Neither the forum nor general Google-Search lead me to an answer to this one, so i choose to open this up for my round and others who maybe have the same question.
Per Spell-Description:
"You summon hundreds of Diminutive spiders, which cling together in the form of a whip made for a creature of your size. You can wield this object as if it were an actual whip, except you make a melee touch attack with it instead of a regular attack. Any creature you strike with the whip takes swarm damage as if it were attacked by a spider swarm (1d6 points of damage plus poison and distraction)."
As far as I see it, the spells description contradicts itself in therms of scoring a critical hit.
"You can wield this object as if it were an actual whip, except you make a melee touch attack with it instead of a regular attack."
Ok, I make an melee touch attack instead of a regular attack, but I do make an attack, roll for it and may score a critical.
"Any creature you strike with the whip takes swarm damage as if it were attacked by a spider swarm (1d6 points of damage plus poison and distraction)."
So, if a swarm attacks, it just moves inside the targets square, and automatically deals damage as a swarm would:
"Swarm Attack: creatures with the swarm subtype don’t make standard melee attacks. Instead, they deal automatic damage to any creature whose space they occupy at the end of their move, with no attack roll needed."
You can score a crit with a whip and per definition it is treated as a whip.
But it takes damage as if attacked by a swarm, and swarms can't crit due to lacking attack roll.
So, could a character score a critical hit with a "Whip of Spiders", resulting in double swarm damage or not?
The second part is describing the damage the whip does, which is the same as a swarm of spiders. But the first part clearly lays out that it is an attack roll and thus can crit.
the way i see it.
it's a spell with an attack roll, these usually (unless stated) can crit (on 20 for X2 ).
it state to work as a whip except specific things :
touch attack, damage dealt, ignoring the no damage to armored enemies and no trip and such.
that mean that on any other regards it should be treated as a whip :
reach, if you have proficiency or not, if it can be further buffed as a weapon (via say arcane strike or a bards spells\buffing to weapon damage) and since they didn't specifically call it out the same crit range and multiplayer as a whip.
let me put it like this - if it was exactly as swarm attack and it also doesn't crit (say the insects are all contributing to the damage by swarming the target but at the same time like a swarm they don't crit) i'd expect the attack roll to be aimed at the square the enemy is in instead of his touch ac (dc 5, like splash weapons vs grids).