34 BONE STREWN CORRIDOR. The floor of this hall is marked by the bony remains of men and small humanoids and by the empty, dry husks of giant insects. At the spot marked '34' there is a covered shallow pit, the lair of a giant trapdoor spider beneath the floor. The cover is made of stones and webbing and so artfully made that an adventurer has only a 1 in 20 chance (1 in 6 for an elf) of realizing the concealed cover is present if they are simply walking by.
Giant Spider (AC 4; MV 3"*12"; HD 4+4; hp 24; #AT 1 bite; D 2-8; SA Webs, poison; XP 435)
The spider will wait for the final steps to pass by, then come out and attempt to bite the last person in line. It is deathly quiet and has a 5 in 6 chance of surprise if its trapdoor has not yet been discovered. It will not attempt to attack creatures of bugbear size or larger.