Blood Crawlers spawn six times more often on tiles that are in front of background walls,[1] hence they are more likely to be encountered in the chasms of the Crimson than on the surface.
Like all spiders, Blood Crawlers cannot travel through a vertical 2-tile wide shaft.
Trivia[]
Blood Crawlers differ significantly from real-life spiders:
They seem to have two eyes, but real spiders have eight eyes.
On the Official Terraria Forums artwork seen below, however, they appear to have two large eyes (seen on the in-game sprite) and six small ones, which complete the eight eyes.
The Bestiary entry for the Blood Crawler: "Most spiders operate independently, but ones touched by the Crimson hive coordinate together to consume life."
Despite this statement, such a particularly coordinated behavior is not present in the AI of Blood Crawlers, in comparison to that of the other spiders.
Desktop 1.4.4: Drop rate of Monster Meat have been decreased from 0.1*1/1000 (0.1%) to 0.07*1/1500 (0.07%) (from 0.5*1/200 (0.5%) to 0.2*1/500 (0.2%) in worlds with The Constant seed).
↑Information taken from the PC1.4.0.5 source code, method SpawnNPC() in Terraria.NPC.cs. There may be inaccuracies, as the current PC version is 1.4.4.9.