The nature of allometry in an exaggerated trait: The postocular flange in Platyneuromus Weele (Insecta: Megaloptera)

The origin and function of exaggerated traits exhibited by a great number of species with sexual dimorphism remain largely unexplored. The usual model considered as the evolutionary mechanism for the development of these structures is sexual selection. The nature of growth of the postocular flange (POF) in three species of the dobsonfly genus Platyneuromus (Megaloptera, Corydalidae, Corydalinae) is analyzed to explore sexual size dimorphism and allometric scaling. Results involve positive allometry of POF in males of two species, and negative allometry in males of one species, in general with a female-biased sexual dimorphism. We suggest an ancestral condition of dual incipient ornamentation in Platyneuromus, with a subsequent departure of size and shape of POF in males, triggered by sexual selection. Different sexual selection intensities may explain the parallel or divergent growth of POF within the scheme of dual ornamentation. Empirical behavioral data as well as a phylogenetic framework are necessary to clarify possible causes of phenotypic development, time of origin, and evolution of the POF.

诸多具有性二态性（sexual dimorphism）的物种所展现出的夸张性状，其起源与功能在很大程度上仍未得到充分探索。目前被广泛视为这类结构演化发育机制的经典模型为性选择。本研究针对广翅目（Megaloptera）、泥蛉科（Corydalidae）、齿蛉亚科（Corydalinae）扁齿蛉属（Platyneuromus）的3个物种的眼后突（postocular flange，POF）生长特性展开分析，以探讨其体型性二态性与异速缩放规律。研究结果显示：2个物种的雄性个体眼后突呈现正异速生长，剩余1个物种的雄性个体则表现为负异速生长，整体呈现雌性偏倚的性二态性。我们推测扁齿蛉属的祖先类群具备双重雏形装饰结构，随后在性选择的驱动下，雄性个体的眼后突在大小与形态上发生分化；不同强度的性选择或可解释该双重装饰结构框架下，眼后突生长模式的平行演化与趋异演化。未来需结合实证行为学数据与系统发育框架，方能阐明眼后突表型发育、起源时间及其演化的潜在成因。