Thomisus onustus

While overall size is smaller, in terms of prey to predator length ratio, juvenile spiders capture larger prey than late instar and adult females.[4] 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.[10]

T. onustus are summer-stenochronus spiders (summer reproductive season) and sit-and-wait predators. They stay in flower corollas and wait for insect prey including bees (Apoidea), butterflies (Lepidoptera), hoverflies (Syrphidae),[10] diptera, hymenoptera, and other spiders. T. onustus are cursorial spiders and do not use silk for prey capture. Instead, they use their long raptorial forelegs to ambush nearby insects. They frequently prey on insects far larger than themselves, ranging from 1.25 to 16.00mm in length.[4]  Males feed less and tend to prey on smaller insects.[8]

While some Australian crab spider species are able to use the reflection of UV light to generate a deceptive signal that attracts prey to host flower species, European species, including T. onustus, lack this ability. Honeybees attracted to the UV reflectance of Australian species, for example, are repelled by the presence of T. onustus.[9]

When T. onustus acts as an ambush predator, it influences the ways in which pollinators, such as honeybees and hoverflies, manage the trade-off between predation rate and resource intake. Honey bees, for example, will avoid resource (nectar) poor habitats as well as those with higher concentrations of crab spiders, preferring to frequent safer areas. However, honeybees are more susceptible to predation by crab spiders and competition is more intense in these areas. Hoverflies, on the other hand, prefer less competitive but riskier resource areas.[11] While bumblebees, Bombus terrestris, avoid T. onustus, they do not learn from previous encounters with spider predators in order to enhance avoidance of heterospecific individuals.[12]

During periods of insect food shortages (ie. during inclement weather conditions), T. onustus is capable of using pollen and nectar as food sources for extended periods of time as a starvation survival strategy. Spiders will actively visit flowers of multiple species, such as those from Asteraceae and Asteroideae, for feeding. As Asteraceae species present pollen in an allotropic manner (pollen grains are exposed on the capitulum surface), spiders are able to acquire them easily. Since pollen grains are unable to pass through the cuticular platelets of spider pharynxes due to their relatively large size (> 1 μm), pollen is consumed via extra-intestinal digestion, with nutrients likely extracted through apertures in pollen grains.[10]

After thorough investigation, the average amount of days that this spider can survive without food was 21.4 days. There were also dietary factors, such as different types of pollen and nectar, that could potentially increase the survival rate of these spiderlings by 1.5-2 times. By extensively evaluating the sitting positions of the spider groups that were pollen-fed, it was discovered that these spiders are known to actively seek out visitations to flowers for pollen. Hence, this tendency is one of the hypotheses that may explain why they are able to survive longer without food in the Spring.[10]

T. onustus are able to subsist off pollen for over 40 days under laboratory conditions, further indicating the importance of pollen feeding in sustaining juvenile spiders that may lack sufficient fat reserves, especially during the spring season, as well as those with limited access to insect prey.[13]

The number of eggs laid varies widely for T. onustus, ranging from less than ten eggs to over 400 per cocoon. Cocoons laid in early spring consist of far more spiderlings than those laid in the late summer. Unlike the more variable developmental stages of T. onustus, the period of the cocoon, or the time between the laying of eggs and emergence of spiderlings, is generally around one month regardless of season. This can be attributed to the insulation provided by the cocoon, making eggs less susceptible to seasonal and/or temperature changes.[8]

T. onustus typically molt at regular intervals up to the third and sixth instars provided they obtain adequate nutrition.[10] After eclosion from the cocoon, spiderling sex differences are not yet visible. By the second molt, the swelling of pedipalp tips distinguishes males. Males typically reach maturity after between three and five molts. Unlike males, females molt far more, reaching maturity after six to nine molts. As such, males typically mature after two and a half months and females after over a year. Due to shorter male life spans, sibling mating is, therefore, impossible.[8]

The maximum life expectancy of these spiders is 600 days for females, and female spiders have a greater life expectancy than males (several months versus several weeks).[8]

Yellow coloration is likely due to the presence of ommochrome compounds and/or their precursors, such as xanthommatin and 3-hydroxykynurenine, deposited on hypodermal layers, which lie above specialized guanocyte cells full of guanine crystals, which lead to light scattering. Incident light is reflected from guanocytes back through pigment-containing hypodermal layers. White coloration is likely due to high concentrations of the transparent ommochrome precursor kynurenine and the reflection from guanine crystals. These explanations account for human-perceived white to yellow changes via differential pigment deposition in the hypodermis, but do not explain variations in UV reflectance.

Thomisus Onustus in Behbahan, Iran

Color change abilities are due to contributions from both epithelial and cuticular layers, with epithelial cells modulating ‘human-visible’ changes. The cuticle of T. onustus is not equivalently transparent across the color spectrum, indicating a role in color variation. The cuticle limits the maximum reflectance that can be produced from the UV range and, as such, offers a barrier against potential UV photo-damage. Guanine crystals, present in the hypodermis, strongly reflect UV light, and, as the only UV-reflective element in crab spider color schemes, are the key determinant of UV-coloration. Guanine crystals are exposed through the partially UV-transmitting hypodermis and cuticle. While kynurenine is transparent to humans, it likely functions as a UV filtering pigment. UV reflectance may have evolved through a change in the metabolic pathway that allowed for guanine crystal exposure through partially UV-transmitting hypodermis and cuticle. As a whole, interactions between the cuticle, pigments, and/or crystals in the hypodermis that exist in variable oxidative stages, and guanocytes combine to produce changes in the observed reflectance spectrum of crab spiders.[19]

These color change mechanisms likely evolved from ancestral crab spiders with UV-reflective abdomens or, if pre-dated by UV-absorbent hypodermal pigments, evolved through the guanine crystal exposure through a clear hypodermis.[19]

Under direct sunlight exposure, color change in T. onustus is determined by the external factor of background color. Such background matching is common in many animals able to undergo reversible color changes (some fish, reptiles, amphibians, crustaceans and cephalopods). However, background matching in T. onustus is less present under non-natural light conditions, suggesting that factors other than background color may play a role in the process. Additionally, color changes from white to yellow occur between 1.43 and 2.14 times faster than changes from yellow to white. Furthermore, molting results in a slower rate of change from yellow to white, indicating a potential link between color change and development. These changes are likely mediated by the hormone 20-hydroxyecdysone. The endocrine system is thought to mediate the transduction of environmental cues into the physiological response of color change.[15]