What is the largest dragonfly .?

Dragonflies (Order Odonata) definitely live more than 24 hours. Although the adults are relatively short-lived (usually no more than four weeks), the aquatic immatures (naiads) may live for up to three years before molting to the adult stage. Your friend may be mistaking them for mayflies (Order Ephemeroptera), in which the adults of some species do not live more than a day or so. Well, I don't know if I'm really improving his/her answer but drogaonflies live for a very little time. They are very weak and fragile, and even though they're tiny they could be pretty noticable by their predators.