noun
water spider; diving bell spider
Refers to the species Argyroneta aquatica, the only spider that lives almost entirely underwater.
ミズグモは水中に巣を作る珍しいクモだ。
The water spider is a rare spider that builds its nest underwater.
noun
ninja water-walking shoes
Historical term for buoyant footwear used by ninja to walk on water; not a modern item.
忍者は水蜘蛛と呼ばれる浮き具を使って水上を移動したと言われる。
It is said that ninja moved across water using buoyant gear called mizugumo.
Compound of 水 (mizu, 'water') and 蜘蛛 (kumo, 'spider'). The ninja tool sense is a metaphorical extension based on the idea of walking on water like a spider.