I read that they are carnivores
from
Dragonflies at Animal Corner
"All dragonflies are carnivorous in both the larval and adult stages of their lives. Dragonflies typically eat mosquitoes, midges and other small insects like flies, bees and butterflies, catching its prey while it is flying. A Dragonflies' ability to manoeuvre in many directions makes them able to out-fly their prey.
Dragonflies also have the advantage of excellent eyesight. Each of their two large eyes is made up of thousands of six-sided units. Together, these smaller eyes enable a dragonfly to detect even the slightest movement. They have large optic brain lobes and 80% of their mental processes are devoted to vision and they can detect colour, ultraviolet light and polarization."
and the average dragonfly can eat its own weight in 30 minutes.
I also found this here -
Odonata: Dragonflies and Damselflies
"Dragonflies are generalists, that is, they eat whatever suitable prey is abundant. Oftentimes, they hunt in groups where large numbers of termites or ants are flying, or near swarms of mayflies, caddisflies, or gnats. According to most studies, the main diet of adult odonates consists of small insects, especially Diptera (flies). Maturing dragonfly larvae feed very intensively, as do females when developing their eggs. Studies show that food shortage may limit reproductive behavior. Dragonflies do not hunt in cold weather. Damselflies, however, are not as limited by temperature and have been observed hunting during cold spells. Males are territorial, sometimes patrolling for prey for hours at a time.
Though dragonflies are predators, they themselves must be wary of many predators. Birds, lizards, frogs, spiders, fish, water bugs, and even other large dragonflies have all been seen eating odonates. However, dragonflies have many adaptations enabling them to avoid predation. They have exceptional visual responses and truly agile flight."
Mia Culpa! 
I must have been hungry at the time 
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