Thomisus onustus is a crab spider belonging to the genus Thomisus. These spiders are found across Europe, North Africa, and parts of the Middle East and Asia. T. onustus reside in flowers in lowland vegetation. Females are distinguished by their larger size and ability to change color between white, yellow, and pink as a means of matching flower color. This cryptic mimicry allows them to both evade predators and enhance insect prey capture abilities. Males are smaller, more slender, and drab in coloration, usually green or brown. T. onustus is also distinguished from other relatives by its distinct life cycle patterns in which spiderlings emerge in either late summer or early spring. Furthermore, T. onustus have developed a mutualistic relationship with host plants where spiders feed on and/or deter harmful florivores while benefiting from the plant's supply of pollen and nectar, which T. onustus spiders are able to use as food sources, especially during periods of low insect prey abundance.
T. onustus is a medium-sized spider that exhibits sexual dimorphism, with females between lengths of 7–11 mm and smaller males ranging between lengths of 2–4 mm. Females are heavy-bodied and mostly stationary, whereas males are slender and more motile. Females have a pink, yellow, or white prosoma and males are brown to green-yellow in color. Both sexes have a triangular opisthosoma. This species can be distinguished from its close relative Thomisus zyuzini by its long ventral tibial apophysis and retrolateral tibial apophysis, the arrangement of the basal tibia tubercle on the male palp, and the circular intromittent orifice, which is oriented anteriad in the epigynum.
