Bug o’the Week – Wetlands Month IV – Water Scavenger Beetle revised

Bug o’the Week
by Kate Redmond

Wetlands Month IV Water Scavenger Beetle revised

Salutations, BugFans,

We’re wrapping up National Wetlands Week with a beetle that you don’t even need a magnifying glass to see!  This is a revision of an episode that first aired in the summer of 2009 – new words; no new pictures.

BOTW hasn’t plunged underwater for several months now, but in this episode we will get a chance to get our collective gills wet again.  Water scavenger beetles are hefty beetles (some measure more than 1 ½ inches) in the family Hydrophilidae that are easily mistaken for Predaceous Diving beetles (family Dytiscidae https://bugguide.net/node/view/1415131/bgimage) of previous BOTW fame (https://uwm.edu/field-station/bug-of-the-week/predaceous-diving-beetle/).  Other than sharing their classification in the beetle Order Coleoptera, they are not closely related.  North America hosts more than 250 species of Water scavenger beetles, including an introduced, non-aquatic species that makes itself at home in dung, where its larvae eat maggots (fly larvae).

The usually-black, dome-shaped Water scavenger beetles https://bugguide.net/node/view/1644233/bgimage look a little less streamlined than the usually-black Predaceous diving beetles, and their flat, ventral surfaces often sport a keel.  , In contrast to the Predaceous diving beetle’s oar-like strokes https://bugguide.net/node/view/1811015/bgimage, the Water scavenger beetles’ swimming involves alternate left-right-left-right strokes of their flattened, hairy, second and third pairs of legs https://bugguide.net/node/view/378043/bgimagehttps://bugguide.net/node/view/1925805/bgimage.  Their swimming may be clumsy by comparison, but scavengers don’t need the speed and maneuverability of predators.  They are good flyers https://bugguide.net/node/view/742111/bgimage that may leave their watery homes and fly to lights at night (just scoop them up in a paper cup and return them to the water). 

Along with their beetle classification, they also share with Predaceous diving beetles the shallow waters of freshwater ponds and quiet stream edges, although Water scavenger beetles like their weedy, algae-choked habitat a bit warmer than Predaceous diving beetles do.  What they do not share is a lifestyle.  Adult Water scavenger beetles (depending on species) may feed on their aquatic neighbors or may be recyclers, with a food pyramid that includes algae and, as their name suggests, decaying vegetation and dead animal tissue.    

The very-carnivorous Water scavenger beetle larvae (https://bugguide.net/node/view/1872987/bgimage) are described as “sluggish” and are found crawling on the pond floor or climbing on underwater vegetation.  The larvae are couch-potato versions of the sleek Predaceous diving beetle larvae/water tigers (https://bugguide.net/node/view/2276347/bgimage), though they sometimes share the same “water tiger” moniker.  Their feeding category is “engulfer-predator” – they use their powerful, hollow jaws https://bugguide.net/node/view/183298/bgimage to subdue and then vacuum out the juices of their prey.  Their food-list includes their brethren, along with other aquatic invertebrates (they love mosquito larvae) and they also go after tadpoles, snails, and mini-fish. 

According to Eaton and Kaufman, in the Field Guide to Insects of North America, some species of Water scavenger beetles can squeak by rubbing their abdomen against the underside of their wing covers.  Wikipedia lists a repertoire of “stress calls, a male courtship call, a male copulating sound, and a female rejection buzz.”

Water scavenger beetles overwinter as adults, and in early summer, females lay eggs in a cocoon-like structure that’s attached to aquatic plants or left to float like a raft.  In The New Field Book of Freshwater Life, Elsie Klots says that the egg case of one genus includes a vertical “mast” that extends above the water’s surface.  The mast may be involved with respiration, but it may also be an escape hatch for larvae – escape being vital in a group whose young hatch from eggs within a case and immediately start chowing-down on their siblings.  A case may hold 100+ eggs at the start, but cannibalism reduces the number of larvae that live to exit. 

They spend a month underwater as larvae and then leave the water and create a pupal cell by scooping away soil with their mandibles.  It takes them 36 to 48 hours to dig a hole that’s three inches deep.  They climb in and pupate, reappearing as adults in a few weeks.

Predaceous diving beetles breathe, as many aquatic insects do (and as Water scavenger beetle larvae do), by backing their rear end up to the water’s surface and taking in air with a tube or pore (some Water scavenger beetle larvae also have exterior, branched gills https://bugguide.net/node/view/1058195/bgimage).  Adult Water scavenger beetles break through the surface film with un-wet-able (“hydrophobic”) antennae that form a funnel through which air is transported.  Oxygen is stored in a space under the elytra (hard wing covers), and the beetle takes that air into its body through its spiracles (breathing pores).  The nickname “silver-beetle” is a nod to its secondary source of oxygen – a film of air bubbles that typically covers the beetle’s flat ventral surface, trapped there in a layer of thick hairs.  Air held in these hairs can be renewed from oxygen suspended in the water, allowing the beetle to stay under longer.    

It seems that Water scavenger beetles have a Super Power – at least, one Australian species does!  It’s the ability to locomote on the underside of the surface film (remember – due to electrical charges, the layer of water molecules at the surface of a body of water is “tougher” than the molecules below it, which is what allows some insects to skate along its surface.  This same surface tension makes it hard for small critters to break through from below).  See the video here https://www.smithsonianmag.com/smart-news/beetle-can-walk-along-underside-waters-surface-180978115/.  Snails and leeches can do this, too.

The air trapped on the underside of its body may help the beetle stay “belly-up” without using extra energy, giving it enough buoyancy to stroll along under the surface film without breaking through, though each footstep makes the water dimple upwards (scientists don’t know exactly how the beetle’s feet get traction).  Researcher John Gould recounted seeing the phenomenon for the first time, “The beetle was casually walking along the underside of the water’s surface with ease while upside down. Every now and then, it would come to a stop, and then kept plodding along across the surface as if it was walking across any regular solid.” 

How does the beetle do this?  Why?  Are there other beetles that do it?  Scientists who collect aquatic beetles report that when they roil up the substrate with their nets, beetles often float up to the surface.  But do they walk around up/under there, or do they return to their normal haunts ASAP?  So many questions – stay tuned.


J. Reese Voshell, Jr, in A Guide to Common Freshwater Invertebrates of North America, says that “beetle” comes from the Old English “bitula” – “to bite” – a reference to the strong jaws of adult beetles. 

Shelly Cox, in her blog called “MOBUGS – Missouri’s Majority,” shares a great (but unattributed) quote about Water scavenger beetles – “This is a water beetle. It is the hardest object in the world to pick up with tweezers. The second hardest is Mount Everest.”  The BugLady can’t speak to either of those.

Once upon a time, a Naturalist named Linda Bower wondered what she would see if she put a camcorder in a pond.  A whole lot, as it turned out.  She has expanded her gaze to include terrestrial bugs and non-insects, as you will see if you check the excellent offerings at https://www.youtube.com/channel/UCJ2iEp9598fAgiqdMwMZX_g.  Glimpses of a world that exists under our radar.  For the Aquatic playlist click on “Life in and Around the Pond.”  

And remember – Every Month is Wetlands Month (and every fifth living thing is a beetle)!  

Kate Redmond, The BugLady

Bug of the Week archives:

Bug o’the Week – Wetlands Month III – Ostracods

Bug o’the Week
by Kate Redmond

Wetlands Month III Ostracods

Salutations, BugFans,

We continue to celebrate Wetlands month with this slightly updated tale about ostracods, which originally aired in 2015.

By now it’s no secret that the BugLady is enthralled by wee aquatic critters, especially those that inhabit the waters of ephemeral ponds.  Who needs charismatic megafauna!  (and reminder – the BOTW definition of “bug” borrows more from that of a first grader than that of an entomologist).

Little bug – big story – put your feet up.

Once upon a time, there were ostracods.  How do we know that?  Because these tiny, aquatic critters, critters that you would never expect to contribute significantly to the fossil record, have, in fact, managed to produce the most numerous fossils of all arthropods.  Of all arthropods!  That’s insects, spiders, centipedes, millipedes, and crustaceans.  Since the Ordovician period, 485 to 443 million years ago.  They’ve even been found in amber (fossilized tree sap), where they may have landed during a flood.  So pervasive are they that a system has been developed for evaluating ancient climates (paleotemperatures) called the mutual ostracod temperature range (MOTR), based on a measurement of the building blocks in the ostracod’s shell.  And, since the shapes of ostracod shells are indicators of their ecological milieu and of their feeding habits, they are used as paleoenvironmental indicators.  And, fossilized ostracods are used to date marine sediments. 

In the etymology department, Wikipedia tells us that the Greek root “ostracon” means “shell” or “tile,” and that “the word ‘ostracize’ comes from the same root, due to the practice of voting with shells or potsherds.”

Ostracods’ most recent family tree seems to read: Phylum Arthropoda (“jointed appendages”) (yes – they have appendages), subphylum Crustacea, and class Ostracoda.  The Monterey Bay Aquarium website explains that because they look like a shrimp inside a seed pod, they are commonly called seed shrimp (and an older European nickname is mussel shrimp), but the BugLady thinks they look like swimming pistachios (OK – like what pistachios would look like if they swam).  Fellow Crustaceans include the familiar pill bugs, crabs, crayfish, shrimp, and lobsters, as well as barnacles, fairy shrimp, and minute aquatic forms like daphnia, and copepods. 

Ostracods are found worldwide, and there are lots of ostracod species, both marine and non-marine, with many more waiting to be discovered.  The BugLady found estimates of 8,000 to 13,000 total living species, 2,000 of which are non-marine (non-salt water), with 420 of those non-marine species being found in North America.  As many as 50,000 additional species have been identified from fossils.  About half of the non-marine species are in the family Cyprididae, and the BugLady suspects that the ostracods she photographed belong in that family because the Cyprididae are noted for their ability to swim and to tolerate stagnant, oxygen-poor waters, and for having drought-resistant eggs, larvae and adults – important adaptations for an ephemeral pond dweller.  There is a World Ostracod Database.  

If you can think of an aquatic habitat – running water, still, permanent, ephemeral, underground, surface, salt, fresh, hot sulfur, brackish, shallow, or ocean abyss – there are ostracods living in it.  They also occupy marginally wet habitats like mud and sand, algal mats, clumps of wet moss, and damp tropical soils.  One species hangs onto the underside of the surface film in open water.  Freshwater species probably evolved from ostracods that lived in brackish waters that flooded frequently, so that they gradually adjusted to lower saline levels.  One source speculated that the biggest jump in new species of ostracods may come when groundwater is analyzed for their presence (the BugLady hasn’t seen any in her tap/well water, but maybe that’s why faucets have those little screen in them)!  

Many species are generalists, not limited to a single habitat, and while most freshwater species are “benthic” (creeping about on the substrate/debris at the bottom of the body of water), some are active swimmers, others live on aquatic plants, and a few species are planktonic – moving passively with the water currents.  In general, ostracods prefer the shallow water at the wetland’s edge, up to a depth of about three feet, and they are more active in light than in shadows.  They can tolerate a wide range of water chemistries and temperatures, but they aren’t found in highly polluted waters. 

In her Field Book of Ponds and Streams, Ann Haven Morgan calls ostracods “another army of minute crustaceans averaging only a millimeter in length, and impossible to tell apart with a simple lens.”  What they have in common is two limy/calcified shells called valves, which are hinged at the back and held together with muscles like a scallop’s.  This makes them look like teeny clams – teeny, hairy clams, because the outside of the valves may be covered with hair-like setae. 

There’s a lot going on inside that shell – the body is somewhat flattened and is not segmented.  The head end has two pairs of antenna-like appendages plus two pairs of mouthparts (mandibles and maxillae).  Depending on the species, those all-purpose antennae, especially the second pair, may be used for digging, climbing, locomotion, and feeding, and males use them to clasp females.  The thorax area has three pairs of legs that are variously used for locomotion, respiration and grooming.  The rear end has two long “tails” called caudal or furcal rami that can also be used for locomotion.  The appendages can be tucked inside the valves and the valves pulled shut if the creature is alarmed. 

Ostracods come in a variety of colors from dark gray to yellow to red to blotched, and species living on green plants are often gray, green or brown.  Most freshwater forms measure between 1mm and 3mm (1/8”), but one South American freshwater species is about a half-inch long, and some marine species are giants at almost 1 ¼” (“as big as a meatball,” says one source).  

Scientists tell us that when they collect ostracods, most of the individuals they find are female.  The number of males in a population depends on an ostracod’s species, and the type of reproduction is determined by the number of males in the population.  Reproduction by parthenogenesis (virgin birth) is common, especially in freshwater ostracods (in some species, males have never been found), but in species with more males, sexual reproduction is de rigueur.  Eggs are laid on rocks and vegetation or simply loosed into the water.  The eggs hatch into active Nauplius larvae (named after Poseidon’s son) that have appendages on their head for swimming and that shed eight times on their way to adulthood.  Eggs that are left in the mud when an ephemeral pond dries up will hatch when water returns, no matter how long it takes. 

A diverse crowd like the ostracods shows up on many rungs of the trophic ladder.  There are carnivores/predators, herbivores, detritivores, and scavengers but ostracods are generally characterized as omnivorous scavengers.  They eat tiny organisms like algae, diatoms, bacteria, mold, and pieces of organic detritus that are present in the water or on vegetation.  Some deliver food to their mouthparts via a current set up by the appendages.  The BugLady found a picture of an ostracod straddling an aquatic leaf, rasping off food on both sides as it moved along the edge of the leaf.  Ostracods are eaten by hydra and other benthic organisms and by small fish, larval salamanders, and waterfowl.  One species has been shown to be able to survive a trip through a bluegill’s digestive tract.

Whether swimming or creeping, ostracods locomote by extending their appendages from between the valves, with the valves “ajar” https://www.youtube.com/watch?v=F32fyIeVBAM.  Their main sense is touch – they move their antennae constantly and also have sensory hairs on their bodies.  A Nauplius larva has a simple eye, as do the adults of some species.

The Ward Science supply company will sell you some ostracods, but they urge consumer responsibility: “Never purchase living specimens without having a disposition strategy in place,” they say, later adding that “In order to protect our environment, do not release any of these organisms into the wild. When you are done with the crustaceans, add bleach to the culture and dump it down the drain.”  Good on you, Ward Science Supply.  Hope everyone is listening. 

Several species of marine ostracods have bioluminescence in their bag of tricks (they use luciferin and luciferase, the same two chemicals that lightning beetles use), and they glow blue at night in the water and on the sand https://oceanwire.wordpress.com/tag/ostracod-crustaceans/.  The light is in a secretion that the ostracod releases when it is disturbed, and some species use it, like fireflies, to attract mates.  The Japanese call them sea fireflies (umi-hotaru), and the BugLady came across an anecdote about how Japanese sailors during WWII read their instruments and charts by the light of bowls of luminescent ostracods, because regular light sources would have revealed their presence to the enemy. 

For a deeper dive into ostracods (including a peek inside), here’s an article about them from the University of Florida’s great “Featured Creature” series https://entnemdept.ufl.edu/creatures/MISC/ostracods.html.

Kate Redmond, The BugLady

Bug of the Week archives:

Tea & Topics: Beavers in the Milwaukee River (In Person and Virtual)

March 14 @ 1:00 pm – 2:00 pm

Tea & Topics Beavers in the Milwaukee River In Person and Virtual

Beavers are ecosystem engineers with the ability to build and maintain diverse wetland habitats. Beaver ponds have the potential to store and filter floodwater and increase the quality and quantity of habitat for fish, birds, mammals, and other wildlife. Cheryl Nenn from Milwaukee Riverkeeper to discuss the current beaver populations within the Milwaukee River Basin, how humans are partnering with beavers around the world to reduce flooding and increase wetlands, and how beavers might be able to play a larger role in our watershed in the future.

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Tea & Topics are always free to join! | Ages 18+

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