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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.emales 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.
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T. onustus are members of the genus Thomisus, which includes around 150 described species, and is well supported as being monophyletic. It is relatively morphologically homogeneous genus, with synapomorphies that include circular scopula hairs (when viewed as a cross section), bulbuses that are subequal in length and width, disk shaped tegulums, sperm ducts that follow a circular peripheral course through the tegulum, and a lack of conductors and median apophyses. However, some subgroupings within Thomisus are not well supported. The family Thomisidae encompasses over 2000 species of crab spiders including the common close relative of T. onustus, Misumena vatia, Thomisus spectabilis. onustus typically reside on shrubs and within lowland vegetation, preferring warmer areas. They inhabit a wide variety of flowers and herbs, usually staying at the flowering peaks. T. onustus is unique among crab spider species in that it prefers to situate itself in flower centers, which have unique spectral properties, over petals. T. onustus are distributed across Europe, North Africa, Turkey, Caucasus, Russia (from Europe to South Siberia), Israel, Central Asia, Iran, China, Korea, and Japan, preferring warm areas.
While overall size is smaller, in terms of prey to predator length ratio, juvenile spiders capture larger prey than late instar and adult females. Pollen feeding is particularly important for spiderlings, as it allows them to survive beyond what yolk reserves would otherwise allow. Due to the lack of amino acids, especially tyrosine, in pollen grains, spiderlings that feed exclusively on pollen are unable to molt versus spiderlings that feed on insect prey.
