In party positions, someone who would be considered a fly on the wall except they are less concerned with the situation at hand, and more concerned with finding any fly on the wall. A spider on the wall's intent could be for benefit, malice, or curiosity.
The more aggressive, attentive or insightful fly on the wall of flies on walls.
Sometimes, one who moves about a party trying to pick up someone who is hardly talking or interacting with anyone.
Synonyms: A wallmower in relation to a wallflower.
Similar to spider on the ceiling, and a toad on the lilly pad.