The Sempervivum arachnoideum is very unique succulent in that it has the appearance of an organized entanglement. If you are not familiar with this plant you might get the impression that they are infested with spiders. On the contrary, those webby structures on the leaves could be a defense mechanism against insects.
Trichomes loosely connect leaves together.
The white webs on the leaves are specialized trichomes that connect the leaves together. As seen from the picture above, there are normal trichomes (hairs) on the surface of the leaves but those that form the webs are coming mainly from the pointy tip and margins of the leaves. These spiderweb-like trichomes stretch as the leaves grow. That is why they are increasingly longer and sparse on the periphery of the rosette. The growth of the trichomes starts with the growth of the leaves as indicated by the concentration of very tight webs in the middle.
Trichomes are "leaf hairs". They come in different forms and in some cases there are different types of trichomes in one leaf as exemplified by the Sempervivum arachnoideum. (Just like in humans and animals - the hair that grows on the head is different from those that grow on the arms, so is in plants.) They have different functions depending on the nature or habitat of the plants. Trichomes can serve as a deterrent to insects. Also they can serve as defensive layer against extreme environmental conditions such as wind, heat, cold, ultraviolet rays. In some cases trichomes also serve as a mechanism for salt tolerance.
Webby trichomes make the plants look frosty.
Sempervivum arachnoideum is a type of succulent that originated from the Alps (1) and I believe that their trichomes play an important role in its ability to adapt in the snowy, windy and cold conditions of the Alpine Mountains. It is an evergreen plant and can tolerate freezing temperatures. Although they do well in dry conditions, adequate water helps the tiny plants to grow at a faster rate.
Pink flowers float from the multitudes of Sempervivum aracnoideum rosettes.
Sempervivum arachnoideum is a plant to give to an arachnophobe. :)
1) Elijah Walton, Thomas George Bonney (1869). Flowers from the Upper Alps: With Glimpses of Their Homes