quick-glancing to the sun

I couldn't even make a guess. It sounds like some kind of Yoga posture, but not in that context.

Wait, maybe it's a quick flash of reflected sun light from part of the insects body.

i felt the same way, but it could also be that it is the insect that cast a quick glance at the sun, right?

My interpretations are
(1) that the observer of these insects follows them with the eye until their path crosses the sun, and the observer then quickly looks away.
(2) that the short life of insects (such as butterflies) implies their activities are governed by keeping track (glancing at) the time indicated by the sun's position.

The poet could imply either or both of these, but the second is more likely because of the anthropomorphic portrayal of the insects as "thinking beings" in the lines above. Further, there is an allusion to the idiom "the quick and the dead" where "quick" means "living". Richness of meaning is what good poetry is about.

Your point could also be valid in the sense of the insects "showing off" their gilding to the sun....striking the sun a glancing blow.