Lice???

so, my second batch of chicks have hatched and I have another batch on the way. with my first batch of chicks, I took them away from the hen to hand raise them and I found these wierd flat, grey things all over the chicks. I googled mites and it's without a doubt, NOT mites. They have these long flat grey bodies and are quite big (about 4mm long). When I give my chicks a dusting of tick, flea and mite powder, they die INSTANTLY and sort of 'reverse' out of the fluff and drop off.
Tonight I checked my new batch of babies and they too have these wierd things. What baffels me is that the hen is as clean as a whistle....not one! It's almost as if the chicks hatched with these thing. can anyone tell me what these things are, where they come from and what is the best way of dealing with them?
The babies are old english game bantams.