When it's hot out, you'll often see dragonflies perched on a stem in the sun with their long bodies (abdomens) sticking straight up towards the sky. It looks like they are doing some sort of insect handstand, but they are really working on thermoregulation, and their strange posture is called obelisking. Not all dragonflies obelisk to cool their bodies, some drop their abdomens downward, some shade themselves with their wings, some circulate hemolymph through their abdominal sections, and some dive into the water...
A male Blue Dasher (Pachydiplax longipennis) dragonfly obelisking in the hot sun to regulate his body temperature.
By raising their abdomens straight up, dragonflies reduce the surface area heated by the sun, which helps them cool their bodies. Blue Dashers are famous for obelisking. They often take the stance even when the temperatures are not that high, and males also seem to use the posture as a threat display when defending their territory. Additionally, if the sun is low in the sky and it's cooler, they use the obelisk posture to heat themselves by exposing more of their abdomen to the sun's warming rays.
A male Blue Dasher dragonfly has several distinguishing field marks--a powder-blue abdomen tipped in black, amazing turquoise-green eyes in a white face, brownish areas on the wings, and very noticeable stripes on its thorax.
Blue Dashers are common in numbers but not in looks! With powdery blue abdomens and bright turquoise-green eyes, it's hard to pass them by without a second look!
Blue Dashers are "perching" predators. They like to perch in one place and fly out to catch their prey, returning to the same perch to eat it. Because they spend so much time sitting and waiting in one place without moving, thermoregulation by adjusting their posture works well for them (source: Obelisk posture, Wikipedia). Blue Dashers are formidable predators and will eat all sorts of insects including mosquitoes, flies, butterflies, moths, mayflies, flying ants and termites (source: Idaho State Univeristy).
Even as a naiad (the nymph form that lives in the water), Blue Dashers are "sit and wait" predators, hiding behind rocks and logs until the prey goes past.
...an interesting fact: Blue Dasher naiads can tolerate low levels of oxygen in the water, so just as lichens are an indicator species of a healthy environment, a lot of Blue Dasher naiads in relation to other species in an area can indicate low water quality (source: Idaho State University).
(I photographed this guy on 6/13/2010 on Pinckney Island in Hilton Head, SC. It was really hot that day and beautiful. The field guide I use to help me identify dragonflies is "Dragonflies and Damselflies of Northeast Ohio," by Larry Rosche, Judy Semroc, and Linda Gilbert)