Bugs in my beans

[Green-Moo]

Last year I grew lots of beans at home, dried them & stored them in airproof glass jars. However, upon opening them many of the beans have been eaten by some sort of small insect. The bugs are dead in the jar. Presumably they were in the beans already as there's no way they could get into sealed jars afterwards, but I really didn't see them & I checked the beans carefully for signs of damage etc first.

Does anyone know how I can spot them & avoid them next time? It's a real shame to have a whole jar ruined!