A Histological Study on Copulation Duration, Patterns of Sperm Transfer and Organization inside the Spermatheca of a Grasshopper, Dichromorpha viridis (Scudder)

Abstract

The mechanism of sperm transfer and sperm organization inside the spermatheca was investigated in Dichromorpha viridis (Scudder). Spermathecae from single- and multiple- mated females whose copulations were interrupted at various prescribed time intervals, were examined histologically. Sperm organization inside the spermatheca from 24 to 120 hours after copulation had terminated was also investigated. The first observation of sperm inside the spermatheca did not occur until 30 minutes had passed into copulation. The majority of sperm transferred into the spermatheca were in the form of sperm bundles, or spermatodesmes, and the rate at which sperm bundles were transferred appeared to decrease after 13 hours into copulation (average mating duration in the laboratory was 28 hours). The occurrence of abundant sperm with sperm bundles was observed only in females that had mated before and less than 9 hours into copulation, and so believed to be from previous matings. As mating continued, less and less individual sperm was observed, and by 15 hours into copulation only sperm bundles were observed inside the spermatheca. The interrupted multiple matings indicate that at least some sperm bundles remain inside the spermatheca while individual sperm were removed by some mechanism, possibly sperm flushing by an excess of seminal fluid provided by the mating male. Some sperm bundles were shown to keep their bundle integrity for at least 7 days after copulation had terminated. The sperm bundles must disassociate into individual sperm prior to fertilizing the female's eggs; therefore, it may be the female that provides the mechanism or chemical stimulus to initiate sperm bundle disassociation prior to oviposition and/or a multiple mating. The results of this investigation implicate sperm competition, perhaps mediated by female choice, as a primary reason for lengthy copulations in D. viridis. Males may also act as mechanical plugs by remaining in copula for an extended duration after a sufficient amount of sperm has been transferred, or males may be overcompensating the process of sperm removal by transferring an excess of seminal fluid to flush-out any sperm present from previous matings.