Female-Above Copulation in Cretaceous Stoneflies: Early Mating Strategies of Plecoptera

Stoneflies (order Plecoptera) are an ancient lineage of insects, yet their reproductive behaviours during earlier geological periods remain poorly understood. This study presents the first fossilized evidence of copulation in stoneflies, preserved in two pieces of mid-Cretaceous Burmese amber (ca. 99 Ma) from northern Myanmar. One amber piece captures a pair of Largusoperla crassus Chen, 2018 in a precopulation posture, while the other features a pair of Largusoperla vena sp. nov. during copulation, showcasing a unique female-above copulatory position. This posture suggests a more active female role in mating compared to the relatively passive role observed in modern stoneflies. Morphological analyses reveal well-preserved genital structures, including elongated male aedeagi, hook-like paraprocts, and bilobed female subgenital plates, supporting the hypothesis that this female-above position may represent an earlier reproductive strategy. The asymmetric male genitalia in L. crassus may indicate a transitional stage in the evolution of copulatory behaviours. Taphonomic analyses suggest one pair was entangled on the forest floor, while the other was preserved in a drier, elevated microhabitat. These findings provide insights into the ecological contexts and preservation conditions of these mating behaviours, shedding light on the environmental factors that facilitated such rare behavioural fossilization.