It’s the start of a new week and you know what that means: Science Sunday! I thought I’d mix things up a little today by talking about some of my own research. The subject of today’s post is this bug, a bug that should be quite familiar to my long-time regular (awesome!) readers:
Giant water bug, Family Belostomatidae, Abedus herberti
This is Abedus herberti, a giant water bug in the family Belostomatidae within the order Hemiptera (true bugs). It’s a large, aquatic insect native to Arizona and northern Mexico that you’ll find in streams, often in the mountains. It’s an interesting bug for many reasons, but it is especially well-known because the male bugs care for the eggs until they hatch (see my post about giant water bug parents for more details!) and they are wickedly efficient predators. These traits make these bugs fascinating for entomologists like me, but they’re not what got me interested in giant water bugs originally. I got excited about giant water bugs because of this:
Abedus herberti at surface, collecting air
Respiration! I never thought I would ever work with either insect respiration or aquatic insects (except dragonflies), but this project opened up a whole new world of possibilities to me and completely changed the direction of my research. So, today I’m going to tell you about the project. It’s too long for one post, so this week I’ll give you an overview of the respiratory behaviors of Abedus herberti and next Sunday I’ll talk about the experiments I did to show that this is a respiratory behavior.
Giant water bugs are aquatic insects and, as such, have several adaptations that allow them to live in water. I’ve talked about aquatic insect respiration before, so I’m not going to go over the respiratory adaptations again here, but note that giant water bugs depend on air to breathe. Water bugs in the genus Lethocerus have a long respiratory tube (called a respiratory siphon) that they stick out of the water that works a lot like a human using a snorkel. They also have a small space under their wings that holds a small amount of air so they can breathe underwater for a short time. (Imagine using a SCUBA tank – same deal!) Abedus herberti does things a bit differently. First, the respiratory siphon has been reduced to short air straps:
Abedus herberti. Arrow points to the air straps.
Second, it has a much bigger space under the wings. That means it can carry more air with it underwater and can remain submerged a lot longer.
So how does Abedus herberti breathe? Let’s trace the behavior from the moment the bug sticks its air straps out of the water, fills the space under its wings with air, and dives into the water to settle near the bottom. The bug then follows one of three behavioral pathways. The simplest is this: the bug absorbs oxygen from the air bubble into the body. When it has used up most of the oxygen, it goes to the surface to replace the bubble. If the bug’s close enough to the surface, it simply raises its abdomen and sticks the air straps out. If it’s in deeper water, it stretches as far as it can to try to reach the surface without letting go by raising the abdomen up, releasing the hind and middle legs, and holding on with only the front claws. If that’s not enough, it will let go completely, float to the surface, and quickly replenish the air store before diving to the bottom again. You can see the behavior in this rather blurry video:
That’s one behavioral pattern. In the second pattern, the bugs add one more step: gaping. The bugs surface, dive, and sit at the bottom, using the oxygen in the air bubble as before. However, after about 5 minutes they expose the air bubble to the water. To do this, they lower the abdomen, creating a space between the abdomen and the wings:
The giant water bug Abedus herberti gaping, exposing its air store to the water. The silvery part is the air bubble.
Gaping is a tiny behavior, one very small movement, but it does so much for the bug. By exposing the air bubble to the water, the bug transforms the air bubble from a simple oxygen store into a physical gill capable of absorbing oxygen directly from the water, tripling the length of time it can remain underwater! The bugs may gape for 20 minutes, then close the gap before returning to the surface.
The third behavioral pattern adds one important step: dynamic gaping. This pattern starts with the bug surfacing, diving, sitting on the bottom, and gaping. After gaping for 5 or more minutes, the bug starts doing this:
They do this motion over and over and over for up to three hours. Gaping allows the air store to become a physical gill, but dynamic gaping makes the physical gill function as efficiently as possible by stirring the water around the bubble. This pushes oxygen-depleted water away from the bubble and draws in oxygenated water. The physical gill is much less efficient at absorbing oxygen from the water when the bug gapes, but does not dynamically gape. Dynamic gaping is thus a form of ventilation that allows the bugs to remain underwater ten times longer than they can without gaping or dynamically gaping! But even a dynamically gaping bug must eventually return to the surface (see my post on better breathing underwater to learn why), so it closes the gap between the abdomen and wings and surfaces.
The advantages of this behavior are clear: gaping allows the bugs to remain underwater 3 times longer and dynamic gaping ten times longer than they can when they do not expose the air store to the water. But why is it important to stay underwater? This is one reason:
Egrets and other wading birds like to eat water bugs!
Many things love eating large, protein filled insects, so staying hidden underwater as long as possible likely helps A. herberti avoid predators. However, the bugs carry a lot of air with them, which makes them very buoyant. If they let go of the bottom, they float immediately to the top. Diving is probably very hard too because they have to fight against their tendency to float to the surface. So, if the bugs benefit from remaining underwater, but it’s hard to stay underwater, then it’s a good idea to stay underwater as long as possible. Gaping and dynamic gaping to the rescue! These two simple, easy behaviors greatly extend the length of time the bugs can remain submerged, but the behaviors probably also require far less energy than diving from the surface. If so, then gaping and dynamic gaping help the bugs avoid predators, save energy by avoiding trips to the surface, and maximize the time the bugs can spend trying to capture food.
So that’s gaping and dynamic gaping! Next week, I’ll discuss how I know that these are actually respiratory behaviors. I hope you’ll check back for part two!
Goforth, C. L. and Smith, R. L. 2012. Subsurface behaviours facilitate respiration by a physical gill in an adult giant water bug, Abedus herberti. Animal Behaviour: doi:10.1016/j.anbehav.2011.12.02. (Published online only currently – will replace this with the print citation when the issue is released)
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