蛛 is a bound form (b.f.) meaning 'spider' — it cannot be used alone. It always appears as part of a compound or fixed expression, most commonly 蜘蛛 (zhīzhū, spider), 蜘蛛网 (zhīzhū wǎng, spider web), and the four-character idiom 蛛丝马迹 (clues or traces, literally 'spider threads and horse tracks'). As a bound morpheme, it must be attached to another element to form a complete word.