Slapsticks are only seen outside and are rarely seen above ground, implying that they live primarily underground, similar to species such as moles on Earth.
In shape they look like brown, wooden poles stuck into the ground with a metal-looking rectangle on top, replicating a common spade or shovel. They rise straight out of the ground, upwards, when they are striking, and they tend to favour slapping the victim in the face. The part of the slapstick seen above ground appears to be around two metres in height, however it may be that this was the optimal height for them to reach Arthur, Zaphod and Ford's faces to strike them, and they may be able to stretch to whichever height is best suited to their task.
They seem to have roots, which can be seen on the back of the shovel-like rectangles on the top of them, as well as the 'pole' seeming to be one long root connecting them to the ground. The long 'pole' is able to be severed, which was shown when Ford wrestled one with his towel and pulled it out of the ground. Although it appears solid, it can bend, snake-like, in order for the Slapstick to slither back underground if attacked or injured.
All of the slapsticks seen are almost identical in shape and size, however their size beneath the ground is unknown.
Behind the scenes
This creature has only appeared in the 2005 film.
Their name originates from slapstick comedy, which involves exaggerated physical violence. A common use of slapstick is when characters are surrounded by rakes or spades and find themselves standing on the end of the handles and being hit in the face by the object, which the slapsticks are a reference to.
In the special features of this film, the existence of the slapsticks is said to explain the squashed appearance of the Vogons faces, at least in the film universe, due to being attacked by the creatures living with them on Vogsphere. It is said that this is why the Vogons' noses are so high up on their faces, due to the slaps in the face forcing them higher.