So I was kinda right, I think? The spider is an entity which consumes the personality of the next host.
But the artist consumed the spider instead, so the spider probably "moved" to her or something, which explains the first chapter.
One thing I've seen is, in older girl younger guy, they deliberately drop the girl's mental age to put them on equal footing. The pairing is so rarely well executed. I could probably find better doujins than manga for it.
Its even more rare in visual novels, which kinda ticked me off since I...
Probably just me, but I wish their mother gave the thumbs up as well.
I've seen a couple of doujins where moms pushed their middle aged daughters onto young guys and the interactions are adorable. I suppose it would adequately scale down here.
This is turning out to be actually good as far as age gap drama goes. I only wish the genders reversed camp gets something this good, without an ambiguous ending.