I'm almost certain I read this story in an Omni magazine collection of short stories. It wasn't the regular run of the magazine, but an oversized version with a slightly thicker, sturdier paper than usual used for the cover. I've been looking on the Internet Archive collection of Omni, but this doesn't seem to be included yet. I read this story in the early 90's, but it was purchased at a used book store so it could have been written in the 80's during Omni's heyday. If my vague memories are correct, the collection's cover illustration was a close-up of a woman's face that was un-human in some way (alien features, or robotic) but this was an extremely common sort of thing for Omni, so don't consider that very strongly.
The story had two brothers on an alien planet. They had to live in a heavily fortified building and only go out wearing protective clothing because of the dangerous animal life. The forest surrounding them was described as being dense and jungle-like. I can't recall if both brothers were hunters, or biologists, or if there was one of each, but I think their mission was to hunt and collect animals. One of the brothers becomes convinced the main life form they're hunting are actually intelligent creatures, but the other is equally convinced they aren't. The story itself is very vague about it, it could go either way, and I believe one brother loses his life while trying to interact with them.
The alien was described as being rather large, the body rounded with a leathery, somewhat oily dark skin, and multiple jointed, arachnid legs surrounding a mouth on the underside. They might have made some kind of webs, too, and had acidic or venomous spit. There was a full-color painting accompanying the story depicting the aliens in a stylized way.
Anyone remember this one?