Different sperm number in the spermatophores of Orchesella villosa (Geoffroy) (Entomobryidae) and Allacma fusca (L.) (Sminthuridae)

Members of the basal hexapod group Collembola perform indirect sperm transfer; males deposit spermatophores on the soil and these are then picked up by females for insemination. Orchesella villosa exhibits a dissociated mating behavior, while Allacma fusca has either associated mating behavior or, more commonly, produces spermatophores without pairing. Sperm ultrastructure in the two species has been studied by TEM and SEM observations to estimate the volume of these cells. The sperm number into each spermatophore has been determined by counting the fluorescent nuclei after Hoechst staining of sperm droplets squash preparations. Sperm droplets at the apex of spermatophores contain numerous spermatozoa of typical appearance, wound up and with a long anterior appendage (peduncle). Sperm droplets of Orchesella villosa are smaller (about 10 times) than in A. fusca, however they contain substantially more sperm cells (about three times). These differences could be explained as follows: (a) sperm of O. villosa are shorter than those of A. fusca (58 and 107 μm, respectively) and thus the volume of a single sperm of O. villosa is about 10 times smaller than in A. fusca; (b) considering the volume of sperm droplets and of sperm cells, the estimated number of sperm cells would be higher in A. fusca than in O. villosa; and (c) the conspicuous reduction of the sperm cells observed in A. fusca is likely linked to the aberrant meiosis present in this species, as well as in all Symphypleona studied so far, that causes a loss of 50% of their sperm.