Larvae of caddisfly, aquatic insects of the order Trichoptera, using substrate particles, build tiny tubes where they protect their fragile bodies and carry out the metamorphosis which will transform them in adults.

Researchers from the University of Granada unveil a curious secret of Nature, unknown until now. When the time for pupation comes, larvae modify the architecture of their shelters, balancing their two halves by means of adding weight to both ends. The goal is that the tubes in which they stay captive until pupation can rest horizontally on the shallow banks of the streams, given that, in case of staying in vertical position, the probability of being exposed to the air would be too high and they could die out of dessication

The researchers carried out this task using microtomography techniques, which allow to obtain images and volumetric reconstructions with high resolutions.

Researchers from the University of Granada (UGR) have discovered a curious secret of Nature unknown to this day: the larvae of some aquatic insects build tiny tube-like shelters using little sand grains and tiny stones, which they the use to protect their fragile bodies.

When it's time to pupate and carry out the metamorphosis (that is, when they enclose themselves in a cocoon-like wrapping in which they transition from larva to adult), they add additional sand grains or tiny stones which they place strategically in both ends of the tubes (be it in the inner or the outer side) with the goal of balancing with precision the weight between the two halves, as if they were perfect scales.

The research has proven that the goal of balancing the weight of these tiny tubes, which the insects build with substrate particles, is to allow these aquatic insects (of the order Trichoptera, and especially the species Anitellaamelia) to lay horizontally on the shallow banks of the streams. The volume of water of said streams decreases a lot at the end of the summer, so they may survive this way. If they stayed in a vertical position, the probability of being exposed to the air would be very high and, therefore, the animal could die out of dessication.

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This work has been carried out by Javier Alba-Tercedor, a professor at the Department of Zoology of the UGR, along with fellow researchers Carmen Zamora-Muñoz and Marta Saínz de Bariaín, using the microtomography scanner Skyscan 1172 (a device which allows to scan tiny animals with high resolution) from the Faculty of Science (UGR). The results have been shown in the Bruker Micro-CT Users Meeting, the most important microtomography conference in the world, which took place in May this year, in Bruges (Belgium).

Tomography is a non-invasive method, well known in the scientific community thanks to its wide use in medicine. With tomography we can obtain high resolutions without altering the samples, so we can study valuable specimens without harming them.

Double tubes

Professor Alba-Tercedor's research team was studying Anitellaamelia, an aquatic insect of the order Trichoptera whose adults look like moths and with scarce populations in some water streams in the northwest of the Iberian Peninsula, when they discovered this curious secret of Nature. Some of the pupation tubes built by the insects caught the attention of the researchers for being doubles, that is to say, two concentric tubes were observed, one of them inside the other, an unprecedented fact.

Intrigued by this fact, professor Alba-Tercedor carried out a research on this aquatic insect using microtomography. The first images showed this double structure and, in addition, they showed that the larva had put additional grains of sand or gravel in both ends before pupating. These grains, which in the images looked like big rocks, were put in the inner or the outer side.

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Professor Alba-Tercedor explains: "searching for an explanation for that change in the architecture of the tubes just before enclosing themselves in them and transforming into adults, we formulated an hypothesis: the function of the weight added in both ends of the tiny tubes would deal with the need of having to balance the weight between both halves of the tubes."

With that change, the tiny tubes can lay horizontally in the shallow banks of the streams where, at the end of the summer, the larvae stay enclosed for several weeks until they complete their transformation and emerge as flying adults.

Avoiding exposure to the air

"During this time, the streams' volume of water decrease a lot and there remain some poodles in which the larvae, enclosed in their tiny tubes, stay submerged. During this time, if the tubes were in a vertical position, the probability of part of them being exposed to the air would be very high and, therefore, the animal would die out of dessication," the professor of Zoology from the UGR stresses.

In order to prove their hypothesis, Alba-Tercedor scanned and made volumetric reconstructions of different tubes, using microtomography methods. Each of them was virtually divided in two halves of the same length. Using software, it was possible to measure the volume of each grain as a way to indirectly measure their weight and, therefore, they could measure the sum of the volumes of all the grains in each half.

The results confirmed the hypothesis: the weight of the two halves was exactly the same thanks to the re-balancing work that they make by themselves adding new substrate particles to both ends.

These UGR researchers assert that, no matter how much they study different animals, Nature's skill doesn't cease to amaze them.

"Such a tiny larva, whose length is only a little more than one centimeter, is capable of being an expert architect building the tubes and adding the exact amount and volume of grains in both ends to balance the weight as in a scale, afterwards. The survival of the species depends on this. Evolution has selected the ones that built the right way," professor Alba-Tercedor concludes.