Beyond venomous fangs: Uloboridae spiders have lost their venom apparatus but not their toxins

Abstract Venom is one of the most potent chemical secretions in nature which has played a significant role in the evolutionary success of many animal groups, including spiders. However, the family Uloboridae has seemingly lost the venom-producing organs, leaving the presence and function of toxins in question. Uloborids employ a distinct hunting method involving extensive silk-wrapping and regurgitation of digestive fluids onto the whole prey, yet the precise mechanism by which prey is immobilized remains unknown. One hypothesis is that toxins may have shifted from the venom to other secretions that come into contact with prey, such as silk or digestive fluids. Here, we use a combination of histology and multi-tissue transcriptomics, to 1) investigate the absence of venom glands, and 2) examine the expression of venom toxins in different body parts of Uluborus plumipes . Our findings indicate that U. plumipes effectively does not possess venom glands, nor the duct opening in the fangs. However, we identified putative neurotoxins that are highly expressed in the digestive gland, suggesting that these may contribute to prey immobilization. Among the most highly expressed were U3-aranetoxins, U24-ctenitoxins, and a defensin, although the latter was not predicted to have neurotoxic activity. These results support our hypothesis that Uloboridae spiders have lost their venom apparatus, but not their toxins. These “digestive toxins” might be responsible for complete prey immobilisation and represent an alternative hunting strategy that replaced the loss of a frontal venom system.