Caddis fly larvae in tiny pebbles. Photo: NOAA Response and Recovery Bureau.
May 20, 2021 –
How do you decorate the underside of a leaf? Why are you hanging your eggs there, of course? How do you catch delicious river bites? Well, you want to make a network. You need a miracle substance to accomplish this. Spiders aren’t the only animals that make silk. From caddis fly larvae to lacewings to inchworms, many insects invade the sticky stuff.
Martha Foley and Curt Stager discuss the many advantages of silk at Natural Selections.
Martha Foley: Let’s talk about silk, but not about spiders. I think we’ve probably covered spiders and the silk they use in different ways. I notice other silks, like when inchworms fall or even wood caterpillars fall from the heights, from the leaves to the ground. Is it the same stuff?
Curt Stager: Yes, we both call them silk. They are both proteins. However, the molecular structure is different. And it’s not surprising that they have different names. The spider silk proteins are called spidroins, the insect proteins fibroins. So you are just another structure. But the same basic idea.
MF: Also strong like spider silk?
CS: Strong and stretchy. They are made in a different part of the body. When an insect does, it comes from the mouth, not the tip of the belly, if you are a spider that does. It’s surprising when you pause and think about how many insects actually make silk. And of course there is the silkworm or the caterpillar cocoon from which the word comes, but many other insects do too.
MF: Like the Inchworms. We talked about caddis flying earlier. You know these little things. I imagine it underwater, the larva, covered with small sticks or small grains of sand, crawling around on the rock surfaces. The same things silk use?
CS: Yes, so these are water larvae, or nymphs, and they spend most of their lives underwater foraging around. Depending on the species, these caddis fly larvae make cases in which they live from different materials.
MF: These are the sticks and stones that I see.
CS: Yes, if there are a lot of pine needles below, you can see those little clumps of pine needles or tiny grains of sand crawling around, but these things need to be glued together with something. And the material they use is a special type of silk that is sticky on all sides. So it’s like double-sided tape sticking all of that stuff around them together.
MF: Is it like a shell for the larva, right?
CS: Yes, they form a little shelter, and if you pick them up from the ground very carefully, the larva will move in, but if you let them sit for a minute the front end will come out and start crawling around again.
MF: Right, kind of neat to do and fun to watch. They’re easy to spot there anyway.
CS: But there are other uses that caddis flies have besides the shelter. You can also use it to catch food. I had never seen this in the Saranac River outside of Saranac Lake until recently. There in the shallows they looked like little silk socks, funnel-shaped things about the length of your thumb.
MF: Like a little bag or something?
CS: Like little bags that rise in the current and stick to the rocks at the bottom. And the wide end of the funnels would point upstream and then taper to a small fine point. But the point would then also point upstream again.
MF: They are like fishing net stuff.
CS: They are like fishing nets and they wobble like wind socks when the current goes by. They basically filter small bits of food out of the water with silk nets.
MF: I’ve never seen this before, I really want to see that.
CS: It is really spectacular. You don’t notice them at first.
MF: As big as your thumb
CS: About the size of your thumb and apparently they vary in size too depending on how fast the current is or how much food is out there. Apparently they make bigger nets when there is less food around so they have a better chance of catching something.
MF: That’s crazy. Are all insects used by the larva for silk?
CS: Mainly. There is one exception, they are green lacewings. If you’ve ever seen these, small green bodies and delicate wings. The adults can make the same material, but not as thread. They attach their eggs to the underside of a leaf by making a small dab of the sticky silk on the underside, pushing an egg in, and then pulling the egg down so that a small thread comes down, and then releasing it. And it hangs there like a little Christmas ornament. And then they do another one and another so there are a number of small, dangling lacewing eggs.
MF: I have to go out and look for it, as well as the thumb-sized Caddis wind sock. That’s crazy. I still picture the little Christmas ornaments hanging from the underside of a leaf.
CS: Yes, the underside of a sheet. So yes, it’s worth getting on your hands and knees in the forest.
MF: Get your little lens out, right? Alright dr. Curt Stager from Paul Smith’s College. I’m Martha Foley from St. Lawrence University.
Natural Selections airs every Thursday morning on NCPR.
