My line in the sand is sentience (the ability to suffer). What's yours?
From a mere "observation" stand point along the lines "behaviorism," well almost all animals including insects "suffer" and "feel pain" because they react to violent stimuli. So, that means we probably can't from your line in the sand hurt almost any mammal or insect. That seems a little extreme doesn't lead to a very pleasant life for more aware species like you and your friends. If you want to live like that, go ahead, but I don't think you have the right to impose that on other humans, who would find living like that extremely difficult.
I personally don't have definite line in the sand. I love rabbits (for I raised rabbits when I was a kid) and most rodents and wouldn't want to see them get hurt, but I used to have rat issues in my apartment, I had to kill them otherwise I'd be constantly sick, and that would impact people who I love and I need to be there for. I owe love to people I know before rodents. And for humans I don't know, would I fight for children on the east side of my city getting sick due to lead in the water? Yes, and before crying for the suffering of rats, cows, chickens, etc.
I think you might be conflating "pain" with "suffering". Pain is simply a reaction to a negative stimuli. Pain response has been shown in plants and insects, for instance. But I don't place any value on pain that doesn't lead to a negative subjective experience. That's what I think is actually bad. Suffering has not been shown in plants or insects.
But there has been a lot of research showing that many non-human mammalian animals do suffer. Here's some research on chickens, for instance: https://rd.springer.com/article/10.1007/s10071-016-1064-4
And I've found this to be a good resource for navigating the sentience debate: http://www.animal-ethics.org/sentience-section/
Likewise, the ability to communicate doesn't imply a need to. Conversely, the ability to communicate doesn't define consciousness. Precisely, the ability to refrain from communication is likewise important. So if an animal can't even shut the fuck up, that's where I draw a line. And I should take that into consideration more often when posting online. Further, action can be involuntary communication. So "A lot can be inferred from how enemies are treated". Concerning invasive surgery, the practical need for analgesics is a smooth procedure, not the suffering of the patient.
Corollary: If something cannot rationalize it's suffering that's another line in the sand. E.g. children vs a society of fish missing a sibling.
This is the same argument as "What's a planet", my definition ensures that we still have nine!
I'll give you another one: "Which people keeps the Temple Mount", my solution says the Jews!
Lines need to be drawn somewhere, and people will be upset no matter where the line is drawn. Sometimes we need to accept that we got one line drawn to far to the left for our liking, another drawn too far to the right for our liking, but the choice is to either accept imperfect lines or spend our whole lives arguing or fighting.
I'll let you have eight planets, but that Temple Mount is mine!