[MUSIC PLAYING] LINDSAY GLASNER: Insects, the most diverse group of animals in the world. But did you know that many of the insects we see flying around today actually spend most of their lives underwater? In fact, without aquatic insects, we wouldn't have fish, frogs, or birds in our streams. Scientists actually use aquatic insects to help tell if a stream is healthy or not.
How do these insects live both on land and in water? How do they find food? What do they do to survive?
Hi, I'm Lindsay.
HOPE BATCHELLER: And I'm Hope. Today, we're going to explore the complex life cycles, the crazy adaptations, the ferocious predators, and the importance of this world of insects that lives under the stream's surface.
Are you ready, Lindsay?
LINDSAY GLASNER: Let's dive in.
You've seen these speedy insects zooming around during the summer, but have you ever wondered where they come from? Believe it or not, many of these insects spend several years under water before emerging as adults.
HOPE BATCHELLER: The juveniles of aquatic insects actually look drastically different from the adults. For example, an adult dragonfly might look like this, but a juvenile might look like this.
LINDSAY GLASNER: Caddisflies also grow up underwater. As adults, they look like moths with really hairy wings. But as juveniles, they actually look underwater caterpillars.
HOPE BATCHELLER: Caddisfly larva live in little cases they make to disguise them self. They make these cases out of things like sticky spit that they produce, little pebbles, twigs, and leaves. This way, they're super-camouflaged.
Can you see this caddisfly?
Imagine how hard it would be for a predator to see that animal.
LINDSAY GLASNER: When caddisflies are ready to turn into adults, they seal their case closed like the cocoon of a butterfly. In fact, butterflies and caddisflies do complete metamorphosis.
HOPE BATCHELLER: For complete metamorphosis, the insects, like butterflies, first hatch from eggs. The larvae, are also known as caterpillars, eat food and grow bigger. They may shed their skin three to five times as they grow.
When they're ready to transition into adult, they shed their last larval skin and become what we call pupae. The pupal stage is when the greatest transformation happens. The insects totally change shape.
After a while, the insects split open that pupal shell and out crawls the adults. All they have to do is spread their wings and take flight.
LINDSAY GLASNER: Caddisflies do complete metamorphosis in the same way as a butterfly. The eggs will hatch in the water and become larvae. The larvae will then eat and eat and eat until they become pupae. They then seal off their cases and undergo metamorphosis. The pupae then cut open their cases, swim to the surface, shed their skin, and become adults.
HOPE BATCHELLER: Some insects have a different kind of life cycle called incomplete metamorphosis. This is the kind of life cycle that things like dragonflies and cockroaches have.
LINDSAY GLASNER: For incomplete metamorphosis, insects will hatch from their eggs and develop into nymphs. Nymphs are like miniature adults without wings. Instead of becoming pupae like caddisflies, the nymphs will slowly develop their adult features. In the final stage, they split open their old skin, crawl out, and inflate their wings.
HOPE BATCHELLER: Since these insects sometimes live underwater for several years before becoming adults, they need all sorts of adaptations to survive.
LINDSAY GLASNER: Animals that live underwater can't breathe air like we do.
HOPE BATCHELLER: Just like fish have gills to get oxygen from the water, some insects have gills as well. Some of these insects have external gills. This means that their gills are on the outside of their body, just like on this caddisfly larva. The water in the stream flows over their gills so the insects can get oxygen to breathe.
LINDSAY GLASNER: However, some aquatic insects have internal gills, like dragonflies. See their large abdomen? That's where their gills are. Dragonflies will suck in water from their abdomen so they can get fresh water to breathe. Their abdomen isn't used for just breathing, though. In fact, they can squirt out water fast enough for jet propulsion to make a quick getaway.
HOPE BATCHELLER: Not all aquatic insects use gills to survive underwater. Some aquatic insects, like the belostomatids, also known as the "toe biters," spend their entire lives underwater. These insects can get as big as five inches wide. That's bigger than the width of your hand.
To survive, they actually have tubes that come out of their abdomen to let them breathe, just like a snorkel. This way, they can keep their body and eggs underwater while still breathing from the surface.
LINDSAY GLASNER: Insects also need to get energy by eating. You know how important eating is for your growth? It's the same for larval insects. Some eat vegetation. Some eat plankton. And some even eat fish.
HOPE BATCHELLER: The "toe biters" are vicious predators. After all, how do you think these guys got their names? Their mouth is like a sharp tube that spears the prey and injects a saliva to paralyze the other animals. These guys can even catch tadpoles and fish that are bigger than they are.
LINDSAY GLASNER: Dragonflies also have a really bizarre way feeding. Rather than just opening and closing their mouth like we do, their mouth is almost like an arm. Dragonflies are ferocious predators that sit and wait for things to swim by, and then they reach out and snag it for food.
HOPE BATCHELLER: The caddisflies aren't as bloodthirsty as the dragonflies or the "toe biters," but they also have a pretty cool way of collecting food. Some caddisflies spin nets to collect the little pieces of plants and algae as they float down the stream. These nets are almost like underwater spider webs. The web funnels the food directly towards the caddisflies, and all the larvae have to do is just sit there and chow down.
Food chains can help us picture who keeps whom in nature. All of the energy ultimately comes from the sun. Plants are special because they can use the sun's energy to make their own food. Animals, like us and these insects, have to eat other things, like plants and other animals to get their energy.
If we start to draw lines that connect the energy flow in the stream, we could develop the stream's "food chain."
LINDSAY GLASNER: The sun gives energy to the plants. The caddisflies eat the plants. The dragonflies will eat the caddisflies. The fish will eat the dragonflies. And then, the birds can eat the fish. That's the steam food chain.
HOPE BATCHELLER: If you'll notice, everything in the food chain is connected. This means that if something goes wrong with one part of the food chain, it affects everything else.
LINDSAY GLASNER: So what if pollutants get in our stream? Trash, pesticides, and fertilizers can all harm our streams and their inhabitants.
HOPE BATCHELLER: Imagine that a nasty chemical washed from a nearby factory into the stream. Once the chemical is in the water, it gets into the plants. When the caddisflies eat the plants, they get contaminated. This causes the dragonflies to get contaminated when they eat the caddisflies, followed by the fish, then the birds. And eventually, all the animals in the stream are contaminated.
LINDSAY GLASNER: Do you notice something else? As the pollutant moves up the food chain, the animals have higher and higher concentrations of the pollutant. This is called bioaccumulation. It's a big word, but it means the higher an animal is in the food chain, the more pollutants it's likely to get.
A lot of times, the pollutants get so concentrated that the animals at the top of the food chain, like birds or big fish, will get sick.
HOPE BATCHELLER: Because pollutants can have such bad consequences, it's important to monitor streams to make sure they're healthy. It's like going to the doctor for a checkup, except that the patients are the streams. The scientists are the doctors who look at different parts of the stream to see how healthy it is.
LINDSAY GLASNER: Aquatic insects help determine if a stream is healthy. Scientists first need to catch the insects, and then determine what they found. The type of insects they find will help determine if the stream is healthy.
HOPE BATCHELLER: Insects like caddisflies and dragonflies only live in really clean streams. So if you go to a stream and find lots of these critters, it means that the water is pretty clean and healthy.
On the other hand, things like fly larva and worms can live just about anywhere. So if you find lots of these critters in the stream and nothing else, it probably means the stream is polluted.
LINDSAY GLASNER: The best part is you can be a scientist, too, and study your local streams. Let's see what sorts of things we might find.
HOPE BATCHELLER: You can grab a net or just start turning over rocks.
LINDSAY GLASNER: There are often lots of critters in the shallow, fast-flowing, rocky parts of streams.
HOPE BATCHELLER: Check out what we found under this rock. Here's a caddisfly in a case made of leaves.
When scientists study streams, they have a special system for collecting insects. They put a net down in the water and kick around the rocks in front of the net. Any insects on these rocks get swept directly into the net.
LINDSAY GLASNER: Let's see what we found. We can use this field guide to help identify things. Check out this crane fly larva. You can immediately recognize it because it looks like a large grub with tentacles on its head. These guys are really important for eating all the leaves that fall in the stream.
HOPE BATCHELLER: There are hardly any worms or fly larva in here, but lots of other insects. And they're really diverse. This probably means that this stream is healthy.
And you can even share your findings with scientists. There are citizen science projects that want you to collect data. So get online and see what you can be a part of.
LINDSAY GLASNER: And be sure to go explore a stream soon. Who knows what you'll find?
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Cornell Naturalist Outreach students explore the complex life cycles, amazing adaptations and ecological importance of the world of insects living under water. This video is a co-production of Dr. Linda Rayor, the Ithaca College Park Media Lab, and NYS 4H.