Biting lice do look like specs of dandruf (I thought they were tine bits of lanolin) - on very close inspection they are a little bit oval and do move (very slowly). The ones I was shown (by vet) were buff coloured specs to naked eye - but unbelievably freocious looking under microscope!. My affected sheep look like they have been beautifully sheared in parts (one looks like a poodle) - perfect short wool coat underneath, no broken skin - but they were itching themselves on posts (in fact, still are so not sure the 'spot on hav been entirely sccessful, or else they have picked up more). Not Herdwicks though. Good luck, Fi