Welcome to part 2 of my insect respiration series! Last time I focused on the basics of insect respiration, the sort you would find in the standard terrestrial insect. The basic type of insect respiratory system has been around for millions of years and works quite well for most terrestrial insects. There are several variations on the basic theme of insect respiration in terrestrial insects, but the aquatic insects exhibit a very wide range of respiratory adaptations. These adaptations, the modifications aquatic insects have made to their respiratory systems, are the focus of today’s post.
First things first. Insects first evolved on land about 350 million years ago. Because they originated on land, they have terrestrial respiratory systems that work best when breathing in air. When insects first began invading freshwater systems, they had this terrestrial respiratory system to deal with. As you might imagine, breathing in water is a very different thing than breathing in air, so insects had to adapt their terrestrial respiratory systems to a freshwater habitat if they were going to live in water. And this is exactly what they did! Aquatic insects have undergone a huge variety of structural and behavioral modifications that have helped change their respiratory systems from those that worked best on land to those that work well underwater.
Let’s imagine a scenario where a terrestrial insect crawls into the water for the first time. Luckily, the pores through which insects breathe (their spiracles) were already largely waterproof, so it didn’t drown instantly. This insect breathed air, though, and it needed to keep doing so to survive. How did it do this? Before I answer this question, first consider how we humans survive when we’re submerged in water. There are three main ways you can prevent yourself from drowning underwater: hold your breath, use a snorkel or other device that maintains your contact with the air, or take air with you in the form of an oxygen tank (scuba diving!) or a submersible watercraft. And why are we thinking of how humans survive in water? Because insects can do these same things!
Snorkels are actually pretty common in aquatic insects and are one of the simplest adaptations for breathing underwater. The image at the right is a picture of the giant water bug Lethocerus medius. This insect uses it’s long respiratory siphon to allow it to remain underwater while still maintaining contact with the surface to breathe. If the bug needs to dive into the water for some reason, such as to avoid a predator or capture prey, it is able to hold its breath for several minutes (close to a half hour!) until it is able to return to the surface, stick its respiratory siphon back out, and continue breathing air. This sort of respiration is also very common in several fly species, including the mosquitoes and the rat tailed maggots.
Other insects use the scuba tank style of respiration and carry oxygen with them underwater. This is very common in many aquatic insect species, including several species of giant water bugs (the members of the Belostomatinae) and the predacious diving beetles. In the image to the left you’ll see the giant water bug Abedus herberti exposing its air bubble, which it carries under its wings, to the water. (I’ll explain why they might want to expose their air bubbles in my next post!) While the bug is underwater, oxygen is drawn through the spiracles and into the respiratory system from this reservoir of air. When the bubble shrinks and the oxygen supply runs out, the bug goes back to the surface to replenish it. Insects using this form of respiration (called bubble gill respiration) are still using the same atmospheric oxygen insects have always used, but they can stay underwater for a long time. Some insects even take this a step further and use what is called plastron respiration. Plastrons are rather complicated to explain, especially before you’ve read my next post, so I’m not going to go into detail. Just know that insects that use plastrons still carry an air bubble and still rely on atmospheric air, but they have special modifications that prevent the bubble’s shrinking so that they almost never have to go to the surface. It’s like having a scuba tank that never runs out of oxygen!
The snorkels and scuba tanks are very simple modifications for the most part. Insects that use these sorts of respiration still rely on atmospheric air and other than rearranging, modifying, or closing some of their spiracles, their respiratory systems are very similar to those of terrestrial insects. These insects also need to be able to get to the surface periodically to breathe. However, this is very difficult in some habitats, such as at the bottom of lakes or in very fast flowing water. In these types of situations, being able to stay underwater all the time, breathing more like a fish than a terrestrial insect, becomes valuable. And, of course, insects have figured out several ways to do this.
Insects that breathe in water rely on the oxygen that is available in water, dissolved oxygen. Dissolved oxygen levels vary widely across aquatic habitats, so different insects have different modifications that allow them to live in their particular habitats. Take a look at the hellgrammite that is pictured at right. These are big insects and need a relatively large amount of oxygen. However, there isn’t a lot of oxygen in water, even under the best conditions. So, these insects live in the best conditions. They are typically found in very fast flowing water in areas with a lot of turbulence (turbulence increases dissolved oxygen levels in water) and in cool to very cold water (cold water holds more dissolved oxygen than warm water). From this fast flowing, cool water, they absorb oxygen directly through their exoskeleton. But they have also modified their exoskeleton to increase their surface area so that they can absorb even more oxygen. Those little pointy bits coming off the sides in the dark brown section in the back half of the hellgrammite aren’t legs – they’re gills! Gills dramatically increase the surface area of insects that absorb oxygen through their exoskeletons, allowing them to breathe more efficiently. Lots of aquatic insects have gills, including the mayflies, the stoneflies, the dragonflies and damselflies (remember that dragonflies have rectal gills!), and the hellgrammites, among others.
The last adaptation I wanted to go over is very rare in insects and is only known in two groups. Take a look at the bloodworms at the left. The color has faded in these specimens because they were preserved in alcohol, but if they were alive they would be bright, vivid red. This is because these insects contain a sort of hemoglobin to help them breathe! For those of you who don’t know, hemoglobin is the substance in human blood that a) allows our blood cells to absorb oxygen from our lungs and b) makes our blood red. Bloodworms are red because their hemoglobin makes them red. They use their hemoglobin to absorb more oxygen into their bodies than they could without it. This is important because a lot of bloodworms live in very low oxygen environments such as the bottom of deep lakes, in polluted waters (where they are sometimes one of the only insects that can survive!), and lakes with a lot of bacteria. These are habitats in which these insects would not be able to live without their hemoglobin helping them bring oxygen into their bodies.
In my next post, I’ll discuss the many things that aquatic insects do to make their respiratory systems work more efficiently in water. Whether they rely on atmospheric or dissolved oxygen, insects exhibit countless behaviors that help them use the tiny amount of oxygen in the water as effectively as possible. Check back soon!
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