The common whitetail or long-tailed skimmer (Plathemis lydia) is a common dragonfly across much of North America, with a striking and unusual appearance. The male's chunky white body (about 5 cm long), combined with the brownish-black bands on its otherwise translucent wings, give it a checkered look. Females have a brown body and a different pattern of wing spots, closely resembling that of female Libellula pulchella, the twelve-spotted skimmer. Whitetail females can be distinguished by their smaller size, shorter bodies, and white zigzag abdominal stripes; the abdominal stripes of L. puchella are straight and yellow.
The common whitetail can be seen hawking for mosquitoes and other small flying insects over ponds, marshes, and slow-moving rivers in most regions except the higher mountain regions. Periods of activity vary between regions; for example in California, the adults are active from April to September.
Like all perchers, common whitetails often rest on objects near the water, and sometimes on the ground. Males are territorial, holding a 10 to 30 metre stretch of the water's edge, and patrolling it to drive off other males. The white pruinescence on the abdomen, found only in mature males, is displayed to other males as a territorial threat.
The nymphs are dark green or brown, but are usually found covered in algae. They feed on aquatic invertebrates such as mayfly larvae and small crayfish, and also on small aquatic vertebrates such as tadpoles and minnows. Because of their abundance, whitetail naiads are in turn an important food source for various fish, frogs, and birds, and also for other aquatic insects.
Some authorities classify the whitetails, including the common whitetail, in genus Libellula rather than Plathemis. This matter has been debated at least since the end of the nineteenth century. Recent molecular systematics evidence suggests that separation of the whitetails from the rest of Libellula may be appropriate.
Every few days, female "Plathemis lydia" dragonflies will go to ponds in order to lay their eggs, or ovipost. Male dragonflies that are defending mating territories at the ponds will then attempt to mate with the female dragonflies. While the females are at the pond to ovipost, they will actively discriminate against the males attempting to mate with them, rejecting up to 48.9% of all mating attempts. While the females prefer to ovipost in the middle of the day, and will actively look for particular parts of the pond to find a more suitable place to lay their eggs. While male rejection is quite high, mate choice appears to be of little importance. It seems that phenotypic characteristics such as body mass, wing length, and first day of reproduction do not directly affect selection in either males or females. Male-male competition on the other hand is very important, as males fight for territories that offer a better place for females to lay their eggs. This competition for territory leads to a dominance hierarchy, where individual males recognize and maintain territorial boundaries. In this hierarchy, dominant males have an advantage of mating with females over their subordinate counterparts. While it is clear that females generally fail to discriminate against males, it is not clear why they do not, given the opportunity. One such reason may be that rapid mating was selected for over evolutionary time. Predation on females during mating periods, as well as risks of males losing their territories during the process of mating may be important factors as to why females fail to discriminate.
Due to the need to fight off other males in order to be more successful in breeding, male dragonflies have developed extensive flight muscles. Much of male Plathemis lydia dragonflies body mass is accounted for by flight muscle, which is one of the highest flight-muscle ratio (FMR, found by flight-muscle mass/body weight) of an animal. Males with smaller FMRs have a slight decrease in mating success when compared to their counterparts. This decrease in success found in males with a smaller FMR can be contributed to a decrease in the ability to compete in aerial contests. However, a tradeoff must be made in order to have a larger FMR. Males that were found to have the highest FMR had the least amount of gut contents, and in turn less fat reserves. This means that the male dragonflies are making a tradeoff between flight ability and longevity, which may affect long-term mating success.
Post mating, male Plathemis lydia dragonflies will perform "non-contact mate-gaurding throughout oviposition". During this time, the male dragonflies will hover roughly 0.5-1m away from their mate, and chase of any rival males that may be attempting to mate with the female. The intensity at which males guard females is positively correlated with the frequency of male harassment that occurs during the oviposition. However, males tend to guard less intensely over the course of the oviposition process, and will stop guarding entire once it has finished. This behavior of mate-guarding relates to the territoriality of dragonflies.
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