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Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co‐evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.

Male genitalia are typically highly variable across species, for which sexual selection is thought to be responsible. Sexually selected traits characteristically show positive allometry and high phenotypic variation, although genitalia seem to be typified by negative allometry due to stabilizing selection. Additionally, while sexual selection appears to be the primary force responsible for genital evolution, the precise mechanism is unclear, but good-genes selection could be involved. If so, male genital variation should correlate with some male quality measure(s). We investigated the allometry of male Nyctalus noctula genitalia and investigated associations between genital size and three phenotypic measures of male quality (body size, relative body mass, and fluctuating asymmetry (FA)). We found that the penis exhibited positive allometry and high phenotypic variation, and was positively associated with male body size and relative body mass, but not with FA. This pattern is more typical of sexually selected display traits, contrasting with general patterns of genital allometry. The baculum was negatively allometric and was not associated with any quality measure. Our results suggest that the N. noctula penis is under directional sexual selection and is a reliable indicator of male phenotypic quality.

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In Drosophila melanogaster, the DDT resistance allele (DDT‐R) is beneficial in the presence of DDT. Interestingly, DDT‐R also elevates female fitness in the absence of DDT and existed in populations before DDT use. However, DDT‐R did not spread regardless of DDT‐independent selective advantages in females. We ask whether sexual antagonism could explain why DDT‐R did not spread before pesticide use. We tested pre‐ and post‐copulatory male fitness correlates in two genetic backgrounds into which we backcrossed the DDT‐R allele. We found costs to DDT‐R that depended on the genetic background in which DDT‐R was found and documented strong epistasis between genetic background and DDT‐R that influenced male size. Although it remains unclear whether DDT‐R is generally sexually antagonistic, or whether the fitness costs noted would be sufficient to retard the spread of DDT‐R in the absence of DDT, general fitness advantages to DDT‐R in the absence of DDT may be unlikely.

Insect genitalia have been extensively studied for taxonomic purposes, but functional anatomy has rarely been examined. We report here on the detailed internal anatomy of the reproductive tract of female yellow dung flies (Scathophaga stercoraria) and the mechanics of copula and sperm transfer. Female dung flies have paired accessory glands, three spermathecae (one singlet and one doublet), each with its own narrow duct, a large muscular bursa copulatrix, which is met by the common oviduct dorso-anteriorly, and paired lateral oviducts and ovaries. The bursa is lined internally with a thick cuticle. During copula and while ejaculating, the male aligns the gonopore with the spermathecal duct entrances to the bursa and pinches the female's abdomen at approximately this point. Sperm packing in the spermathecae appears quite orderly, and structurally the sperm appear typical of many insects. Aedeagus withdrawal appears to remove some bursal sperm. The results are discussed in relation to other Diptera.