How can tunnels and chambers be built in trickling sand? In order to be able to build nests in the finely crumbly subsoil of desert and dune areas, some species of pollen wasep have developed a clever strategy: They combine grains of sand with silk threads, researchers report. The biologists have discovered previously unknown glands and structures on the head of these important pollinator insects of the Mediterranean region, which enable them to use the unusual concept of silk spinning.
From experience from the sandpit era or from building castles on the beach, we know that stable structures cannot be built out of dry sand, and it is certainly not possible to dig tunnels in the trickling material. To make this possible, people have to resort to structural stabilization measures. This is exactly what tiny inhabitants of the deserts and coastal dunes of the Mediterranean and South Africa do, report Dominique Zimmermann’s researchers from the Natural History Museum in Vienna. These insects, which are only a few millimeters in size, belong to the pollen wasp family.
Similar to bees, all representatives of this group of insects feed their larvae with pollen, which they collect from the plants in their home. However, it is not a question of pollinator insects that form colonies – they live alone and only build their nests for their own offspring. Most pollen wasp species dig tunnels and brood chambers in loamy soil or use the loam to build free brood cells above ground on rocks. But the representatives of the genus Quartinia, which occurs mainly in the Mediterranean region and southern Africa, have also opened up habitats where these nest building options are not available: They can also build their nests in loose sand.
Cleverly stabilized nest tubes
It was already known that the cavity structures of the nests of the Quartinia pollen wasps owe their stability to fine silk webs. But so far little details have been known about how the insects produce these structures. Zimmermann and her colleagues emphasize that while the production of silk is quite common in larvae, it is hardly known in adult insects. The biologists have now been able to show which adaptations the female Quartinia pollen wasps can use to build their nests using self-produced silk as a binding agent in the sand.
As they report, they struck gold in their search for special body structures for silk production on the insects’ heads. This required extremely fine work: “To examine the tissue, the tiny heads were first embedded in plastic and then series of thin cuts were made with a diamond knife on an ultramicrotome,” says Zimmermann. “We then systematically searched the surfaces with the scanning electron microscope and the examinations were supplemented with the use of a micro-computer tomograph,” reports the researcher.
Newly discovered spinning structures on the head
In their investigations, the scientists compared the head structures of females of different Quartinia species with those of the males and other representatives of the pollen wasps. “We were able to identify a previously unknown gland in the mouthparts of the females that is associated with silk production. We also discovered an unusual appendix at the tip of the lower jaw, which is probably used to spin the threads. These structures are missing in representatives of other genera as well as in the males, ”reports Zimmermann.
The newly discovered structures are therefore key adaptations of the genus Quartinia, say the researchers: They made it possible to settle sand habitats, and thus an ecological zone that is completely new for pollen wasps. This is also reflected in the success and importance of this family: With over 140 known species, some of which form large populations, they play a central role in pollination and thus for the reproduction of flowering plants in the sandy ecosystems, the scientists emphasize.
As you finally emphasize, one aspect remains to be clarified: You also want to break down the chemical composition of the silk threads with which the Quartinia wasps stabilize their nest tubes. Perhaps the future research results could even be of interest to materials researchers.
Source: Natural History Museum Vienna, specialist article: Arthropod Structure & Development, doi: 10.1016 / j.asd.2021.101045