Are you trying to measure the collective light or the light from each worm?
If the second, you could try taking an Avi of them and instead of averaging the frames, add them. That should get them bright enough to take a reading you can work with. You might have to buy a 16" dob to shoot through though, to get enough light catching power.
Seriously though you never know with the right gain and exposure my may be able to do it with a ToUcam and still average them by stacking in registax. There are plenty of programs around that will then measure the brightness.