Theridion grallator, also known as the happy face spider, is a spider in the family Theridiidae. Its Hawaiian name is nananana makakiʻi (face-patterned spider). The specific epithet grallator is Latin for "stilt walker", a reference to the species' long, spindly legs.
The spider is up to 5 millimetres (0.20 in) long. Certain morphs have a pattern uncannily 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.
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. The variation is possibly a kind of camouflage against birds, their only natural enemies of significance, to counteract pattern recognition by predators. As the pattern may change according to what food the spider has eaten and as T. grallator is very small, hides during the day, and is thus not a significant prey item for any species of predator, it is more likely that the bizarre variety of patterns serves no significant adaptive purpose at all.
T. grallator lives beneath the leaves of plants, where they also spin their much reduced webs. The females guard their eggs until they hatch, and catch prey for their young. T. grallator hunts mainly during the evening.
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.
- Rosemary G. Gillespie & Bruce E. Tabashink (1989). "What makes a happy face? Determination of colour pattern in the Hawaiian happy face spider Theridion grallator (Araneae, Theridiidae)". Heredity 62 (3): 355–363. doi:10.1038/hdy.1989.50.
- Rosemary G. Gillespie (1989). "Diet-induced color change in the Hawaiian happy-face spider Theridion grallator (Araneae, Theridiidae)" (PDF). Journal of Arachnology 17 (2): 171–177.
- Miquel A. Arnedo, Ingi Agnarsson & Rosemary G. Gillespie (2007). "Molecular insights into the phylogenetic structure of the spider genus Theridion (Araneae, Theridiidae) and the origin of the Hawaiian Theridion-like fauna" (PDF). Zoologica Scripta 36 (4): 337–352. doi:10.1111/j.1463-6409.2007.00280.x.