An Imposter Bug is a sub-species of Arachnids created by Zander Barcalow after he was transformed into a Bug. They were created when Nurser Spiders injected Arachnid flux-larvae with human blood, resulting in the Imposter Bug sub-species.
Imposter Bugs are first seen during the Klendathu Campaign on the planet. While at a temporary camp on Klendathu, the Roughnecks are attacked by a new and dangerous form of Arachnid, an Imposter Bug that approximates the size and shape of a human. Imposter bugs are unique because they are able to compact their bodies allowing them to fit inside stolen M-1A4 Power Suits and infiltrate enemy lines. Once ensconced they return to their true form and unleash havoc in their enemies' midst. They are known to be fast and deadly and represent a significant threat to the security of Troopers everywhere.
One odd note, is how Private Higgins was able to outrun a single Imposter Bug, most likely this was due to his fear getting the best of him.