Shorty has a casing from the dragonfly group commonly called Darner and technically called Aeshnidae (Family) with several different genera and numerous species. This group has large sized individuals as nymphs and without good dense habitat for them, they don't survive very long due to predation in ponds with high densities of sport fish, thus you usually don't see lots of the nymphs in these types of ponds. Various interesting names are Blue darner, Canada darner, Spatterdock darner, Black-tipped darner, Green -striped darner, Comet darner, Fawn darner, Swamp darner, etc. For some interesting pictures see:
http://www.pbase.com/tmurray74/darner_dragonflies http://www.fcps.edu/islandcreekes/ecology/green_darner.htmhttp://www.azdragonfly.net/species/giant-darnerI think the adult hatched from the casing because if it molted underwater the casing would not have floated. When the dragonfly molts out of water it traps air in the casing and it will float. The casing probably fell in the water after the adult hatched.
