A spider gag is a medical or ring style gag that holds the mouth open. There are several different designs but most are metal with a ring that goes in the mouth and curved metal pieces that hold the lips out of the way. These are combined with a strap or buckle that latches behind the head.
More About Spider Gag
Many spider gags are big enough to allow objects, body parts, or toys to be inserted into the mouth while the gag is being worn. Spider gags, and other medical style gags, lend themselves to doctor/patient role play, or dental play. Spider gags can also be used as part of humiliation play, because the wearer of the gag is likely to drool quite a bit.
Because a spider gag places metal against the teeth, care must be taken to not damage or chip the teeth while the gag is being worn.