I haven't seen a spell or rule that indicates transformation into an insect is possible or impossible. While I could state some philosophical reasons for impossibility, it is clear that you can transform into a plant or stone.
So what level are we thinking of?
-Lvl 20 to become bird or fish (clearly major changes beyond land animal)
-Lvl 25 to become a plant
-Animal seems the most clear choice due to "Summon the Ravenous Swarm" and other Animal spells involving insects.
I would assume Base Level 20, like turning into a bird or fish -- there's a sufficiently large gap between that and a plain land animal (2 magnitudes) and a sufficiently small one between that and a plant (1 magnitude).
I'll derail the question slightly and ask: what about a single magus turning into an insectswarm*? Is that possible?
Well, Animal is clearly the right requisite, since it does include insects, as you said. As far as I understand it, in Mythic Europe insects are lumped together with mice, eels, frogs, and the like as 'vermin' or 'worms', while other animals are classified as 'of the earth', 'of the air', and of the 'water' (land animals, birds, and fish).
Now, to get back to your initial question, let us look at some Animal spells: Growth of the Creeping things, and Transformation of the Ravenous Beast to the Torpid Toad, both MuAn, both dealing with 'vermin', and both using the generic 'change an animal' guidelines without extra magnitudes. I would say that that justifies a lvl 20 MuCo(An) guideline 'change a human into a worm', worm meaning any vermin (like fish means any animal living in the water, including cetaceans and squids).
As for turning into an insect swarm... I would say that at least require some Original Research, probably leading to a very oddball Hermetic Virtue, if your ST allows it at all. Well, maybe if the swarm cannot separate, in which case this is more of a very odd version of becoming insubstantial, almost a cosmetic effect of Cloak of Mist: a cloud of gnats, all flying together in a roughly man-sized volume, who cannot really be hurt or do anything physical... yes, that might be seen as just a variant of Cloak of Mist. But a swarm of rats, able to act independently ? Your mind has to be in one place, I don't think you can just split it like that under current Magic Theory. And if you can, then what's to prevent you doing a pack of wolves next ? Or a herd of cattle ? Lots of problems in sight.
Well, technically the level 30 MuCo(Au) guideline IS 'turn a human into an insubstantial object', the very term you use, but I think I see what you mean, you don't want to be a gaseous substance (not that the medieval man had a clear understanding of gases), but more of a ghost.
PeCo 40 'destroy one property of a person, such as ... their solidity) might be a possibility.
MuCo alone cannot be enough, as a ghost has no body and so is not covered in any way by Corpus. I would allow MuCo(Au), with the proviso that you are changing your body so it has the solidity (or rather lack of) of air, rather than turning into a mist: you'd keep your general shape, perhaps become invisible (or not, your choice when you design the spell), and be able to go through anything that's not airtight. Or
MuCo(Me) could give you the (lack of) physical body of a ghost. I'm not sure about the level though, but at least 30 and possibly higher, 35 or 40.
Perdo implies (to me) complete destruction, not temporary cessation of an essential property. At least when applied to the body itself (unlike invisibility simply destroying the species). Couldn't Muto change that one property of a person (solidity), with a Mentem requisite to anchor the mind/soul of the caster? Becoming an actual ghost/spirit would be barred but simple transparency could be possible. Of course, you may need to add a Terram req to keep from slipping into the earth
Transforming oneself into species may also be a hazardous way of obtaining insubstantial form...
I´d say at least 1 Magnitude higher than bird/fish. I think insects is guaranteed to be "further away".
A swarm, raise an extra magnitude again maybe and you have to use Group Target of course, yeah sure.
If you can turn into water or air, no reason why not insects as well i think.
Why would it have to be split? If a person can walk around with his mind flying around in the shape of a bird(and i dont think there´s anything preventing it to be multiple parts either), well i think your logic may have been thrown out from the start, even if i agree with the essence of it.
And for the swarm thing, there´s nothing saying that the "mind" have to be inside all parts of the swarm.
That´s the downside of it yes. Which was why i said it should probably go up in level a bit, if its +1 Magnitude plus Group Target, that means it would at minimum be 15 levels higher, that´s almost enough to make it something exclusive to specialists. And for specialists to be able to do extraordinary things, well that´s not THAT bad.
Perhaps setting a size limit that in this case, Group equals your real size instead of normal base size? Would make it harder to go totally munchkin at least.
You´re not alone in that. Perdo to destroy a property to me feels quite quite wrong. Muto for the win.
The destination size of the combined creatures is a good idea and very logical. Makes for thousands of bees, maybe 75-80 rats, or a few large dogs per average human. The SG could rule that that each 20% missing is a body level when the mage reforms, or that it is also a great way to let an enemy have an arcane connection to you.
I'm not sure that this spell effect would be munchkin-ish though or what the problems would be. It seems that separating a target into lots of smaller parts makes them a far better target for area effect spells that inflict damage. It would be easier to kill a group of rats with a single AoE effect than a single person due to the number of body levels and the damage they can survive.
I can follow that the "mind" of the magus is a factor, but no more than some effects that allow a mage to become stone or a tree, or other changes which remove the mind from a flesh and bone mammal. I could see that as a prime hand-wave or saga ruling right there, and can easily concede that SGs might say no way.
And yup, a specialist being able to do this is not a big deal, given the firepower that a ignem specialist can bring to combat. 4-5 wolves is pretty trivial by comparison.
I would only add, that a swarm can be considered a single entity, as can a stone or a tree or a salmon or a owl. However, to my way of thinking, a magus could not become more than one rat, or wolf, or bird, ect. because while those being can and do flock and swarm, they are still considered individual minds.
I do not expect to be correct on this, only that as a magus can become a mist or a fog or a pool of water, they cannot become multiple fires, a few lightening bolts, or a dozen goblets made of frozen water. I think or some reason a magus would need to remain a single entity.
And a swarm of bees is recognized to behave as a single unit, unconsciously of the individual actors of the swarm.
I suppose I am speaking more to my view of the magic than from mechanics. As I reread some posts I would say that with magic anything is possible, so why not with a maga who truly practices the Arts with such creative flair and brilliant insight with a specialty in becoming multiple animals... I could see one doing so.
I made a mage shapeshifter who would then cast spells to make copies of himself... I never thought to mentally control them though.
Morph into a herd of cattle by the use of a few plus to size, conduct a perfectly controlled stampede across a town, bye bye town. Or army.
Morph into 10 big nasty tigers, attack 10 different opponents in 10 different locations 10 days travel from each other(by using Month Duration)...
Infiltrate 10 cities at the same time using wasps.
It can certainly be abused. The above is just the basics i came up with in a few seconds of thought.
Certainly true enough.
Exactly, you can already do wilder things, expressively stated by RAW so why not this?
Yup, it´s certainly abusable but it´s not quite THAT horrible, so meh... why not... Can certainly make for a fun character.
I dont think there is such a need. If there is, i´ve missed it.
apply a guideline that says the target's final form is costed into the spell in the same way as the form. Thus splitting into separate creatures will work if group is applied, and that target form must maintain a "group" to keep the spell active. Otherwise its like removing the cloak in Shape of the Woodland Prowler. To let a spell divide the targets far away from each other the R and T must be scaled to Arc?
or, the character's mind follows a member of the group (the alpha wolf, queen bee, or bull) which the rest of the heard follows?
For what it's worth, TMRE pg. 68 calls "allowing the magus to take ghostly form" MuCo(Au) 30. The reverse, allowing a spirit to become corporeal, is MuMe with a form requisite appropriate to the corporeal form. No Cr or Pe required here either way.
I looked at that last night since I was expecting to see the "this is not a Hermetic theory and only available through Mystery" like the Greater Elixir. I did not find that language though so it does work as a benchmark. I prefer a Mentem requisite to Auram though... what does spirit have to do with air? Turning into mist or fog, sure, but straight up ghost doesn't scream Auram to me.
I am thinking you have the Au req instead of Me, because you are taking the ghostly "form" not actually becoming a ghost. While when you are making a ghost become corporeal you are affecting a ghost, thus the Me.
Taking a ghostly form is simply you becoming the ethereal ghostly image in which so many believe, literally the misty vaporous image of yourself with no substance. Thus you are simply changing with Muto your corpus into a misty hazy version of yourself. I do not believe you gain any abilities of a ghost as with this spell you are not becoming a ghost, but rather a being that looks like a ghost.
Making a ghost become corporeal is you affecting a actual ghost which is as you said requires Mentum. One spell is doing something to a true spirit being, the other is simply making you look like one.