Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

Giant Diving Beetle & Water Tiger (Dytiscus spp.)

Giant Diving Beetle
Credit: Terry Thormin

Water Tiger (Giant Diving Beetle larva)
Photo CD 0023 3291 1601, Image #14 - Credit: Terry Thormin

INTRODUCTION

Giant Diving Beetles are members of the family Dytiscidae whose members are collectively called predacious diving beetles. The Giant Diving Beetles all belong to the genus Dytiscus. They are all aquatic and are voracious predators that will often take prey as large as or larger than themselves.

IDENTIFICATION

There are about 150 species of predacious diving beetles in Alberta, ranging in size from about 1.5 mm to 40 mm. Identification of these species is tricky and best left to the professional. Any species that is over about 20 mm will undoubtedly be in the genus Dytiscus and thus can be called a Giant Diving Beetle. The beetle illustrated here is a female Dytiscus alaskanus. The Water Tiger is an unidentified species. Most species of predacious diving beetles are dark brown or black, often with a lighter brown or reddish brown border, and some having a lighter band across the back near the hind end. The hind legs, which are used for swimming, are the longest, and have long hairs on them. The larvae, which are called water tigers, are elongate, with a large head and powerful jaws. They swim using all three pairs of legs and usually have the tail end sticking up in the air.

DISTRIBUTION

Found throughout the province wherever there are ponds, sloughs, lakes, streams and rivers. They are not found in fast-moving water.

TIME OF YEAR

Adults and larvae of various species can be found throughout the spring, summer and fall. The adults of some species move from shallow bodies of water in the fall to deeper lakes where they remain active all winter long under the ice.

HABITAT AND HABITS

As mentioned before, these insects are aquatic in the larval and adult stages. The larvae do, however, come out onto dry land to pupate. Both adults and larvae are voracious predators, and will devour a wide variety of insects and other invertebrates, as well as vertebrates such as frogs, toads, salamanders and small fish. The adults have mandibles that allow them to tear their prey apart. The larvae have jaws like hypodermic needles that allow them to inject digestive enzymes into their prey. These enzymes dissolve the body tissues and the water tiger sucks up the resulting liquid. Both adults and larvae come to the surface to breathe by sticking the back end out of the water. Adults carry air under the wing covers, and often have a small bubble attached to the back of the abdomen.

Adults come out of the water at night and fly around. Because they use the moon to navigate, they are attracted to any bright light. For this reason they are often found under porch lights, street lights and gas station lights.

SIMILAR SPECIES

In the case of predacious diving beetles it is a closely related family that must be discussed. Water scavenger beetles, members of the family Hydrophilidae, look very similar to predacious diving beetles. They tend to have a higher dome appearance than the predacious diving beetles. They also swim by moving their hind legs alternately as opposed to simultaneously the way the predacious diving beetles do, and they come to the surface for air by sticking the head out, not the back end. Unlike predacious diving beetles, they keep air on the underside of the abdomen, which gives them a silvery appearance.

COMMENTS

Giant Diving Beetles, when they turn up under a porch light or street light, often amaze people because of their large size. Although they have biting mouthparts, and can certainly give one a bit of a nip, they are not capable of doing any real damage. Gas stations with their bright lights and car lots, with the combination of bright lights and shiny reflective car tops, can attract large numbers of the medium-sized predacious diving beetles, to the point that they may even become a bit of a nuisance.

For more information on Dytiscus alaskanus you can visit the University of Alberta’s entomology collection species page.

[Royal Alberta Museum Home Page] [Invertebrate Zoology]

For more information on this bug, please contact the Bug Room.
© Copyright 2000-2008, Royal Alberta Museum - ALL RIGHTS RESERVED
Last update: December 10 2004