Thank you! Just what I needed.
The description in A&A does says Spiders are "worms of the air" so that means they do spontaneously generate from the air, but A&A is a little fuzzy on how much air you need to create 4000 lbs of spiders.
The problem with rego'ing them to attack still stands though. A creature that is controlled magically is under a magical effect. If that effect doesn't have a high enough penetration to overcome a Mages parma, then the creature still can't hurt the Mage.
So you're suggesting that if a Mentem spell causes a grog to attack a mage, the grog first must penetrate the mage's MR? I'm not sure every SG would agree with you on that one, if only due to the "mind/body" split involved. Starts to sound a bit like the "pink dot" argument.
(Hey, magi! Pondering how to screen your grogs or visitors for unwanted enchantments or other magical effects? No need for bulky InVe effects that can only detect 1 Realm each - just shake their hands!) 
Except a grog is likely to attack with a sword that isn't under an effect so MR wouldn't apply in that case. Otherwise yes that is what I'm suggesting, you can't just take control of people or animals and use them as a way to get around MR. It's not "pink dot" it's how MR is meant to work IMO. MR is built the way it is so it's hard (not impossible just hard) for attacking spell casters to leverage small effects into massively powerful attacks. Being protected from something that is under a magical effect, whatever that effect might be, is an integral part of that mechanic.
"Pink Dot" is an abuse of the MR rules where an attacked magus uses a ridiculously low powered spell to leverage his Parma into an indestructible force field.
How would shaking hands with something under a magical effect feel any different from shaking hands with something entirely mundane?
The underlying principles of the pink dot could apply here though.
The pink dot does make a mockery of the MR rules, but RoPM does mention MR and the walking dead where the dead are animated by the magic. The force of harm is magical. If a magus bites another magus, his parma doesn't bounce the bite. Equally if one magus compels another to fireball his sodalis, they use MR vs Ignem NOT MR uses mentem.
The Animals are NOT magical media, they are real natural animals just as if god's works had made them.
A
Yes but that's because a Magus, even though he can cast spells, is not necesarily under the effects of magic.
Yes because the spell is not part of the magus being affected by magus if the same Magus was compelled to bite then in that case the bite would bounce.
Sure right up till you compell them to bite someone then they are under a magical effect.
No one seems to have picked up on the timescale for 'naturally occuring vermin caused by rego instead of creo'.
Creo: gives instantaneously appearing spiders/animals of your choice. Magically summoned, ergo resisted by MR whatever they do. Rego commands needed to make them do anything useful. No need to consider if rego effects mean they are resisted or not.
Rego: spontaneous appearance, similar timescale (I presume) to how long it would take them to appear from whatever your source material is - a few days to a week. Spell might expire before any/all of them have appeared. Extra rego effects to make them do something useful anyway. Will this make them resistable?
My instincts suggest that if there is an on-going effect, which is changeable by a controlling magus from one moment to the next, then that should be resistable. I am undecided about a one-off effect.
Rather than read about the 'pink dot' problem (again), I'd like to hear general views on what the consequences would be for either option.
ie
Situation A: a rule is made that 'command-type' spells do not cause the recipient's actions to invoke MR. eg1 I mind-control that archer; he looses a series of arrows at that magus. eg2, I ReAn that dog; he bites that magus. Neither are resisted. One of these 'seems reasonable', but the other doesn't.
Situation B: the opposite rule, the archer's arrows are then resisted! And so is the dog. One of these 'seems reasonable', but the other doesn't.
What do the rest of you think?
Gilarius
I always see rego effects as happening instantaneously by default. It magically transforms a thing from one natural state to another. Requiring it to take the natural amount of time is like requiring a teleportation spell to take the same amount of time walking the distance.
That's how the rules read. Rego-Craft magic produces stuff "in a trice" (aka "instantly"). (Covenants, p 50, col i, et al.)
I can't find it atm (Covenants? C&G?) but I want to remember that the Ease Factor goes up significantly for effects that duplicate longer efforts. (This may just be "larger efforts" - without finding the citation, I'm not 100% sure.)
If so, then using Rego-Craft for brewing beer (a multi-week process) is easier than using it for fermenting wine (a multi-month process), and the Ease Factor for long-term spontaneous generation would be higher.
Also - I have no idea how the rules address making a Finesse Roll with an autonomous item like this, or if it's even possible. There are examples in Covenants, but all those assume the "trigger" effect is intentional and triggered by someone who wants to create the item in question. If someone had no idea what was about to happen, I'd guess it would be a d+0 roll vs. the Ease Factor to create that item.
(Somewhere - might only be in these forums - I remember a comment about the need to teach grogs Finesse if they are to use such enchanted items with any reliability.)
And, natch, there are no guidelines for EF for Rego and spontaneous generation, "intentional" or otherwise. 
(EF could arguably vary from "automatic" to "3" to "24+, Almost Impossible" to "breaks the limit of the Divine"*.)
(* If you rule that only God, and not magi, can achieve spontaneous generation of this type, creating "something" from "(almost) nothing".)
I would suggest that the Rego would fall into the same natural changes as water to ice, or fruit out of season, and therefore instant.
I would use the numbers limit in the the sidebar about spont generation, but I see no reason why a rego would need to take time.
A
Thanks. I've read Covenants, but not for a while.
A very good point, made succinctly.
However, what do people think about whether 'command spells' are resistable?
Gilarius
Well there is this.
Of course reading the rules on that page you'd need twenty-thousand Auram base individuals to make ten ponies worth of spiders so I don't think Rego'ing that amount is practical.
I don't think it's the matter of their actions of someone commanded being blocked as much as it's them being blocked. A dog that's being controlled is under a continuing magical effect as are his teeth. A mind controlled archer is also under a continuing magical effect but his arrows are not. So MR can't protect against the arrows.
That's not an EF, but it's good to know that that, at least, is "somewhere". I don't have A&A - is that CrAn or ReAn?
ReAn and the sample spells don't require a finesse roll. I guess because their natural not manufactured things
If it was generating out of air, wasnt it ReAu rather than ReAn(as its stated that vermin is NOT Animal)? (and no i dont have the book)