Blood Crawler

A Blood Crawler is a monster found often in the Crimson. It can crawl on the background walls freely in any direction, but will traverse the terrain like the player if there is none. It follows the Spider AI. They are rough counterparts to the Eaters  of the  Corruption, in that they tend to attack in swarms and have greater mobility than the player, but they do not come in a variety of sizes like eaters do.

Trivia
Blood Crawlers appear to be inspired by ticks, given the bloody theme of the Crimson. Ticks are however invertebrates, in which case it makes little sense for Blood Crawlers to drop the Vertebrae item.