Drinking behaviour of the orb web spider Argiope bruennichi (Araneae; Araneidae)

Water is essential for survival in terrestrial animals. Balancing the water budget can be achieved by avoiding water loss and gaining water. In arthropods drinking as a process of water gain is well investigated in insects. In spiders drinking has only been shown to be present in cursorial spiders but not revealed for web builders. However, some orb web spiders were observed to occasionally ingest water droplets in the web. We here tested whether this reflects drinking. We subjected individual Argiope bruennichi spiders to two different treatments-'water deprivation' vs. 'water saturation'. We conducted drinking tests by recording the spider's behavioural response to spraying the web with defined amounts of water. After spraying A. bruennichi searched the silk-overstitched web hub for water droplets and ingested them. Individuals that experienced the water deprivation treatment showed significantly more water ingesting behaviours, revealing that this response represents a true drinking mode. All individuals exclusively searched the covered web hubs. We further demonstrated that this structure can retain water for up to 40 min providing an effective substrate for the spiders to drink from. Hence, without the need of leaving the web the silk-covered hubs may help A. bruennichi spiders to balance their water budget.