Behavioral Ecology of the Dragonfly Libellula pulchella Drury (Odonata: Anisoptera)

Abstract

The behavioral ecology of the dragonfly Libellula pulchella Drury was studied at four ponds in Minnesota. Males of this species are highly aggressive and defend a particular portion of the breeding site from all other males. Territories are established only in areas of the pond which have water free of surface vegetation, a sunny exposure and perch sites offering an unobstructed view of the territory. Males normally remain in the same territory throughout the day and are rarely displaced by other males. They are, however, seldom present at the site for more than 1 day. Ter- ritorial defense consists of patrol, pursuit and circle flights and physical attacks. The response of a territorial male to an intruder depends largely on its behavior. Non- territorial males are more likely to evoke intense aggression. A number of other odonate species, particularly those similar to L. pulchella in size and behavior, are also the objects of aggression. At high population densities the frequency of aggressive interactions in- creases and territory size decreases. High densities also result in increased interference with mating. Interference is reduced to some extent by the brevity of copulation and by male guarding and female avoidance behavior. Certain areas of the breeding site are more attractive to females, and males with territories there achieve the most matings. A number of abiotic factors including time of year, time of day, light intensity and tem- perature have a strong influence on the pattern of male and female occurrence at the pond. Peaks in abundance of both sexes occur within similar ranges of temperature and light intensity. The temporal peak in female abundance is shorter than that of males. Time of day and light intensity have a significant effect on abundance only within the range of temperature favorable for breeding activity. Beyond this range few dragonflies appear at the pond. Even when temperatures are favorable, a large proportion of a male's time is spent thermoregulating. By varying its perching posture in relation to the sun, this dragonfly is able to maintain thoracic temperatures within the range necessary for reproductive behavior and thus achieves a certain degree of independence from fluctuation in the ambient temperature.