The Forgotten Planet — Murray Leinster

Beneath dense gray clouds through which no sun shone lay a forgotten planet. It was a nightmare world of grotesque and terrifying animal-plant life. Gigantic beetles, spiders, bugs and ants filled the putrid, musty earth -- ready to kill and devour anything in sight.
There were men amidst this horror -- men who cringed and ran from the ravening monsters and huddled in the mushroom forests at night.
Burl was one of these creatures. But one day inspiration hit Burl. He would find a weapon -- he would fight back.
And with this idea the first step was taken in man's most desperate flight for freedom in this most horrible of all worlds. But it was only a first step.