my pits were like that:
one small room (2x1) with a pit directly above.
the room is connected to the main hive through a drawbrige which is closed when the infesting takes place (for obvious reasons)
first throw one larva into the pit (this is important, multiple larvae will kill the host and die themselves)
then the host and wait for the larva to attack
if the larva wins, wait for the host to transform. then and only then you can lower the bridge
if the larva loses, throw another one
here's how it looks like
pit:
----------
----x-----
----------
(the x is the pit, while the - means floor)
directly below:
||||||||||
||||--b---
||||||||||
(the | means wall, the b means raised drawbridge)
I hope this was helpful. I'll try posting an actual screenshot later.
