The spiders are aggressive hunters of intrusive psychic forces, and are attracted in vast numbers to the presence of any invasive psychic entities. As the wraithbone core is supportive of psychic consciousness in a similar way to warp space, it is possible for it to become corrupted by the fragmented consciousness of lost human or other non-Eldar souls.
In theory at least, it would also be possible for a daemon of Chaos to find its way into the wraithbone core, although such a thihng would be immediately apparent. The tiny warp spiders ensure that this doesn''t happen, hunting and destroying alien psychic fragments much as white blood cells in the human body attack and neutralise foreign bacteria.
p. 10, 2nd edition Eldar Codex
Wraithbone or even the Infinity Circuits are not inherently anti-Daemonic. They are an environment supportive of psychic entities in much the same way that the human bloodstream is a far more hospitable and attractive environment for bacteria than the hostile outside environment. Wraithbone is essentially warp energy transmuted to solid matter. Any anti-Daemonic properties are from other sources, such as the crystal spiders, the active of Eldar souls rejecting foreign psychic entities, or additional psychic wards applied to the wraithbone.
As for what a skyrunner might be, see the below image from 2nd edition Codex Imperialis:
http://www.dakkadakka.com/wiki/en/Image:Kid_Kyoto-Codex_Imperialis_Eldar_3
