ADULTS: The most reliable way to distinguish a damselfly (sub-order Zygoptera) from a dragonfly (sub-order Anisoptera) is based on the wing venation. However, as a rough guide, damselflies tend to be more slender-bodied that dragonflies, and many (but not all!) rest with their wings held vertically. Their compound eyes tend to be widely separated, in contrast to dragonflies where the eyes tend to meet in the middle of the head.
LARVAE: The larvae of nearly all species are aquatic, with just a few tropical species found in moist leaf litter. Damselfly larvae are slender-bodied and have terminal gills ('tails'). Many larvae are readily identifiable to family, based on the shape of the terminal gills. To identify a larva to genus or species usually requires microscopic examination.