While looking for caterpillars on the branches of an ancient basswood I spied what I thought was a yellowish slug, maybe 6-7 cm long, creeping slowly up the trunk of the tree. I wandered over to have a look and was quite surprised by what I found: a slug-shaped phalanx of black-headed, white larvae (looking very maggot-like) was creeping en mass up the tree. The formation moved steadily, at about 2-4 cm per minute, and the individual larvae remained in constant contact the whole time I watched them (i.e., no one larva got ahead or behind the others although they did reshuffle themselves with respect to their relative positions en route). My first thought was that these may be sawfly larvae, but I could be very wrong. I'm hoping that someone can help me identify this organism by my photos and description of this behavior. I'm befuddled! ;-)