Hello, tiger,
You narrowed it down to the correct answer choice and one other - great job!
That's the stage you want to be at, where you can consistently narrow it down to two, at least in the early parts of each LR section. Don't get discouraged if you repeatedly narrow it down and pick the wrong one - you're nearly where you need to be, just one more step. (And if you don't, congratulations!)
Answer choice (A) doesn't really weaken the argument. With all Weaken questions, you're not trying to weaken the conclusion itself, you're trying to weaken the link between premises and conclusion. You're trying to show that, even if all stated premises are true, this additional information would mean that the conclusion could not be validly arrived at from the premises.
So for answer choice (A), it's not necessary that all insects be flyers in order for the painted spider to be a successful predator. The respective proportions of flyers to non-flyers isn't really that relevant to how the painted spider competes with other predatory spiders in the area. No matter how many non-flyers there are, the painted spider and the other predators in the area are only fighting over the flyers - it doesn't matter if there are almost none or way too many, all the spiders are taking slices out of the same pie of flying insects. (Ew, insect pie.
)
Answer choice (D) weakens the link between the premise that the painted spider's webs are stickier and the conclusion that the painted spider is a more successful predator. Even if the other premise is true, that stickier webs are more efficient at trapping insects, that efficiency is wasted if insects can see the webs and therefore more easily avoid them.
Your point about insects flying into flames is astute, but slightly off base. Insects will sometimes fly into sources of light, but rarely will insects fly into objects that are merely reflecting light. (Except mirrors, sometimes.
) Anyway...since this is the very first question in the section, and sections are roughly curved by difficulty, I wouldn't suggest spending too much time thinking about relatively small holes in the argument like that.
Answer choice (D) may not be a perfect answer, but it is the best answer. Better than answer choice (A).
Hope that helps,
Lucas