There is a similar effect related to Delirium Tremens, caused by extreme alcohol withdrawal. Apparently people across cultures report seeing the same "Hat Man" in their peripheral vision, who disappears when looked at directly, but everyone seems to report the same ominous feeling about him. Also there are reports of people seeing a bunch of spiders everywhere, when going through alcohol withdrawal.

The mind and body are always in sync somewhat, even when under great stress like that. Anticholinergics at high doses act almost mechanistically on the PNS to create the parasthesia/formication on/under one’s skin. I think the visual perception of the insects is in that case secondary to the tactile sensation.

That is quite characteristically distinct to the “endogenous” appearance of other figures when using serotonergics, that almost certainly arise from intra CNS activity

Those are also common hallucinations reported for recreational Benadryl dosages.