Creeping about in the margins of the wetlands, you settle on a simple body plan of head, thorax and abdomen, with six main legs used for locomotion. It’s a neat, efficient plan because you can keep three legs down in a stable triangle shape while moving the other three, then repeat, alternating between sets of legs.

Your remaining legs mostly reduce to little stubs, and are well on the way to being removed altogether, except for two at the front and two at the back.

Your frontmost legs, which gradually move up and forward, and become more sensitive to chemicals around them, until they form antennae[1].

Your last two legs at the back though… they could be used for two different purposes.

They could also become antennae, giving you more direction finding capability for food, or they can become levers to help you bump up where you can’t walk.

[1] Why do insects need antennae to smell? Why don’t they just have noses? Well, the answer to that is simply that they don’t have lungs. We evolved noses because we already had a structure with air moving past it constantly, so it was a good place to capture as much passing chemistry as possible. Insects didn’t have that.
So they wave their (highly adapted) front legs in the air to achive the same thing.

You can think of antennae as inside out nostrils, if you like.