This effect does not target.
If "Ground Spider" is no longer on the field when its effect resolves, the monster will still be switched to Defense Position.
If your opponent Special Summons multiple monsters and at least one of them is Summoned in Attack Position, you can activate this effect.
If your opponent Special Summons multiple monsters and you activate this effect, all of the Summoned monsters are changed to Defense Position.
You cannot activate this effect during the Damage Step.