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龙虱科（Dytiscidae），亦作龙虱科，为水甲虫（英语：water beetle）的一种，隶属于鞘翅目之下的肉食亚目。生活在田野、水沟、小溪等水体中，是一种掠食性水生昆虫。其学名源于希腊文δυτικός（dytikos），意思就是“肉食性的潜水甲虫”。本科物种几乎见于全世界所有淡水栖境，但亦有部分于落叶堆中栖息。成虫体长一般在1和2.5 cm（0.4～1.0英寸）之间，尽管物种间的差异会比较大。当中见于欧洲的Dytiscus latissimus（英语：Dytiscus latissimus）及巴西的Megadytes ducalis（英语：Megadytes ducalis）有最大的体型，可分别长至4.5 cm（1.8英寸）到4.75 cm（1.9英寸）。与之相反，拥有最小体型的物种应当为栖息于澳大利亚地下水中的Limbodessus（英语：Limbodessus） atypicali，只有约0.9 mm（0.035英寸）长。 大多数物种的颜色都在深棕色、黑色或深橄榄绿，某些亚科的物种会在其边沿带有金黄色的条状斑纹。

。 They have short, but sharp mandible (insect)（英语：mandible (insect)）s. Immediately upon biting, they deliver digestive enzymes. The 幼体e are commonly known as water tigers. The family includes more than 4,000 described 物种 in numerous 属.

When still in larval form, the beetles vary in size from about 1 to 5 cm (0.5 to 2.0 in). The larval bodies are shaped like crescents, with the tail long and covered with thin hairs. Six legs protrude from along the 胸, which also sports the same thin hairs. The head is flat and square, with a pair of long, large pincers. When hunting, they cling to grasses or pieces of wood along the bottom, and hold perfectly still until prey passes by, then they lunge, trapping their prey between their front legs and biting down with their pincers. Their usual prey includes 蝌蚪s and glassworm（英语：glassworm）s, among other smaller water-dwelling creatures.

As the larvae mature, they crawl from the water on the sturdy legs, and bury themselves in the mud for 蛹. After about a week, or longer in some species, they emerge from the mud as adults.

本科物种的成虫，特别是Cybister属的物种，均为可食用昆虫。在美国内华达州洪堡干湖（英语：Humboldt Sink）的一个山洞内，还在当地史前人类遗下的粪化石内发现有Cybister explanatus（英语：Cybister explanatus）被食用的残骸。在墨西哥，他们在吃C. explanatus时会以墨西哥夹饼夹着来吃。在日本多个地区（例如：长野县）有以日本龙虱（Cybister japonicus）为食的习惯；在中华人民共和国广东省，除了日本龙虱以外，亦会食用C. bengalensis、C. guerini、C. limbatus、C. sugillatus、C. tripunctatus等物种。D. marginalis（英语：Great diving beetle）很可能也包括在内，但这个物种的肉很少。也有报告指在台湾、泰国及新几内亚亦有食用本科物种的习惯。

Large but slow on land and not particularly fierce as adults, they are also eaten with relish by many midsized 鸟s, 哺乳动物s, and other larger 捕食s. The larvae are usually safer, due to their 保护色 and ability to escape by water jet; they can be quite hard to catch and may become 顶级掠食者s in small 池塘s.

The diving beetle plays a role in a 切罗基人 creation story. According to the narrative, upon finding nowhere to rest in the "liquid chaos" the beetle brought up soft mud from the bottom. This mud then spread out to form all of the land on Earth.

Adult Dytiscidae, as well as 豉甲科, are collected by young girls in 东非. It is believed that inducing the beetles to bite the nipples will stimulate breast growth.

螨 of the genus Dytiscacarus were found to be are highly specialised 寄生 of 鞘翅目 in the family Dytiscidae, undergoing their entire life cycle while inhabiting the space beneath the 翅鞘 of their hosts.

本科包括下列各属：