Dragonflies and damselflies are in the same order, Odonata, but they are taxonomically separated on an obscure level not built into this site, the suborder. Dragonflies are in the rarely mentioned suborder Epiprocta, and within that suborder is the infraorder Anisoptera, the scientific name by which they're best known. None of that will help you catch trout, but it explains what the hyphen in this page's name is all about.
Dragonfly nymphs emerge by crawling up various objects out of the water, so the emergence is not important. Freshly emerged flies may have a little trouble flying and end up on the water, but emergence is so sporadic that this activity never causes a good rise of fish, let alone selective feeding. The lone, struggling stray may draw an explosive rise.
Adult dragonflies are so good at flying that they very rarely end up on the water and never in good numbers. They live for a long time as adults and feed heavily on other insects in mid-air. It's great fun to watch one twist its way through a cloud of tiny midges with aerial maneuvers beyond Lockheed Martin's wildest dreams. Dragonflies are also good indicators of impending mayfly spinner falls: just like swallows and waxwings, they swoop among the spinner swarms at treetop level long before the smaller mayflies are low enough to be spotted.
Diet: Other insects, small fish
Dragonfly nymphs are extremely good predators, and they're by far the most important stage of this order for trout. They are especially common in slow or still water, where they crawl around or swim by intaking water through their mouths and propelling it out their posteriors like a tiny jet. A dead-drift presentation or a jerked retreive is ideal.
This specimen of Libella saturata, the flame skimmer dragonfly, is the only invertebrate in this site's collection that I caught by hook and line. It swooped down and attacked my dry fly on the Firehole River, where it lived as a nymph in the hot water near the geothermal springs and geyser runoff.
This is a realy big, dark dragonfly nymph. Notice how "fuzzy" it is--it's covered with small hairs that collect sediment and debris and camouflage the nymph to match its natural surroundings. The greenish cast in some of the photos of this specimen is due to strange lighting during my first night photographing things for this site.
Our great discussion of hellgrammites included a very interesting little tangent about the feeding habits of dragonflies which I'm copying into the dragonfly section.
This is pure trivia, and non-hellgrammite related, but Brett's comments on the discards of bats and nighthawks reminded me of an experience on the Delaware. I was fishing with a guide and we saw several mayfly wing and thorax combos floating along with missing abdomens. At first we hypothesized that a small trout must have bitten the abdomen right off. Then we saw a dragonfly intercept a sulphur mid flight, chew up the abdomen almost instantly, and let the wings and thorax fall, helicoptering like a winged maple seed. This is probably not news to the entomologists here, but some everyday anglers might not have yet noticed this phenomenon. Gonzo, do we have a wing and thorax tie yet?
I haven't found trout to be that interested in dragonfly discards, but you never know. I am reminded of an evening on Mud Run when my fishing partner's fly mysteriously ascended skyward, towing a good portion of the tip of his fly line. His fly, while having a minimal effect on the trout we were pursuing, was sufficiently convincing to a large dragonfly--a testament to the powerful flight and lifting capabilities of these insects!
I love watching dragonflies feed on mayflies, but I admit I've never noticed the falling discards. I'll keep an eye out for them next season. A good photo sequence of the phenomenon would be awesome, but I think my photography skills would need to jump several orders of magnitude to capture such a thing in action.
This goes for all Gomphidae. If the prementum is flat, and the animal has 4 segments in each antenna, and two-segmented fore and middle tarsi, and a ligula with no median cleft--Gomphid. The Corduligastridae have a spoon-shaped prementum and a cleft in the middle of toothlike processes on ligula. Now you'll know whenever you catch more Odonata.