Structural and Functional Analyses of Stabilimentum in the Garden Spider, Argiope bruennichi (Araneae: Araneidae)

In the web construction of the garden spider, Argiope bruennichi, the decoration known as the stabilimentum forms a zigzag band after completing the orb web. Based on ecological observations, the spider A. bruennichi was observed to consistently maintain an inverted posture on the stabilimentum of the web, with its position consistently fixed on the open side of the web. The stabilimentum of the spider web can be basically divided into a hub region and a band region; the hub region measures approximately 2.5 times the spider's body length, while the band region measures approximately 2.7 times the spider's body length. The hub silk is constructed after the formation of band silk from periphery towards the center, and that the silk in the hub and band regions originate from numerous pairs of aciniform gland spigots on the median and posterior spinnerets. Our fine structural analysis revealed significant differences in the silk morphological properties between stabilimentum band silk in closed and open sides. Particularly, the silks of the stabilimentum were found to be dried fibers less than 500 nm in diameter, devoid of adhesive substances commonly found in prey capture. Moreover, the zigzag band pattern of the stabilimentum silk fibers is expected to play a role in attracting prey animals through light scattering by forming light dispersion at various angles from the light source.