I fully expect the following spell to be controversial though I think it's neither against the rules, nor particularly unbalancing.
Muto Corpus Level 3 - Sustenance of the Spider
R: Self, D: Conc, T: Ind
The magus gains the ability to feed on air, as spiders do*. Feeding takes the same time as eating a normal meal would - thus, a magus can survive indefinitely through this spell without incurring warping, as he is only affected by the spell a small fraction of the time.
Base 2, Conc +1
Hmm is it possible that when the sources say a spider takes it's nourishment from the air it means it's food supply is airborne. (i.e. flies and moths) After all spiders do hunt according to Pliny and not just for fun but as labor. Translations can be tricky that way.
Beyond that a spell that gives a human the ability of an animal may not always be a base 2 like for Eyes of the Cat. So you might have to up the base effect.
I am with Maine75 here. It says "The spider is an aerial worm that takes its nourishment from the air " not that the nourishment is air. Quite a different thing. I would assume that Spiders eating flies and other small insects was well known, since it is readily observable at plain sight.
Interesting link. Of course, while Isidore says they live on air, Pliny says they hunt. And it's not a difficult observation for a magus to make - it's not as though all the natural history authorities disdained observation for collecting folklore - Aristotle's Zoology was excellent. The general question of how mythic everyday phenomena in Mythic Europe should be is a thorny one. And what people believed is hard to pin down - different people believed different things and wrote different things for different purposes. A bestiary intended to glorify God's creation and provide moral parables might very well contradict what a contemporary guide to keeping and using animals has to say.
Back to the spell:
A Muto spell to gain the ability to feed on something non-standard is a good idea but it needs to last until whatever that sustenance is has been fully digested and incorporated or there will be a hungry magus with something odd in his gut at the end - i'd suggest sun duration or a non-standard duration eg. three bells of the same magnitude as sun.
Especially, even if we allow the "feeding on air", i would definitely not call that a "minor ability".
Base levels for MuCo are still fairly on the low side so base 5 or 10 should be enough?
Well i think its quite obvious that the majority of people will have seen spiderwebs set up to catch flies.
And enough have probably seen more to make it reasonably common knowledge that Yes, spiders DO eat, just like every other living thing.
Actually, I think that the medieval understanding was that the spider did indeed feed on air. Check out the Aberdeen bestiary at http://www.abdn.ac.uk/bestiary/translat/72r.hti, as well as Timothy Ferguson's old (but still very flavourful) Book of Beasts at http://www.durenmar.de/articles/beasts/chapter8.html. While many people have seen spiderwebs catch insects, and a few have seen spiders kill insects caught in their webs, the spider obviously (from the point of view of an ancient observer) does not eat those insects that get caught in its web, since it leaves the corpses intact.
In terms of the ability of feeding on air being "minor" and appropriate for the Level 2 MuCo guideline, it's clearly very subjective; but from my point of view, it's no more valuable - and probably less so - than being able to breathe underwater or to see in the dark.
The point about feeding requiring digestion, and about digestion requiring some time (how much?) is a good one, and I'd be grateful to anyone who could point any reference on the subject, either in game (canon Ars material on digestion) or out of game (common understanding of digestion in the middle ages). Without any such source, my interpretation through the "lens of antiquity" would be the following. When one eats, one generally feels invigorated by the end of the meal. Drinking quenches thirst within a few heartbeats. Thus, one can draw sustenance from food pretty quickly even if not really immediately.
Retreat as Flying Vermin
Mu(Co)An 35
R: Voice , D: Sun , T: Circle
This spell allows Alexander to turn a group of people ,
and their Animal-based equipment , into bats.
Base 20 , +01 Touch , +02 Sun.
According to pg 112 a target: Circle spell effects everything in the circle at the time of casting and says the spell ends if the circle is broken even if thats before the duration would normally end but says nothing about the targets being unable to leave the circle.
Ring duration says that the spell ends when the target leaves the circle or if the circle is broken. So leaving the circle doesn't seem to be the same thing as breaking the circle else the two different conditions wouldn't be stipulated. Since the spell has duration sun leaving the circle doesn't end the spell.
In other words, the spell affects everything within the Circle, which must be drawn by the magus at the time of the casting. Eligible targets that enter the Circle are affected by the magic, which stops affecting them if and when they leave the Circle.
This has been discussed at length in another thread.
Apparently the writers of Magi of Hermes didn't read that thread because the spell description clearly allows the effect to continue after the targets leave the circle.
Note that the spell does not end when someone affected by the spell leaves the circle. However, that someone ceases to be part of the spell's target, which is everything within the Circle.
The spell was created to allow the magus's retinue to escape from danger and the description points out the grogs equipment that is not animal based will be left behind.
Personally I think that both kinds of spells are allowed to be designed. Of course any given spell has to be one or the other; but a maga can invent one Circle/Sun(say) spell that affects everyone who was initially in the circle when the spell was cast, and will continue to affect them all day - and also invent another Circle/Sun spell that affects anyone who enters the circle at any time that day, for as long as they remain in the circle. I think the same is true for Room, Structure, and Boundary spells. I imagine going through example canon spells would uphold the existence of this choice (but Serf's Parma).
Well, that's not really that explicit though. The magus retinue may simply fly up (staying within the circle) and hide. Less effective than what you'd get with a Group target, but hey, it's also two magnitudes lower.
Honestly, if you assume that Circle affects anyone initially within the Circle, regardless of whether they stay within the Circle after the casting, you are either making Circle quite different from all other "area" targets, or just breaking a ton of spells from the corebook itself.
I've stated before that MoH was a disappointment, ruleswise. Not all spells followed guidelines or gave correct calculations.
Atleast one spell has been hashed through on this forum - do not take MoH as gospel, merely as suggestions.
Sorry I was a bit busy yesterday and I'm afraid I sacrificed clarity for brevity. Both interpretations of duration seem to exist for other targets as well as circle in RAW so I'm actually with Gerg on this one.
Someone else summed up the effect better then I can when it was explained to me on another thread.
As far as I'm concerned this spell is another example from RAW of an effect that appears in other books for a variety of targets and durations not just in Magi of Hermes. As far as the specific spell goes while what I wrote might not have been explicit the text of the description did seem to be. I could direct quote more of the full text if you like. The spell is clearly designed to allow the party to escape from danger as bats and not merely fly straight up.
I'm not sure this is fair to the writers, editors, and play testers that put work into this product. Certainly there will be errata and almost every book makes mistakes on spell calculations. I for one think one of the main purposes for the book was to take some of the places where the rules are less clear and clarify them by example. Certainly for me the book provides many examples of what can be done with hermetic magic that I never thought of.
Either way good bad or ugly Magi of Hermes deserves to be included in any discussion of RAW as RAW. Not just out of respect for the people who worked on it but also for the people who spent good money to get it. If in your saga you don't wish to use it as such fine but that is what House Rules are for.
though I think it's neither against the rules, nor particularly unbalancing.