Apparently, arachnids lose all their gained levels if you fold them for repair and deploy them again. They go back to level 25 and begin levelling up again from there. Very odd, I say.
That's because Arcanum has to delete the creature and spawn an item whenever you pack up the arachnid. The item's script is designed to generate a level 25 creature, so when it retriggers... I don't think the levels mean too much though, as I don't believe arachnids have a level scheme. Besides the marginal hp and fp gain each level up, the points just accrue and sit stagnant.
I've never NOT folded them up after individual use, because they go so damned slow. For that matter, I rarely ever use them... Usually just have them for novelty purposes....