Theridion grallator

Female

The spider is up to 5 millimetres (0.20 in) long.[3] Certain morphs have a pattern resembling a smiley face or a grinning clown face on their yellow body. Each spider has a unique pattern, and the patterns differ from island to island. Some lack markings altogether.[3]

On the island of Maui, the happy types seem to follow simple Mendelian inheritance rules, while on other Hawaiian islands the body inheritance patterns seem to be sex-limited.[4] The variation is possibly a kind of camouflage against birds, their only natural enemies of significance, to counteract pattern recognition by predators. As the background color may change according to what food the spider has eaten[5] and as T. grallator is small, hides during the day, and is not a significant prey item for any species of predator, it is possible that the large variety of patterns serves no significant adaptive purpose.

T. grallator lives beneath the leaves of plants, where they spin a much reduced web.[5] Instead of using the web as a prey-detection medium, it has been demonstrated that vibrations of prey transmitted through the residence leaf are detected by the spiders and guides their orientation toward potential prey. Mature males actively move through forest vegetation seeking out females which are more sedentary. A courtship dance involving somatic movements and web-plucking on the part of the male is assessed by potential female mates. Copulation occurs at night, while both spiders hang from the underside of the leaf. Males die soon after mating, but females live longer, and guard their eggs until they hatch, and catch prey for their young. T. grallator hunts mainly from dusk, through the evening, to dawn. By day, the spiders assume a remarkable posture that flattens the body against the residence leaf, with legs splayed out and all joints held flat against the leaf.

The colonization routes of Theridion grallator on the Hawaiian archipelago. Dark purple lines indicate colonization occurring in conjunction with island age. Light purple indicates a reverse colonization. T. grallator is not present on Kauai or Niihau so colonization may have occurred from there, or the nearest continent.

T. grallator is endemic to the Hawaiian archipelago, and have been reported from Oʻahu, Molokaʻi, Maui and the island of Hawaiʻi in rainforests at elevations of 300–2,000 m (980–6,560 ft).

The closest relatives of T. grallator are other Hawaiian species, such as T. posticatum and T. kauaiense. This "T. grallator clade" may be more closely related to the genus Exalbidion than to any other species currently classified in the genus Theridion.[6]

• The Happy-Face Spiders
• Different morphs
• A Theridion grallator in its nest
• Image of T. grallator
• Photo of the Day (with text) - National Geographic - November 12, 2007