Natural born divers! (Predacious Diving Beetles)

This week we look at Predacious diving beetles which are some of the largest aquatic beetles in the world and uncover their methods for staying underwater, as well as what they're diving for. 

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Transcript: 

 

Welcome to episode 58 of insects for fun! The weekly podcast on all things entomology hosted by me Mitchel Logan. I actually just realized recently that it’s been over a year already since starting this podcast. I genuinely can’t believe it went by that fast, but here we are! And before we get into the topic I want to thank this month's patrons Caden, Kirsty, Tina and Todd as well as all the people choosing to rate and review this podcast! 

 

If you’re a weekly listener and have not yet hit the star on spotify or dropped a review and rating on Apple it is one the best ways you can show your support for the show and helps the algorithm push it to more insect fans!

Okay, today we’re quite literally diving into a topic (badum chuck) sorry. But yeah let’s just jump into it! (splash)

 

There are quite a few different kinds of diving beetles, but today we will focus on just one family, and those would be the predaceous diving beetles in the family Dytiscidae. The family name comes from the greek word dytikos which means able to dive. With over 4000 species the diversity in this group is quite vast and they can be found in every part of the world, well, every place but one… 

 

At any rate, these beetles all have a relatively similar shape being oval or football shaped with long legs built for pushing water. The legs are usually all crammed close together as well which is different from terrestrial beetles. Oftentime they have very fine hairs on their legs too, specifically the hind ones which make them appear like long brushes or feathers underwater. It’s not easy to see these when they’re out of the water though, and yes they do fly! (wow) I actually found a really nice diving beetle in a closely related family while black lighting with a friend the other night. The vlog is available for viewing at Patreon.com/insects for fun. 

 

Predaceous diving beetles are also referred to as true diving beetles because they are indeed professionals when it comes to diving. Their body is built for it, and they can stay submerged underwater for hours at a time. It’s hard to say what their average time underwater is because each one is different. Some last 30 minutes, some last 3 hours, and apparently the longest recorded dive was 36 hours. I tried getting an exact species for this claim but was unable which makes me wonder what they actually meant by 36 hours… because the thing is these beetles store air under their elytra or outer shell, and a diving beetle can easily replenish this air by simply floating up to the surface and poking the back end of its body out. I did however find a research article stating that they can spend up to 24 hours fully submerged so who knows. 

 

One really incredible evolutionary mechanism for these would be the ability to use this air bubble as a crude gill. The bubble itself can actually replenish some oxygen from the water in the same way fish gills filter the oxygen. If you’re wondering how a bubble can do that it comes down to the size, the larger an air bubble is the more oxygen it can naturally pull from the environment, and some diving beetles are nearly 1.5 inches long and 1inch wide which for our metric listeners would be 4cm by  2.5cm.

 

So we got these beetles that are natural born divers, but what are they diving for anyway and why do we call them predacious? Beetles in the family Dytiscidae are carnivores, and they hunt anything from frogs to fish and of course other insects. They’re also scavengers which feed on sick and or dead animals at the bottom of water beds. They really are quite handy to have around! Even the larvae are predaceous and are actually referred to as water tigers! These larvae are not your traditional larvae at all and kind of look like a mix between the nymphs of other insects and ladybug larvae. They spend their days preying upon anything from tiny fish and tadpoles to other insect larvae including other water tigers. They are top predators within their niche and are known to be quite beneficial at keeping mosquito populations down! What I think is really cool about them is that they can catch prey passively by floating still near the surface and then grabbing on to anything that gets close enough. The larvae, unlike the adults, do not have elytra to store their oxygen in, and instead must rely on their elastic trachea to store the air they take in from their abdomen. 

Some species though do stay on the bottom or hunt while hiding on rocks. 

 

Predaceous diving beetles can live in many different bodies of water including brackish water which means its got some salt in it, but many of them prefer to stay on the edges or where water is still, because let's face it, trying to dive in a current doesn’t sound like a good time at all, and there’s no way a water tiger can passively catch anything if the currents are too fast. There is one species though which has evolved to live in faster bodies of water, and that would be deronectes aubei which has actually evolved tracheal gills on its elytra to pull oxygen from currents and can stay completely submerged for 6 weeks! This beetle doesn’t really look like a diving beetle either. It actually looks more like a terrestrial beetle that walked into a river and just kept going. 

 

The life of a diving beetle usually begins in the water as a female will deposit eggs either on or inside submerged plants, but sometimes eggs are laid on the surface as well. The larvae hatch and develop through the summer until they are ready to pupate, but before they do the beetles make their way to land and bury themselves in the soil. It makes sense when you think about it because as a pupa these insects have no way at all of getting the oxygen they need if they’re in the water. Even being on the water's edge is dangerous because they could be swept away. In Canada various water beetles hibernate in the winter as eggs, larvae or adults but none of them overwinter as a pupa.

 

Now let’s get into some odd facts regarding the use of these beetles in humans and some lore because thankfully we’ve actually got some this time around. Similar to the giant water bug, predaceous diving beetles are used as a food source in different cultures from Asia to Mexico! In fact species in the genus Cybister were used in tacos both pre settlement and even today! They’re also a source of fried food in China, and other Asian countries. This genus of diving beetles is also used for Traditional Chinese Medicine and Traditional Korean Medicine specifically to help with blood circulation and the treatment of polyuria which means you pee way more than normal. It's also used to help with bed wetting. I guess the beetles contain steroids, and if you didn’t know steroids are used to treat a lot of various conditions that aren’t related to building muscle mass. 

 

As for folklore, the Cherokee native americans have a story about the earth's creation that says  a water beetle came from the sky realm and when he arrived on the surface he had no place to rest because the earth was covered in water. To fix this he started bringing mud and soil up from the depths to create land. 

 

Now for my favorite fun fact, in parts of Asia they have an aquatic form of roulette that uses a dytiscid beetle as the ball. Your money literally wages on the whims of a diving beetle. 

 

If you’re left at the end of this episode thinking these beetles are really cool and wish to have one as a pet then fear not! It is absolutely possible to get yourself one of these diving beetles. Not from a pet store, but from nature or an insect vendor. In fact the University of Florida has a handy pdf dedicated to teaching you all you need to know about keeping adult diving beetles in the family dytiscidae! The TLDR is to have a tank at least 10 inches deep, de chlorinated tap water and lots of aquatic plants. You want this tank to mimic what the insects could use in nature including a large piece of wood or stone that comes out of the water for the beetle to rest on when needing to air out. Having a gravel base is highly recommended to provide more nutrients to the plants and tank. No filtration is required for these nor any specific ph parameters. They’re quite hardy animals, even more so than shrimp. They’ll happily feed on frozen blood worms which you can buy from the pet store or any wild insects you find or catch including flies etc. The tank I currently have is perfect for one of these guys, and now I’m regretting not taking home the diving beetle I found while black lighting. I would not recommend keeping fish and predaceous diving beetles together though. You never know who might kill who, and you also want a lid on your tank because as I mentioned earlier these guys can fly and this usually happens at night.