I know your challenges here Harsha, it is *just not easy* ! That said, I would still think of possibilities to light it with an external light source to get it better defined. I think key is you need have a light source such that the intensity of reflected light from an external source should be less than emitted light from these flies. May be you can try blue LED flash light from a far off distance or with weak batteries in it. Blue light also may enhance the feel/mood of night. Best of luck next time !

I loved your firefly image and also what Ganesh suggested. I think you can try it out and am eagerly waiting to see the results.