Wikia

Spider Eye
Type	Vanilla
Stackable	Yes (64)
EMC Value	128
Mod Included	Vanilla

The Spider Eye is a poisonous food and Vanilla brewing item. It is dropped by either Spiders or Cave Spiders when killed by the player, but is not dropped by spiders that die from environmental damage (such as in a mob grinder). Spider Eyes are used to craft Fermented Spider Eyes for brewing various potions. It can be eaten, but will poison the player for four seconds, causing two hearts of damage. This damage cannot kill the player. (It cannot reduce health below 1/2 hearts).

The Spider Eye's primary function in Tekkit is to make Grin Powder through Maceration. Grin Powder, in turn, is used to make Weed-EX for Cross Breeding.