Giant Water Scavenger Beetle or Giant Black Water Beetle
(Order: Coleoptera; Family: Hydrophilidae)
Hydrophilus triangularis (photo by: Huan Huanlia)
(Kansas: Sedgewick County, Hellers Park. 37°45'40.20"N, 97°20'47.52"W. On ground. September 27, 2013)
Adult diagnosis: This beetle is recognized by its large size, 38 mm to 40 mm in length, and 15 mm to 16 mm in width. The size makes the beetle to be the largest aquatic beetle in the United States. The thickness of the thorax is 8 mm to 9 mm. Besides by its large size, the males and females are identified by their large glossy black elytra with a greenish accent. The adults have long sharp spine on the central of the ventral side of the thorax. The antenna is short and clubbed. The tarsi of the middle and hind legs are strongly compressed and fringed with hairs.
The hind tarsi (left image), and the spine of the water beetle (right) (from: http://jimmccormac.blogspot.com/2012/03/giant-black-water-beetle.html)
Adult/Larvae Natural History: the adults are often attracted to lights at night (usually found around lights in spring and summer). They use their legs alternately while swimming, unlike the dytiscids. To obtain oxygen while diving in the water, the beetle carries air bubble or film under the elytra. Adult females will use structures that resemble the spinnerets of spiders to make pouches of their eggs.
Larvae feed underwater. In late summer, fully grown larvae leave water to prepare pupal cells in moist earth.
Some adults creep under litter on land to overwinter; some active under ice all winter, and live more than 1 year.
Distribution: Found from coast to coast within the United States, ranging from New York to Illinois, across Wisconsin, Nebraska, Oregon, and down to Texas, New Mexico, and Louisiana.
Distribution of Hydrophilus triangularis in the United States. Map generated by: http://www.gbif.org/species/4988136
Habitat: This beetle is found in diversed habitat, especially in shallow ponds with abundant vegetation, ponds and streams. This beetle is also found in a 36.8 C hot springs in the northwest.
Diet: The adults are omnivores, consuming detritus materials, even though the larvae are known to be predacious (ambush predators) - eating mosquito larvae. In some circumstances, the species will consume larger organism such as fish (young fish). Adults sometimes congregate in large numbers and would then become a serious menace if they choose to attack young fish (Wilson, 1923).
Conservation Status: Stable
Hydrophilus triangularis eating an earthworm: http://www.youtube.com/watch?v=qMgI6x-lxK8
Hydrophilus triangularis walking upside-down Beneath Ice in Spring Pond: http://www.naturefootage.com/video_clips/TSC008_0659
Bug Guide: Species Hydrophilus triangularis - Giant Black Water Beetle: http://bugguide.net/node/view/37654
Dunn GA. 1996. Water Scavenger Beetle. In: Insects of the Great Lakes Region. University of Michigan Press. Pp 168.
Hallmark MDM, Life History and Life Process Studies of the Water Scavenger Beetle, Hydrophilus Triangularis Say (Coleoptera: Hydrophilidae). Thesis: Texas Tech University Graduate School.
Milne L, Milne M. 1980. National Audubon Society: Field Guide to insects and Spiders. Alfred A Knopf Books. Pp. 546
Taber SW, Fleenor SB. 2003. Beetles. In: Insects of the Texas Lost Pines. Texas A&M University Press. Pp.153-154.
Wilson, C B. Life history of the scavenger water beetle Hydrous (Hydrophilus) triangularis, and its economic relation to fish breeding. US Government Printing Office, 1923.
Submitted by: Huan Huanlia. November 2013.