From Halopedia, the Halo wiki
Trace evidence bots, colloquially known as Spiders, are small arachnid-like robots that analyze crime scenes for homicide investigators. These single-use machines crawl around the area of a crime inspecting traces of physical evidence such as fibers, skin particles, print tracks, and specks of DNA. Once a spider completes its analysis, a three-dimensional map is compiled of every piece of evidence gathered at the scene. While thorough and effective, the spiders themselves were expensive and at times quite slow, sometimes taking a full week to process a large crime scene.
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