One Trick Magician

Kelly’s Field Notes

Common Name: Water Scavenger Beetle

Order: Coleoptera

Family: Hydrophilidae

Genus: There are 169 genera of water scavenger beetles

Species: There are 2,835 described species of water scavenger beetles

Notable species:

• Tropisternus ellipticus - females buzz and shake if they are not interested in a male’s courtship behavior. This species is found in the central and southeastern portion of the U.S. as well as parts of Mexico.

• Anacaena lutescens - this species can reproduce via parthenogenesis (an embryo develops from a non-fertilized egg. This species is found throughout North America.

• The Japanese water scavenger beetle (Regimbartia attenuata) - this little guy made headlines when a frog swallowed one then it walked through the frog’s digestive tract and out the anus, unharmed. This species is found in Japan and throughout east Asia and northern Australia.

Description: 

Adult water scavenging beetles are shiny and usually black or dark in color, about 6-40mm (0.25-1.5 in) in length. Their legs have long hairs on them which help the beetle to swim. Larvae are larger reaching 60 mm (2.38 inches) in length, resembling a long segmented grub with impressive grabbing mandibles.

Life Cycle: 

Females bury their egg cases, containing up to 32 eggs, in damp soil near streams. Interestingly, unmated females also produce egg cases which they bury but do not hatch (unless they are parthenogenic like An. lutescens). Some of these egg cases will be empty. Larvae eat their way out of the egg, plop into the stream, then continue eating (sometimes each other) until their second insta (molt) which takes about 9.5 days. Larvae lift their prey out of the water before consuming it. We’re not really sure why but it might immobilize aquatic prey. Larvae are also great mosquito larvae hunters!

A total of about 20 days of voracious hunting and eating will take place before they pupate. Pupation lasts about 14 days, in a burrow in the sand. Once they emerge as adults the beetles will make their way back to the stream where they will spend the remainder of their lives (though some species are terrestrial). Most adults feed on vegetation, but it varies by species.

A Note About Surviving Being Eaten:

Other invertebrates have survived the trip through a predator’s digestive tract! Snails have been known to seal up for the long trip through a bird or fish’s digestive tract, but our friends, the water scavenging beetles, are the first documented to actively crawl their way out of a predator. 

Super Powers:

• Acoustics - Beetles of genus Tropisternus have been studied using sound to communicate. They have specific calls to mates, to indicate stress, males have a copulation sound and females reject suiters with a specific buzz. Other water scavenging beetles may also use sound, but more studies need to be made!

• The Escape Artist - The Japanese water scavenger beetle (Regimbartia attenuata) has been noted walking out of a frog’s anus after the frog swallowed the beetle. Neither the frog nor the beetle were harmed in the process. The trip took about 6 minutes at its fastest but most took several hours. Approximately 90% of swallowed beetles successfully escape.

• Swimming - both larvae and adults can swim, though the adults are not very good at it. They prefer to crawl along vegetation. Adults swim by moving their legs alternately.

• Underwater Bubble - water scavenging beetle adults trap air under their wings, allowing them to breathe underwater. Some larvae have gills and others pierce vegetation utilizing the oxygen found within.

• Flight - adult water scavenger beetles can fly.

References:

“Hydrophilidae - an Overview | ScienceDirect Topics.” Science Direct, www.sciencedirect.com/topics/agricultural-and-biological-sciences/hydrophilidae.

“Hydrophilidae.” Wikipedia, 29 Jan. 2023, en.wikipedia.org/wiki/Hydrophilidae.

Lambert, Jonathan. “Water Beetles Can Live on after Being Eaten and Excreted by a Frog.” Science News. 3 Aug. 2020, www.sciencenews.org/article/water-beetle-frog-eaten-alive-escape-death-butt-excretion. ‌. Accessed 3 Aug. 2020.

‌McGreevy, Nora. “When This Beetle Gets Eaten by a Frog, It Heads for the “Back Door.”” Smithsonian Magazine, 5 Aug. 2020, www.smithsonianmag.com/smart-news/when-beetle-gets-eaten-frog-it-forces-its-way-out-back-door-180975484/. Accessed 10 May 2025.

“Water Scavenger Beetles.” Missouri Department of Conservation, 23 Jan. 2024, mdc.mo.gov/discover-nature/field-guide/water-scavenger-beetles. Accessed 10 May 2025.

‌‌“Water Scavenger Beetle - Field Guide to Common Texas Insects.” Field Guide to Common Texas Insects, 6 July 2017, texasinsects.tamu.edu/water-scavenger-beetle/. Accessed 10 May 2025.