Speculation time.
I feel the mother's ghost looking like a spider/praying mantis has a few meanings to it. I can mostly think of correlations to spiders like keeping her son tightly in her web, keeping him in her "web of lies" if you will. Another thing prominent in spiders is that the mother is usually killed by her children for food once they hatch. Maybe he killed his mother, maybe as she lay dying she blamed him the whole time making him believe he's the reason she died. As for the praying mantis all I can really think of is how they latch on to pray with there scythe nubs and stalk pray? Lemme know if there's something I don't know about mantises that works here lol