4 questions:
1. But within the past decade researchers have realized that some brain regions can pinch hit for other regions, for example, after a damaging stroke.
What is "pinch hit"?
2. The researchers think the additional role conferred an evolutionary advantage: having a visual system primed by sound to see the source of that sound could have given humans an extra step in the race for survival.
"Primed by sound" means sound is followed by visual system, right?
3. In the past couple of decades, for example, Mediterranean butterfly and dragonfly species have been found flying around places previously off limits to them—new northern climes such as Germany.
Why there should be an "off" before "limits"?
4. Lighter-colored insects thus function well in warmer climates. They don’t overheat as easily and can stay active longer, giving them a leg up—well, six legs up—in our warming world.
"a leg up—well, six legs up" what is it?

Question

4 questions:
1. But within the past decade researchers have realized that some brain regions can pinch hit for other regions, for example, after a damaging stroke.
What is "pinch hit"?
2. The researchers think the additional role conferred an evolutionary advantage: having a visual system primed by sound to see the source of that sound could have given humans an extra step in the race for survival.
"Primed by sound" means sound is followed by visual system, right?
3. In the past couple of decades, for example, Mediterranean butterfly and dragonfly species have been found flying around places previously off limits to them—new northern climes such as Germany.
Why there should be an "off" before "limits"?
4. Lighter-colored insects thus function well in warmer climates. They don’t overheat as easily and can stay active longer, giving them a leg up—well, six legs up—in our warming world. 
"a leg up—well, six legs up" what is it?

Answer

#4 - "giving someone a leg up" is an idiom.  It means giving someone an advantage.
Because insects have 6 legs, the writer is making a joke that they don't get *a* leg up, they get *six* legs up.

Answer

1)What is "pinch hit"?

"Pinch hit" means substitute or "fill in" in this case.  Essentially, they are saying that the brain has certain areas that generally control different functions in the body, but that if one of those areas is damaged, another area of the brain can control the damaged parts' function instead.  "Pinch hit" comes from the baseball term "pinch hitter," which is used to describe when a player is replaced at bat with another player.

2. "Primed by sound" means sound is followed by visual system, right?

Yes. In this case, the article is referring to how a person will often picture an image based on a sound.  So if you hear a dog bark, you will "see" a dog in your brain, even though you only heard the noise.  "Priming" something generally refers to preparing that thing for the next step, so in this case your hearing the sound readied your brain to see a dog.  Hearing the dog came before the visual of the dog so the sound effectively "primed" your vision.

3) The presence of "off" before "limits" here is to indicate that the places (new northern climes such as Germany) were inaccessible to the insects before.  They were not allowed in those areas for some reason. From the sentence, I am guessing that the insects are warm-weather insects who would not traditionally have survived in northern climates (hence those climates being off-limits because they were unsuitably cold).  However, due to climate change the insects can now survive in these areas, so they are no longer "off-limits."

Answer

#3 - a place is "off-limits" when it's not allowed to enter there.
Look at:
https://www.merriam-webster.com/dictionary/off%E2%80%93limits
http://dictionary.cambridge.org/dictionary/english/off-limits
Hope it helps,
      tano