Static allometry suggests female ornaments in the long-tailed dance fly (Rhamphomyia longicauda) are not deceptive

Despite their prevalence in nature, the evolution of sex-specific female ornaments is still not well understood. Although in some cases (often carotenoid-based ornaments) they appear to honestly signal quality, such as fecundity, it has been suggested that some female ornaments have evolved to deceptively to obtain matings. We address these two hypotheses in the long-tailed dance fly ( Rhamphomyia longicauda ), where females possess two sex-specific ornaments: pinnate scales on the hind femur and tibia and abdominal sacs that are inflated in female-biased display swarms. Although several studies have suggested that female ornaments in this species are deceptive, evidence is mixed and requires further investigation. Here, we use static allometry (with body size as a proxy for condition) of both ornamental and non-ornamental traits in females (and homologous non-ornamental traits in males) in order to determine whether they are honest or deceptive signals of quality. Most male traits scaled isometrically with body size, however male leg hairs showed positive static allometry, probably because they are involved in nuptial-prey capture or in grasping mates. Ornamental traits in females (abdomen area and tibia scale length) showed significant positive allometry and had steep slopes relative to non-ornamental traits. As larger females invest more in ornamentation relative to smaller females, this suggests that these traits are likely honest, condition-dependent signals of quality. We note that honesty and deception are not mutually exclusive hypotheses. Individuals may vary in their signalling strategy, resulting in, for example, deception from some low condition individuals but honesty overall. Although our finding of positive allometry makes it unlikely this occurs in long tailed dance flies, simultaneous honesty and deception should be considered in future studies of female ornamentation.

Competition for access to mates predicts female-specific ornamentation and polyandry

Sexually selected ornaments are highly variable and the factors that drive variation in ornament expression are not always clear. Rare instances of female-specific ornament evolution (such as in some dance fly species) are particularly puzzling. While some evidence suggests that such rare instances represent straightforward reversals of sexual selection intensity, the distinct nature of trade-offs between ornaments and offspring pose special constraints in females. To examine whether competition for access to mates generally favours heightened ornament expression, we built a phylogeny and conducted a comparative analysis of Empidinae dance fly taxa that display ornate female-specific ornaments. We show that species with more female-biased operational sex ratios in lek-like mating swarms have greater female ornamentation, and in taxa with more ornate females, polyandry is increased. These findings support the hypothesis that ornament diversity in dance flies depends on female receptivity, which is associated with contests for nutritious nuptial gifts provided by males. Moreover, our results suggest that increases in female receptivity lead to higher levels of polyandry and sperm competition among males. The incidence of both heightened pre-mating sexual selection on females and post-mating selection on males contradicts assertions that sex-roles are straightforwardly reversed in dance flies.

Empidine dance flies pollinate the woodland geranium as effectively as bees

Most flowering plants species rely on insects for pollination, a successful mutualism allowing them to reproduce over wide areas while flower-visitors are rewarded with food. This association is so conspicuous in the case of bees that other groups of potential pollinators, especially flies, have long been underestimated. However, visitors are not always pollinators. While the importance of flies in plant–visitor networks is now acknowledged, their pollination effectiveness has hardly been investigated. In this study, we assessed the pollination effectiveness of Geranium sylvaticum flower-visitors using single-visit seed set experiments, in a subalpine meadow where flies are predominant. We found that: (i) empidine dance flies were the most frequent visitors of G. sylvaticum ; (ii) a single-visit by an empidine dance fly produced the same average number of seeds as a visit by a bee; (iii) large pollinators were more efficient than small pollinators irrespective of their identity. As a conclusion, large empidines were the main pollinators of G. sylvaticum . Considering the high diversity and abundance of flower-visiting fly species, such results showing their ability to be as effective pollinators as bees should encourage further studies to develop a better understanding on their role in plant–pollinator networks.

Trichoclinocera (Diptera: Empididae): a new aquatic dance fly record for Missouri

We report first record of the occurrence of the aquatic empidid genus Trichoclinocera Collin in Missouri based upon aquatic macroinvertebrate samples collected during March 2016 from two riffles in East Fork of the Black River in Reynolds County in the Ozark Highlands. Substrate characteristics and notes on habitat from where larval specimens of Trichoclinocera were collected are also provided.