Now that I’ve made a quick detour to talk about ants and other stinging insects and the dragonflies at a local wetland for a few posts, it’s back to the giant water bugs! Today I want to go over parental care in giant water bugs. If you remember from my post on insect child care, giant water bugs use a special type of parental care: paternal parental care. This means that only the father participates in the care of offspring, and it is a very unusual behavior among insects. So let’s go over how the water bugs care for their eggs! First, though, I need to provide a little background information about giant water bug taxonomy (the organization of biological organisms) so everyone can make sense of it all.
I’ve already gone over the order, family, and American genera of the giant water bugs in previous posts, so I’m not going to go over them again here. (Please see my taxonomy page for more information if you get confused along the way.) However, there is another taxonomic group that falls between the family and the genera that I haven’t discussed yet, one that is important when considering paternal care behaviors in the giant water bugs. This group is the subfamily. You can easily tell when you are looking at a subfamily, at least when dealing with animals, by the suffix -nae at the end of the name. Most giant water bugs belong to two big subfamilies, Lethocerinae (which includes only the genus Lethocerus, the truly giant water bugs) and Belostomatinae (which includes everything except Lethocerus, usually the smaller, rounder, and/or less robust water bugs). There is one genus, however, that is very rare and only found in a very small part of South America, Horvathinia. Horvathinia is so rare, in fact, that researchers don’t even know where to look for it in the wild or whether it does any sort of paternal care like its close water bug relatives. Horvathinia is generally placed within its own subfamily, Horvathininae, but some researchers think it might belong to either Lethocerinae or Belostomatinae instead. Time and more DNA analyses will answer this question, but for now we’re going to ignore it. After all, we don’t know what sort of parental care it uses, so we don’t need to talk about Horvathinia more here.
So, why do we need to know the subfamilies? There are two basic known types of paternal care in giant water bugs. These behaviors are known collectively as brooding behaviors – the word brood refers to a group of offspring all cared for at one time – and they are divided along the subfamilial lines. This means that Lethocerus, a lethocerine, uses a different brooding behavior than the belostomatines, such as Abedus and Belostoma. Let’s go over belostomatine brooding first as it is generally more familiar outside of the entomological community. This is Abedus herberti:
Isn’t he a handsome father-to-be? This is one of my favorite aquatic insects – I think they are gorgeous, amazing insects! Take a look at those brown, round things on this bug’s back. Those are the eggs that this bug fathered! The belostomatines are back brooders, which means that the males care for the eggs attached to their backs. How to the eggs get there? In the belostomatines, the male and female mate, and then the female lays a few eggs on the back of the male. The male then insists that they mate again (more about this in the next post), and then the female lays a few more eggs. Several hours later, the female finishes laying whatever eggs she has available (up to about 150 in A. herberti) and leaves the area.
The male cares for the clutch of eggs on his back in several different ways. All of the belostomatines carry their clutches to the surface periodically. This allows the embryos developing inside eggs to breathe more efficiently – it is a lot easier to get oxygen from the air than from the water. (In fact, providing oxygen in this way may be the primary function of back brooding behaviors.) In the the bug you see here, Abedus herberti, the father further cares for his clutch by doing push ups underwater. The eggs are able to absorb some oxygen directly from the water, so the push ups are probably a way to stir the water around the eggs and help the developing embryos breathe more efficiently when they are submerged. Other species of belostomatines will do other underwater behaviors. Eggs that are abandoned (male belostomatines can abort their eggs if they aren’t developing properly) or deposited anywhere other than on the backs of the male never hatch. In contrast, almost 100% of brooded eggs hatch. Brooding is thus an obligate behavior, one that is necessary for the continued survival of these species.
Now let’s take a look at how a lethocerine broods and compare their behaviors to those of the belostomatines. This is Lethocerus medius:
As you can clearly see, the eggs are not on the back of the male in this species. Instead, the eggs are laid on a stick above the water line. Can you see them? If not, take a look at the stick below the bug – those light colored, rounded blobs are the eggs. (You can see a previously hatched clutch under the back end of the bug as well.) This bug obviously cares for his eggs very differently than the belostomatine we looked at above. If he’s not carrying his eggs around on his back, how does he care for them?
Lethocerus medius is an emergent brooder. This means that the eggs are laid on vegetation above the water line instead of on the backs of the males. However, like in the belostomatines, the eggs still need care and will die without it. Giant water bug eggs have likely been brooded for millions of years. During that time, it seems they have lost most of their ability to retain water. Lethocerine eggs that are left out of water without parental care dry out so badly that the embryo inside dies. So, lethocerines, like the one you see here, care for their eggs by bringing them water. The male will typically remain attached to the stick that holds his brood, but is usually found at the base of the stick underwater, using his respiratory siphon to breathe. Every now and again, the male will climb up the stick to his clutch and let all the water on his body drip down onto the eggs. There is evidence that suggests that the males of some species might also swallow water that they then regurgitate onto the eggs. Once the eggs are nice and wet, the male then climbs back down his stick and waits underwater until he has to water his eggs again.
So there you have it. One group of giant water bugs cares for their underwater eggs by bringing them to the surface to get air and the other cares for their eggs, which get plenty of air, by bringing them water. Pretty cool, eh? Next time, I’ll discuss some of the costs and trade offs associated with parental care in giant water bugs. In other words, I’ll be talking about why brooding is bad for dad. Stay tuned!
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