The Fen Striders had a decent drop rate - took 15 or so kills to get the tentacles.  The Spore Bats, however, were a completely different story.  I started out needing 50,000 xp to level, and at about 1000 xp per Spore bat kill I had the 4 eyes one kill after I leveled, which means I killed about 50 Spore Bats (NOT younger or greater) to get the 4 eyes.  Worst drop rate I've ever seen.