https://classic.wowhead.com/npc=922/silt-crawler and https://classic.wowhead.com/npc=1088/monstrous-crawler drop https://classic.wowhead.com/item=5938/pristine-crawler-leg for https://classic.wowhead.com/quest=1258/and-bugs but the spots that appear on the map are lacking. Especially Silt Crawlers don't appear at all:

v5.7.0

There is nothing in the v5.7.1 changelog that mentions this quest. In any case I've completed it already so I can't check. Indeed looking for "Silt Crawler" shows lots of spawns, but they didn't appear when I was on the quest.

@wodim Could you update to v5.7.1 and try again? There're a lot of spawns in the DB: