The tropical spirostreptid millipede Alloporus uncinatus has a polygynandrous mating pattern that is apparently shaped by sperm competition Dangerfield 1990, 1993a). In the present study radioisotopic labelling of ejaculates was used to quantify the temporal effects of double mating sequences on sperm precedence patterns. Ejaculates of successive males mix completely within the sperm storage organs and are concentrated close to the site of fertilisation, the spermathecal-oviduct junction. When matings follow each other immediately (experiment 1), equal volumes of the ejaculates of each male are stored within the spermathecae, and both males have equal probability of paternity. Where matings are separated by a 24-h delay (experiment 2), the volumetric contribution of the first male is reduced by 54.8% and last male precedence operates. Although the distal ends of the gonopods play a primary role in the redistribution of ejaculates (Barnett and Telford 1994), by using a single mating sequence we show that they are not responsible for the reduction in ejaculate volume. The most likely explanation is absorption and/or ejection of the first male's ejaculate by the female. Genital functional morphology is used to reconstruct the mechanism of ejaculate distribution.
Male and female genital morphology of the spirostreptid millipede Orthoporus pyrhocephalus was examined by conventional and scanning electron microscopy techniques. The relative syncopulatory positions of the genitalia were established through dissections of pairs in copula. Male gonopods bear scoops and regions of pitted spines. both of which are similar in form to morphological structures implicated in insect sperm displacement. Consideration of the position of the oviduct‐spermathecal junction in females predicts that displacement is restricted to a sperm removal mechanism. The importance of spermathecal shape in determining sperm precedence patterns is discussed. The orientation of the genitalia during copulation indicates that sperm displacement is plausible and a mechanism of gonopod action is suggested.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.