Blue-Tailed Damselfly

Ischnura elegans — the blue-tailed damselfly — is one of the most familiar damselflies in temperate Europe and one of the most-studied species in evolutionary biology of female color polymorphism. The species is widespread across Europe, North Africa, and into central and eastern Asia. Adults are 30-35 mm body length with slim damselfly bodies. Males are entirely dark with a distinctive bright blue 'tail spot' on the eighth abdominal segment (the source of the species' common name). Females are the truly extraordinary part of the species' biology — females come in THREE GENETICALLY-DETERMINED COLOR MORPHS with dramatically different appearances. Form A ('androchromatype' or 'andromorph'): female-with-male-coloration — these females look essentially identical to males, with dark body and bright blue tail spot. Form B ('infuscans'): tan-brown with green-and-black abdominal markings. Form C ('rufescens'): pinkish-orange or salmon coloration. Newly-emerged females of all three morphs go through a brief 'tenerals' color phase that doesn't reflect the genetic morph; the genetic morph emerges over the next 1-3 days as the cuticle develops fully. The polymorphism is maintained in populations by NEGATIVE FREQUENCY-DEPENDENT SELECTION via SEXUAL CONFLICT: males search for and attempt to mate with females, and males have an evolved 'search image' for the most common female morph in their local area. Rare female morphs are encountered less frequently, experience less male harassment, and have higher reproductive success per unit time. As soon as a rare morph becomes more common, male search images shift toward it and the previous rare morph becomes harassed instead. The result is a stable polymorphism with all three morphs persisting in the same populations across generations. The androchrome (form A) particularly benefits from the strategy — males that don't recognize her as female ignore her entirely, allowing her to forage and lay eggs without harassment. The species is one of the most-cited examples of intra-sexual color polymorphism in modern evolutionary biology research.