Oh, no. When the swarm is dispersed. (I missed the page break and hadn't seen your post - turns out I'm agreeing with you.)
Well we appear to be more-or-less agreed so I might as well rough something out.
For the standard gossamer, we just need to add a size for the puddle.
Deliquescence (Ex): 1d3 rounds after a gossamer dies, its remains melt into a foul-smelling puddle 2½ ft. across. The viscous liquid of the puddle is corrosive and poisonous. Contact with the fluid does 1 point of acid damage and exposes the touching object to the gossamer's poison (see below). The dangerous properties of the fluid evaporate away 1d10 minutes after the creature deliquesced.
For the Gossamer Swarm, it'll be a bit more complicated, since we need to modify the Voidborn trait too.
Deliquescence (Ex): Whenever a gossamer swarm takes damage, some of the gossamers in the swarm die and melt into a foul-smelling puddle of toxic fluids 5 feet across. Contact with the fluid does X point of acid damage and exposes the touching object to the gossamer swarm's poison (see below). If the entire swarm is destroyed the resulting puddle is 10 feet across and causes Y acid damage on contact. The dangerous properties of the fluid evaporate away 1d10 minutes after the puddle is formed.
Voidborn (Ex): Gossamers can exist comfortably in the harsh conditions of space. They take no damage from radiation or vacuum. However, gossamers are exquisitely sensitive to gravity and pressure.
If a gossamer swarm enters the air envelope or gravity field of a creature or object larger than Diminutive size, a portion of the gossamers in the swarm will collapse in on themselves and die. If the obstacle weighs more than the total mass of the swarm (roughly 300 pounds), the entire swarm is destroyed, otherwise the swarm takes damage as indicated in the following table.
[TABLE="width: 200"] [TR] [TD]
Size[/TD] [TD]
Damage[/TD] [/TR] [TR] [TD]Tiny[/TD] [TD]X[/TD] [/TR] [TR] [TD]Small[/TD] [TD]XX[/TD] [/TR] [TR] [TD]Medium[/TD] [TD]XXX[/TD] [/TR] [TR] [TD]Large[/TD] [TD]XXXX[/TD] [/TR] [TR] [TD]Huge or bigger[/TD] [TD]XXXXX[/TD] [/TR] [/TABLE]