The blue dasher (Pachydiplax longipennis) is a dragonfly of the skimmer family. It is the only species in the genus Pachydiplax. It is very common and widely distributed through North America and into the Bahamas.^[1]

Although the species name longipennis means "long wings", their wings are not substantially longer than those of related species. Females do, however, have a short abdomen that makes the wings appear longer in comparison.^[1] The blue dasher grows up to 25–43 mm long (1-1.5 in).

The males are easy to recognize with their vibrant blue color, yellow-striped thorax, and metallic green eyes. Females are somewhat less colorful than the male, an example of sexual dimorphism. While they have a matching yellow-striped thorax, their abdomen has a distinct brown and yellow striping that sets them apart from the male, along with contrasting red eyes. Both sexes develop a frosted color with age.^[2]

Pachydiplax longipennis exhibits aggression while finding mates and foraging,^[3][4] and they are not under any conservation threats.^[5]