许多种水蛭以脊椎动物或无脊椎动物的血液为生，有的或多或少地固定生活于一个动物的个体上，圭接近于体外寄生虫；有的只是一时性的侵袭一下寄主，吸饱血液后掉下来；但也有属于普通的掠食性或腐蚀性的。
以海南山蛭例，它的觅食行为如下:当牛渐渐接近山蛭时（1. 5；米范围），牛的身体发出的辐射热、特殊的气味和呼出的暖湿气流刺激山蛭，它头部开始转向运动，身体前段左右摆动进行探测。
Hainan mountain vermiculite example, foraging behavior is as follows: when cows gradually close to mountain vermiculite (1. 5 meters range), bovine radiant heat from the body, special smell and exhale warm current stimulation mountain, it is turning to sports, head body swinging around the front.
随着牛边吃草边接近，刺激越来越强，山蛭身体前段摇动加快，最后转向牛的方向快速摆动，感觉到牛的方向和位置时，山蛭即停止摆动并快速向牛的方向爬行，幼体、亚成体或饥饿的个体爬得较快。
As the cattle are eating grass and close to stimulate more and more strong, mountain vermiculite recently moved to speed up the body, and finally to the direction of the cow fast swing, feel the direction and position, mountain vermiculite swing and quickly stop crawling towards the direction of the cow, larva and adult individual climb faster or hunger.