The nature of hallucinations, at least as far as we know, ties into the nature of abstract art as well as, animism, totems, masks and the way heightened imagination and visualization connects with weariness, fever, starvation, insanity, on the level of vision - the way our eyes fill in blind spots for a full view so too we add and perceive things to move, to breathe, people in pictures to nod their heads, statues to slowly wave their arms, the pencil lines in sketches to flow around figures like fine root systems or webs.
The spider web was important in my cosmic symbol book because as a rock and roll youth my bedroom window was completely covered by a spider web, with the huge black spider in the dead center, never moving. And stone tripping at dawn, I'd watch the spider watch me through the glass, for (what seemed like) hours on end, our brains merging. That spider was my beacon, my transistor 2-way radio to the alien intelligence that unites insect and plant life. Then, in the winter, he was gone. Sadness pervaded my snowy room and mind.
Anyway, the nature of hallucinations stems from a heightened sense of danger, activating the pineal gland for a dose of full brain alertness; if we're gonna die, may as well do it with all the houselights on. So now the program in the vision centers of your brain which fill in the blanks in your normal vision are suddenly lit up and working over time, looking for danger and seeing it wherever they look - on the hunt for movement, life forms, things edible and/or threatening. Anything remotely anthrophomorphic starts to twitch and vibrate, energies are perceived all around, both benign and threatening in an endless push-pull of life and death, time and space, all shrouded over by this extra dimension that reveals all as pure perception, illusion, the eye making sense of a world spiked with madness and chaos. Children don't see puppets at a puppet show, they see the characters. They don't need CGI because they bring the intricate details to the bland surface. All they need is a guy in a wolf mask, and they're scared, because their imagination fills in the rest, they see the wolfman with more hair and wilder-eyed than the mask can actually deliver on its own, to, say an adult.
This is not to say that some visions -- grays, mantises, demons, plant spirits, aren't "real" per se, but that they challenge our notion of what real is, making the point moot in the end. The equivalent metaphor for us would be dogs chewing on math textbooks and trying to understand calculus via tasting the ink and wood pulp.
At this point we're still chewing on the wood pulp. The best we can hypothesize is that the symbols on the pages mean something, and it's probably profound. Our reductionist scientists would say (continuing the dog metaphor), that because the math book is made of wood pulp and ink and is therefore worthless as food, it cannot have a higher meaning. Bless the scientists for refusing to consider hypotheses which would undermine their previous research.