By Austin Bartell
Humans often use handstands to demonstrate both athletic prowess and bravado. But the handstand is more often and arguably, more critically used, by dragonflies. Consisting of the Families Gomphidae, Libellulidae and Calopterygidae of the Order Odonata, these dragonflies often assume the equivalent of the human handstand: the obelisk posture (May, 1976).
Some dragonflies, including Pachydiplax longipennis, even assume this posture as they fly! When an invasive male, identified by its gender-distinctive abdominal colouration, enters another’s breeding site, he is immediately confronted by the resident male. As they face each other, with one slightly higher above the other, they each raise their abdomens high. Consequently, the bluish-white pruinescences, or dusty coatings, of their dorsal surfaces are exposed. As they exchange positions with each other by flying high and dropping back down in the opposing male’s former position, each male seeks the desired lower position. This territorial dispute ends in the displacement of one of the males from the contested area, with the loser flying either vertically or laterally out of the territory (Johnson, 1962). Since the resident male wins the majority of these competitions due to his typically larger size, he then pursues the loser for a short time (Johnson, 1962; Mccauley, 2010). Since the resident males use 85% or more of their energy reserves to defend the most favorable breeding sites, more females would be attracted to these prime places to copulate and lay their eggs (Fried and May, 1983; Jacobs, 1955). Due to the increased success of the eggs hatching when not clustered together, the dispersion of the extremely territorial dragonflies and their breeding sites throughout the ecosystem increases population reproductive success (Jacobs, 1955).
When male P. longipennis are perched in their territory, their wings are slanted down and forward while their abdomens are generally vertically oriented. By exposing their pruinescences to the fullest while at rest, these males evoke a threat display that may prevent, or at least reduce the intensity of, energetically-costly territorial disputes (Jacobs, 1955; Johnson, 1962).
Due to the evolution of the obelisk posture multiple times, some dragonfly species also use this posture to regulate their internal body temperature. By finely adjusting the alignment of their abdomens to modify the surface area that is directly exposed to solar radiation, both females and males of P. longipennis can regulate their radiative heat absorption and internal body temperature. Even a small dragonfly, like Perithemis tenera, which can gain much convective heat due to its small body size, can use the obelisk posture to effectively counter untoward increases in internal temperature (May, 1976).
Through the obelisk posture, many members of the Order Odonata compete for territory and breeding grounds, while others also maintain an optimal internal body temperature to optimize predator/prey as well as intra-species interactions (May, 1976). Consequently, the obelisk posture is an important facet of an Odonate’s evolutionary adaptations and is probably much more relevant to its fitness than handstands are to the fitness of humans!
Fried, C.S. and May, M.L. (1983). Energy expenditure and food intake of the territorial male Pachydiplax longipennis (Odonata; Libellulidae). Ecological Entomology, 8, 283-292.
Jacobs, M.E. (1955). Studies on territorialism and sexual selection in dragonflies. Ecology, 36, 566-586.
Johnson, C. (1962). A Study of Territoriality and Breeding Behavior in Pachydiplax longipennis Burmeister (Odonata: Libellulidae). The Southwestern Naturalist, 7, 191-197.
May, M.L. (1976). Thermoregulation and Adaptation to Temperature in Dragonflies (Odonata: Anisoptera). Ecological Monographs, 46, 1-32.
Mccauley, S.J. (2010). Body size and social dominance influence breeding dispersal in male Pachydiplax longipennis (Odonata). Ecological Entomology, 35, 377–385.