For today’s Friday 5, I’m going to share something near and dear to my heart: aquatic insects that carry bubbles of air with them underwater. These bubbles are important in the respiration of many aquatic insects and have some cool properties (e.g., they can act like gills!). I can spend hours watching aquatic insects breathing, so I’m going to share some of the love with you all today! Let’s start with a couple of simple, very standard types of bubbles. This beetle is a predaceous diving beetle:
Now it’s a little hard to see the bubble here (it’s just barely visible at the back end), but that’s because this beetle holds its bubble under its wings. It acts like a SCUBA tank: the beetle uses up the oxygen and then has to go back to the surface to get another bubble. However, if the beetle exposes that bubble to the water by squeezing a little part of it out the back end (like in the image I posted on Wednesday), this beetle can take advantage of some nifty tricks of physics and turn that bubble into a gill. Without going into too much detail (read the post linked at the top of the page for details!), oxygen can flow into the bubble from the water and extend the length of time the beetle can remain underwater significantly, but only if the bubble is exposed to the water. It is thus very common to see predaceous diving beetles of many species swimming around with big bubbles protruding from their posteriors.
Other beetles carry their bubbles on the outside of their bodies, such as in this water scavenger beetle:
Aquatic insects with bubbles on the outside of their bodies expose their bubbles to the water all the time and can often remain underwater for extended periods. The bubble won’t last forever though, even when it’s constantly exposed to the water, so this beetle and most other insects with belly bubbles still have to go to the surface to get a refill every now and again. Unlike the predaceous diving beetle above that goes to the surface butt first, this beetle pops up to the surface and exposes the top of its head and thorax. I can only presume that there are some cool air channels that allow the air at the surface to flow around the side of the beetle and into the air space under the body. Might have to look into that more closely someday!
Beetles aren’t the only insects with this style of bubble either! This is a water boatman:
As you can see, it’s got a very similar bubble to the water scavenger beetle above. It also exposes it’s thorax at the surface when it needs to refill. However, water boatmen have a really interesting behavior associated with their bubbles. Because oxygen moves incredibly slowly in still water and takes ages to get from the surface to the locations where insects are living, insects such as water boatmen that hang out at the bottom of ponds are exposed to a rather low oxygen environment. That also means that the bubble’s gill-like properties are diminished because once the oxygen close to the bubble is absorbed, it takes a while for more oxygen to reach it. Water boatmen solve this problem by using their huge, oar-like hind legs to stir the water around their bubbles. This creates turbulence in the water, pushing the oxygen poor water away from the bubble and bringing new, comparatively oxygen rich water into contact with it. Awesome behavior!
Here’s another belly bubble, this time on a creeping water bug nymph:
Just another belly bubble you might be thinking, but hear me out. A lot of aquatic bugs hold air stores under their wings. Unfortunately for the nymphs (= the immatures), they don’t have wings, so they are missing the neat little compartments for air storage their elders have. Many species store air in belly bubbles instead. That means that, in several groups of aquatic bugs, the entire respiratory system moves from the bottom of the bug to the top when they undergo their final molt into adults. Now that’s just cool!
And finally, we come to this gorgeous, tiny beetle:
That’s a crawling water beetle, and it holds air under its wings like a lot of other beetles. What makes this beetle special is its hind legs. If you’ve ever identified beetles using the entomology textbook An Introduction to the Study of Insects (originally by Borer and DeLong), one of the first couplets you come to mentions expanded hind coxae that are fused to the metasternum. If that didn’t make any sense to you, this means that the portion of the legs where they attach to the body has been modified into a large flattened plate that is fused to the body. The rest of the leg sticks out from under the plate. These beetles use the space between that plate and the abdomen as a backup air store! They pack some little air bubbles in there that are thought to supplement the main bubble held under the wings, and they’re right out there where they’re exposed to the water. With a name like crawling water beetle, it should be obvious that these beetles are not strong swimmers, so they like to stay underwater as long as they can. Carrying little leg bubbles likely gives them a valuable respiratory boost.
So there you have it! A bevy of bubbles for your enjoyment. Next time you see an aquatic insect, I encourage you to look for a silvery sheen on the body. That’s a good indication that you’re looking at an air store, and you’re one step closer to understanding how that species breathes! I don’t know about you, but I find that terribly exciting. :)