T. grallator gets its vernacular name of "Hawaiian happy-face spider" from the unique patterns superimposed on its abdomen, specifically those that resemble a smiley face.

Spiders with depigmentation or polymorphic colors and patterns can avoid predation by birds that use a search image when scanning for prey.

[8] The alleles that are associated with black, red, or white pigments are arranged in a hierarchical structure and exhibit dominant effects.

[8] The White morph is produced by a massive deposit of guanine below the hypodermis, a structure derived from the ectoderm.

These two mechanisms respond to the presence or absence of guanine and send chemical signals between the hypodermis and digestive diverticula to adjust morph pigmentation.

One example of this is the red and black opisthosoma pigmentation with guanine deposits, showing the benefit of the visual effects of these color polymorphisms.

[11] At least nine species in the Hawaiian islands have been identified to be members of the T. grallator-clade based on the analysis of genitalia patterns.

There has been much debate on how to organize clades and construct an appropriate phylogenetic structure of Theridiidae, and work is still being done to properly classify these species.

In addition, on Maui, all polymorphisms are attributed to individual alleles while on Hawai’i, there are two pairs of color morphs that may depend on one single locus that is differentially expressed in males and females.

[9] Because of the presence of some sex-selective morphs in Hawai’i - a phenomenon not observed in Maui - it is likely that a shift in inheritance pattern occurred due to evolutionary pressures.

[15] Despite the difference in genetic backgrounds and the rare exchange of individuals, hybrid matings between islands can still produce viable offspring.

[15] The evolutionary significance of the color polymorphisms of T. grallator is elusive, but there are selection pressures acting on the various morph proportions.

Because T. grallator resides on the underside of green leaves, the Yellow morph provides them a degree of conspicuousness under the sunlight.

[2] They prefer to reside on the underside of plant leaves such as the native Broussaisia arguta and Clermontia arborescens and the introduced Hedychium coronarium.

H. coronarium is a particularly tactical plant to reside on as its large, slippery leaves allow T. grallator to better evade predation.

Sparsely distributed populations 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).

A common color change is from the translucent yellow to orange, most likely due to the high level (approximately 70%) of dietary consumption of dipterans.

Eleutherodactylus coqui is an invasive species of frog originally from Puerto Rico that preys on T. grallator.

[20] T. grallator lives beneath the leaves of plants, where they spin a relatively small two-dimensional web.

The high level of rainfall damages the glue of the web's silk threads, leading to ineffective prey capture.

[18] Often, the building of small webs is associated with a specialization in prey type, but this is not observed to be the case in T. grallator.

During the day, T. grallator spiders tightly cling to the undersides of leaves to evade predation by gleaning birds.

[21] At night, when diurnal predatory birds are asleep, these spiders will hang by silk threads under the leaf.

[18] In addition, maternal T. grallator spiders may use webs to guard their egg sacs or store the prey they have caught for their young.

During winter months, specifically October to March, there is a higher proportion of smaller sized and immature spiders.

Males die soon after mating, but females live longer, and guard their eggs until they hatch, catching prey for their young.

Thus, female spiders' preference for males with these rarer patterned morphs is not attributed to physical attractiveness but instead to this advantage from predators.

[8] A maternal female T. grallator is notably aggressive against intruders right after the hatching of her young, while she is guarding her egg sac.

Spiderlings who lose their mother either leave their resident leaf by dropping down a silk thread or climbing down the stem or stalk of the plant.

[21] Adult females are usually sedentary and located on the underside of leaves while males are often more mobile as they may move about in the search of mates.