[lee martell]

Poor Spider, trapped in the Twilight Zone!
It may be a trick, but I don’t think it’s being faked.
I sure don’t know how to explain the phenomenon, do you?
I’ll bet someone at Google recognizes this feature.

[Seattle]

It’s probably feeling the infrared light from the camera’s focusing system.

[Deaf Smith]

Spiders have multiple eyes and detect slight movement.

Tapping the phone screen causes the lens to move, exciting the spider of possible prey to eat.

...Per Cliff Clavin.

[Salamander]

People can be real dicks.