This can be done without bait (for example ground beetles and spiders) or with bait (for example dung beetles).
One or more fence-lines of some sort may be added to channel targets into the trap. This is done to reduce the amount of rain and debris entering the trap, and to prevent animals in dry traps from drowning (when it rains) or overheating (during the day) as well as to keep out predators. The opening is usually covered by a sloped stone or lid or some other object.
A little detergent is usually added to break the surface tension of the liquid to promote quick drowning. The fluids that can be used in these traps include formalin (10% formaldehyde), methylated spirits, alcohol, ethylene glycol, trisodium phosphate, picric acid or even (with daily checked traps) plain water. Wet pitfall traps are basically the same, but contain a solution designed to kill and preserve the trapped animals. Dry pitfall traps consist of a container (tin, jar or drum) buried in the ground with its rim at surface level used to trap mobile animals that fall into it. They come in 2 main forms: dry and wet pitfall traps. Pitfall traps come in a variety of sizes and designs.