Water harvesting during orb web recycling

Before constructing an orb web, an araneoid spider takes down and ingests the previous day's web. Nocturnal species do this either during the early evening or in the early morning and diurnal species during the early morning, times of the day when relative humidity (RH) is high. Under these conditions the hygroscopic glue droplets of a web's viscous prey capture thread attract atmospheric moisture, providing a net gain of water when a spider ingests its web. The contribution of web recycling to a spider's water budget has been estimated, but not determined empirically. This study does so for adult females of four species (Araneus marmoreus Clerck, 1757, Argiope trifasciata (Forskal, 1775), Leucauge venusta (Walckenaer, 1841), and Micrathena gracilis (Walckenaer, 1805)) by first determining the total volume of a web's glue droplets at 20% and 37% RH to estimate the water invested in web during construction. Subtracting each of these values from the web's total droplet volume at 90% RH approximates the range of water harvested when the web is recycled. High estimates of the net water gained range from 0.45% of a spider's body mass (3.01 µl water gained) in A. marmoreus to 0.88% of body mass (0.19 µl water gained) in L. venusta. In A. trifasciata, the water gained when a web is ingested is equivalent to 29% of a spider's daily evaporative water loss.