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What Time is it in Nature: Giant Water Bug

February 15, 2014

With the recent cold weather, we’ve had a dearth of insects at Prairie Ridge!  Apart from the usual winter suspects we find in the office (Asian Multicolored Ladybugs and Brown Marmorated Stink Bugs), you will be hard pressed to find an insect wandering around Prairie Ridge on a cold, winter day.  However, you don’t have to look any further than the pond to find insects that are still out and about, even in the dead of winter – even when the pond freezes over!  One of the most fascinating insects you’ll find in the water in winter is a species of giant water bug without a common name, Belostoma flumineum.

Belostoma flumineum

Giant water bugs are large, aquatic insects in the true bug family Belostomatidae.  This particular species is found throughout most of North America, so it is very widespread and relatively common.  B. flumineum are large insects, topping out at just under an inch long.  They’re shaped as you see in the image above, a sort of flattened oval pointed at both ends.  They’re almost always the brown color of the individual above as well, a color that helps camouflage the bugs in their aquatic environments.  The legs are characteristic of the giant water bugs, flattened middle and hind legs adapted for swimming and bulky forelegs that are usually held out in front of the body:

Belostoma flumineum profile

That style of leg is a raptorial foreleg, a strong and powerful limb that is well adapted for reaching out and grabbing things.

Giant water bugs are predators and feed on a variety of aquatic animals, including other insects, tadpoles, and small fish.  Like other giant water bugs, B. flumineum is a sit-and-wait predator, so it uses its hind legs to hold on to a rock or other perch in the water and waits for something to swim by.  When an unlucky prey animal approaches, the water bug will quickly reach out with its raptorial forelegs, grab the animal, and pull it back to its mouth.  This happens very quickly, often in less than a second.  That quick grab for food is the only fast part of the process though!  Giant water bugs, like other true bugs, have piercing-sucking mouthparts, a long, pointed “beak” that you can see under the head in the photo above.  They cannot chew, so they have to liquefy their food to eat.  Using its forelegs, the bug will hold the prey animal near its head, stab it with its beak, and inject it with a paralyzing chemical to knock the animal out.  Then the bug injects its prey with digestive chemicals, turning the tissues into a soup-like substance that the bug can suck up through its mouthparts like a straw.  It’s a pretty gruesome process really, but it’s also one that serves the giant water bugs well: they are often able to capture and consume prey species much larger than they are.

Scientists have long marveled at the amazing predatory abilities of giant water bugs, but they also display a behavior that makes them virtually unique among insects, and indeed the entire animal kingdom.  When most insects reproduce, the female will lay her eggs somewhere, then leave them to develop, hatch, and grow on their own.  In a handful of insect species, the mother, sisters, or (rarely) both parents will care for their offspring throughout at least the egg stage.  Giant water bugs are fascinating because not only do they care for their young until they hatch, but it’s the father that provides the care!  This is a very rare behavior among animals, but there is only one single other species with a similar strategy in insects.

Here’s how it works.  A male and female B. flumineum find each other in a pond and mate.  The female will then climb on the male’s back and lay a few eggs.  Soon the male will kick the female off, they’ll mate again, and she’ll lay a few more eggs.  They’ll keep this up for several hours, until either the female runs out of eggs or the male runs out of space on his back.  At that point, the female leaves – her part is done.  The male then carries around a cluster of eggs glued to his back for a couple of weeks until they hatch.  During that time, we will carry his eggs to the surface periodically and complete a couple of underwater behaviors, all things that help the tiny bugs growing inside the eggs get the oxygen they need to complete development.  If the father does his job correctly, nearly all the eggs will hatch and up to 100 new, baby giant water bugs are released into the pond.

Giant water bugs are often top predators in aquatic habitats without fish, and help regulate prey species populations in the ponds they inhabit.  Our B. flumineum are an important part of the Prairie Ridge pond ecosystem!  Next time you visit, take a stroll down to the pond and peer into the water among the cattails.  They don’t move much, but B. flumineum have to come to the surface periodically to breathe, so look for a little brown football shaped creature quickly darting to the surface and back into the depths.  In the summer, you might even be lucky enough to see a father with eggs on his back, floating near the surface on a warm day.  It’s a pretty amazing sight, so come see if you can spot one!

What Time is it in Nature is a weekly feature highlighting the current plants, animals, and other wildlife at the Musuem’s public outdoor facility, Prairie Ridge Ecostation.  Find out more about the natural happenings at Prairie Ridge at our What Time is it in Nature Archive!

(Photos by Chris Goforth)

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