system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do bugs know to sting/bite humans incase of feeling threatened, but not other large moving things such as branches?

Given the sheer difference in size, how are they even able to perceive us as living threats to them?
gpt
Humans are warm-blooded, so the heat we emit is one of the easiest ways to differentiate us. 

We also have human-specific smells. Sweat, our breath, and hormones are all ways bugs and other animals identify us. For example, mosquitoes are attracted to carbon dioxide from our breath, so people who breathe more heavily are more likely get mosquito bites (along with other factors). 

Animals and insects use these cues - and more - to determine if they're in a potentially dangerous situation. If an insect hasn't been primed to feel threatened, it's much less likely to react to a potentially threatening movement. 

That doesn't mean they can't feel attacked and attempt to defend themselves from an inanimate object, though. If you took a long branch and swiped at a wasp nest, they would likely still attack the branch. A branch attached to a tree moving with a wind, however, is a natural motion they would be accustomed to.