There are a few standard suggestions as to what determines the difference between a dragonfly and a damselfly. These include:
- dragonflies are larger
- dragonflies sit with wings outstretched; damselflies with wings held along the abdomen
#1 is not so. Even just in the UK the Demoiselles, which are damselflies, are larger than our Darters, which are dragonflies. It would be fair to suggest, though, that dragonflies are more sturdily/robustly built. Damselfly abdomens tend to be narrow affairs.
#2 is often true but again there are notable exceptions; the so-called Spreadwing damselflies (Emerald damselflies) usually sit with their wings spread, as does the Odalisque (Epallage fatime) on continental Europe which, I regret, I am yet to see.
The technical difference is explicitely stated by the scientific names of the two sub-orders, at least, if you speak Greek, it does:
- means unequal or dissimilar wings – a dragonfly’s hind-wing is broader than the fore-wing.
- means equal or similar wings – a damselfly’s fore and hind-wings are the same shape.