Giant Water Scavenger Beetle

Giant Water Scavenger Beetle in Photo Tank, 4/7/2021

At one and a half inches long (up to 40mm), the giant water scavenger beetle (Hydrophilus triangularis) is the largest aquatic beetle in the United States.

Yes, aquatic beetles. Beetles (insect order Coleoptera for those into that kind of thing) are the most diverse group of animals on the planet. By far. Numbers vary and are always changing as more species are identified, but the Scripps Institution of Oceanography as of April 2020 gave the numbers as 400,000 beetles species worldwide of which they considered 13,000 species to be aquatic (see link below).

One powerful adaptation to the aquatic lifestyle you can see evidenced in the picture above is the ability to carry an air supply underwater trapped in small hairs along the underside of the body. The trapped air shows as silvery sections under the beetle. The blacker areas toward the rear of the beetle are areas it doesn't have hair to hold air. The areas of trapped air cover the beetle's spiracles -- the openings in insect bodies through which they breathe -- so that while the beetle is swimming around or hanging out underwater, it is breathing air. Amazing.

Another adaptation seen above are the feathery-looking legs that serve for better propulsion through the water.

As adults, the beetles scavenge on dead plant material, but also prey upon smaller aquatic organisms as well. And they fly. For these guys as adults, if the pond dries up, they can take off and find another pond, so being amazingly adapted for life underwater has not deprived them of their aerial abilities, which also serve them well in times of need. As for surviving winter, I have only found that they overwinter as adults, which accounts for the size of this guy so early in the year.  We will likely encounter giant water scavenger beetle larvae later this season, so stay tuned!

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