Ok, the reason the drop rate seems so bad on these is because, though you are in a party and so have a slightly better drop rate in general (unproven), you're only averaging 1/6 the drops of the whole party.
Another thing I've noticed. The initial damage Poison Breath deals you is related the amount of HP the Crawler has left, more HP, more initial damage. I don't think it affects the poison effect of the attack though.
It is not fun in Crawler's Nest when these cute little guys link, they play follow the leader and are preety good at doing whatever the rumble or soldier crawler does, I myself did not move and died because unlike any other person Im not sellfish and zone so any unsuspecting person can die, all and all my first entrance to Crawler's Nest was fun and Horrifying.
Actually, its a well known fact that back when, a dozen crawlers broke free from the tyrannical rule of the crawlers nest and actually built rafts and crossed the ocean, living on a meager diet of water and rolanberries. They fatefully arrived on the coast of sarabuta after 5 long days of drifting. Barely alive, tiny mandragoras came to their rescue and nursed the crawlers back to health.