Planet of Dread by Murray Leinster

They went back to the _Nadine_ for weapons more adequate for encountering the local fauna when it was over. Blast-rifles were not effective against such creatures as these. Torches were contact weapons but they killed. Blast-rifles did not. And Harper needed to pull himself together again, too. Also, neither Moran nor any of the others wanted to go back to the still un-entered wreck while the skinny, somehow disgusting legs of the thing still kicked spasmodically -- quite separate -- on the whitish ground-stuff. Moran had disliked such creatures in miniature form on other worlds. Enlarged like this.

It seemed insane that such creatures, even in miniature, should painstakingly be brought across light-years of space to the new worlds men settled on. But it had been found to be necessary. The ecological system in which human beings belonged had turned out to be infinitely complicated. It had turned out, in fact, to be the ecological system of Earth, and unless all parts of the complex were present, the total was subtly or glaringly wrong. So mankind distastefully ferried pests as well as useful creatures to its new worlds as they were made ready for settlement. Mosquitos throve on the inhabited globes of the Rim Stars. Roaches twitched nervous antennae on the settled planets of the Coal-sack. Dogs on Antares had fleas, and scratched their bites, and humanity spread through the galaxy with an attendant train of insects and annoyances. If they left their pests behind, the total system of checks and balances which make life practical would get lopsided. It would not maintain itself. The vagaries that could result were admirably illustrated in and on the landscape outside the _Nadine_. Something had been left out of the seeding of this planet. The element -- which might be a bacterium or a virus or almost anything at all -- the element that kept creatures at the size called "normal" was either missing or inoperable here. The results were not desirable.