What does it mean when a Goa'uld makes his eyes glow? Does it signify some type of physiological change? Or is it just an intimidating/dramatic gesture?
I can't really find a consistent pattern for this behavior.
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EDIT: I found a claim from Gateworld Forum back to 2005 that the glow is the symbiote taking over the host view.
The symbiote is able to grow an additional optical nerve channel for itself that is connected to its regular one which it can expose and then attach to the humans extra nerve channel.
And the switch mention.
[..]they still need the photophores in order to trigger the optical nerve in the human host to switch over to the path which leads into the symbiotes optical nerve input which the symbiote is able to grow[..]
An alternative is that the symbiote blinds the host and takes full control of the vision by switching it to his own channel. What I can't clarify is that it shares the view of the host or not, but I'll guess yes, and maybe he needs to switch to see something that human eye can't.
That's what I found in their wikia:
The wild Goa'uld on their homeworld have been shown to cannibalize fellow symbiotes, and also lack Naquadah in their blood. The symbiote is also capable of making its eyes glow in the same way it can with a host. (SG1: "Children of the Gods", "The First Ones")
So it means is one of the natural capacities of the original symbiotes. and when you combine with later information (also from the wikia)
[..]Within a host, a Goa'uld parasite has complete control, and the degree of control to exert at any given moment is also up to the parasite. Typically entering in through the neck and more seldom through the throat, the Goa'uld will attach itself to the spinal cord which allows access to the brain, leaving an entry mark. When within a host, the Goa'uld gives perfect health, increased strength, and increased intelligence. When in control of the body, the symbiote is capable of making the hosts eyes glow, and also speak in a deep, booming voice.
They transfer this capacity to their host. Now to answer why they glow, I'll say the same as the voice, to intimidate. There's a mention in the wikia regarding to the voice. So is a valid assumption to extend to the eyes, as there's no natural purpose for it (that we know). Also don't forget that they rule based on the premise that they are Gods. That can be used as a sign of distinction with the host's peers.
[..]The voice, however, is unnecessary. It may be used to differentiate between parasite and host, as the Tok'ra do, or simply be used to intimidate slaves and opponents. (SG1: "Children of the Gods")