蛛 is a bound form that doesn't appear alone in modern Chinese. It combines with 蜘 to form 蜘蛛 (zhīzhū, spider), and appears in 蛛网 (zhūwǎng, spider web) and 蛛丝马迹 (zhūsīmǎjì, spider silk and horse tracks = clues/traces).
Left insect radical — originally a pictograph of a curled-up snake or worm, generalized in Chinese to cover all small crawling creatures including arachnids. Indexes 蛛 in the bug family alongside 蚁 ant, 蜂 bee, 蛇 snake, 蝶 butterfly. Spiders aren't insects biologically, but Chinese taxonomy lumps them under 虫.
Right 朱 supplies the sound, identical pinyin zhū with no drift. Same phonetic family: 株 tree-trunk, 珠 pearl, 诛 to execute. 朱 itself originally pictured the red core of a tree — here it carries no meaning, just the sound clip for the spider word 蜘蛛.
