Theridion is a genus of tangle-web spiders with a worldwide distribution.[6] Notable species are the Hawaiian happy face spider (T. grallator), named for the iconic symbol on its abdomen, and T. nigroannulatum, one of few spider species that lives in social groups, attacking prey en masse to overwhelm them as a team.
Spiders in this genus are about as long or longer then they are wide. Their chelicerae have two or less teeth on the front edge and none on the back edge. The front leg is the longest in both genders, but the next longest is the second leg in males and the fourth leg in females. The epigyne can vary, but the pedipalp has a median apophysis and a colulus is absent from both genders.[7] In several species, the males have mastidia (projections) on the chelicerae.[8]
Theridion has the anterior eye row slightly curved and with a gap between it and the posterior eye row. This distinguishes it from Enoplognatha, another theridiid genus in which the anterior eye row is almost straight and the gap between eye rows is very small.[8]
Theridion build tangle webs, usually under leaves and among vegetation. At night, spiders hang upside-down in these webs. During the day, they hide close by their webs.[9]
Philodromus cespitum, a species of running crab spider, preys on Theridion spiders in European fruit orchards.[10]
Mating behaviour has been studied in several Theridion species. It involves pseudocopulation, numerous sperm inductions and copulatory sequences. The male approaches the female to mate, usually in the latter's retreat (a hood-shaped part of the web where the female lives in). Male Theridion perform contralateral insertions, inserting their left palp into the female's right introductory duct and vice versa for the right palp. Female Theridion are less aggressive after mating compared to other genera of theridiids.[11]