Predaceous Diving Beetle

by Carl Strang

Ponds and marshes are fascinating places. Children love them because they are full of diverse and hidden life. Dip in a net and who knows what you might catch? This week I began monitoring some amphibian traps in Mayslake Forest Preserve’s marshes. I haven’t caught any frogs or salamanders, yet, but on the first day a few traps contained some large beetles. I thought I recognized them, but aquatic invertebrates are a weak point and I needed to do a little background study. I went back yesterday equipped with a water container, and when I caught one of the beetles I plopped it in.

The beetle swims with the hind pair of legs. The feet are fringed and act like oars. You can see the air bubble at the tip of the abdomen. The beetle breathes air from a bubble held beneath the elytra, or hardened top wings.

I first needed to see the antennae. There are two families of swimming beetles with some species reaching an inch or more in length. As you can see, the antennae are long and threadlike. That places this insect in the predaceous diving beetle family, Dytiscidae. The large size, color pattern and other details as well as their abundance in this marsh led to an identification of Dytiscus verticalis.

This gives some sense of Dytiscus’ size, an inch and a quarter long. They fold up tightly when lifted out of the water.

The top photo reveals rows of striations down the length of the elytra, which indicate this is a female. I learned in my reading that adults can live 2-3 years, impressive for an insect. The larvae, also aquatic, are predaceous like the adults. They are biters, so I wouldn’t have held a larva in my hand. They feed on small invertebrates, but also on tadpoles and tiny fishes. They are themselves on the menu for a variety of herons, larger fishes and other predators. The larvae have been shown to be flexible foragers, sitting and waiting when prey densities are high, going on the prowl when prey are fewer. Though both larvae and adults are aquatic, pupation takes place on land, and pupas can drown if water levels rise above them in that stage.

I’m looking forward to finding what else is in Mayslake’s marshes.

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2 Comments

  1. April 21, 2011 at 6:34 am

    […] features were by now familiar thanks to my recent experience with a larger member of its family in the nearby marsh. This is a predaceous diving beetle. There is a pale marking at the posterior edge of its elytron […]

  2. March 12, 2012 at 6:25 am

    […] some interest. Last year I put them out later in March, and right away began catching some of the large Dytiscus water beetles. So, I expect to learn something about them this year, either that there is a later time when they […]


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