It won't be happy to be kept in a jar for too long. Damselflies eat small insects, which they usually catch in long grass or reeds around a pond. You didn't say where you actually found your damselfly as this species is a northern one.
Heaven
12-Jun-2007 19:07
this is the dragonfly I caught today 6/12/07 I would like to know what it eats we kept it in a jar and put holes in it and a bunch of twigs and grass hardly twigs though! I am studying it right now as I'm writing about it! I found it in the grass then it flew away and came back my cousin saw it whaen it came back and thats when I put a jar on it. We found two of them but we didnt catch it it was brown and black I think!