrees of complexity, and spermatozoa may differ in length and arrangement of organelles. For some species we can correlate the modifications of the spermatheca with the function of the spermatophore and the performance of spermatozoa.
A preliminary phylogenetic analysis along with a molecular tree suggests that sexual selection has driven the coadaptive evolution of male-female interactions, resulting in functional analogies, convergences and parallelisms, reductions and exaggerated traits. Further phylogenetic studies are needed to establish these evolutionary trends