Thomisus onustus

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Thomisus onustus
Crab spider (Thomisus onustus) with prey nettle tree butterfly (Libythea celtis).jpg
with prey nettle tree butterfly (Libythea celtis) on spearmint (Mentha spicata), Pirin National Park, Bulgaria
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Infraorder: Araneomorphae
Family: Thomisidae
Genus: Thomisus
Species:
T. onustus
Binomial name
Thomisus onustus
Walckenaer, 1805 [1]
Subspecies [2]

T. o. meridionalisStrand, 1907

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.

Contents

Description

Purple camouflage Thomisidae - Thomisus onustus.JPG
Purple camouflage

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. [3] Females are heavy-bodied and mostly stationary, whereas males are slender and more motile. [4] [3] Females have a pink, yellow, or white prosoma and males are brown to green-yellow in color. Both sexes have a triangular opisthosoma. [3] 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. [5]

Phylogeny

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. [6] The family Thomisidae encompasses over 2000 species of crab spiders including the common close relative of T. onustus, Misumena vatia, Thomisus spectabilis. [7]

Habitat and distribution

T. onustus typically reside on shrubs and within lowland vegetation, preferring warmer areas. [8] They inhabit a wide variety of flowers and herbs, usually staying at the flowering peaks. [5] 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. [9] 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. [3]

Diet

Spiderling

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]

Adult

Predatory feeding

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]

Non-predatory feeding

During periods of insect food shortages (i.e. 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]

Reproduction and life cycle

In the summer of their second year, toward the end of their lives, female spiders weave between two and four cocoons for egg-laying. Spiderlings from the first egg sac emerge during late summer. This gives them access to more abundant prey resources, allowing them to obtain sufficient energy reserves to hibernate in vegetation outside of the cocoon during winter months. Spiderlings from egg sacs woven later, on the other hand, remain in the egg sacs through winter and emerge in early spring of the following year when prey is far scarcer, necessitating the use of pollen feeding to supplement nutritional and energy needs.

With two generations per year and the spring generation larger than the summer one, T. onustus females of both generations generally develop throughout the year whereas spring generation males grow faster, reaching maturity with second generation females from the prior year. Summer males, on the other hand, develop for a longer period, molting several times, and reach maturity in the summer of the next year with first-generation females. Developmental rates for both sexes are highly variable, with spiders at different developmental stages found throughout the year. [10] Overall, the rate of development, duration of instars, number of molts, and molting times are all highly variable within the species. [8]

T. onustus usually attach egg sacs to leaves. Unlike some other cursorial species, females do not enclose themselves within the sac, but continue to catch prey during egg-guarding. [4]

Brood size

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]

Molting

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]

Life span

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]

Enemies

T. onustus are primarily preyed upon by insectivorous birds. While their crypsis is imperfect, meaning that they do not perfectly match flower color, making them slightly detectable, T. onustus generally suffer little from bird predation. This is because it does not pay birds to specialize on crab spiders due to their uneven distributions and crypsis. T. onustus tend to prefer flowers with colors they can match (usually white or yellow), even when they could attain greater hunting success on other flowers. This is due to the increased predation risk of residing on flowers that would make them more conspicuous. [14] While there are relatively few observations of specific predator species of T. onustus, mud-daubers and spider wasps do prey on the spider species. [15]

Protective coloration and behavior

Thomisus onustus on Orchis purpurea Thomisus onustus on Orchis purpurea.jpg
Thomisus onustus on Orchis purpurea

T. onustus females are able to change their entire body color as a means of mimicking the color of flowers where they reside and capture prey. Possible colors include pink, shiny yellow, and white, sometimes with a bright medial stripe. Female color changes usually take several days in order to adjust to flower backgrounds. Males are usually yellow-green to brown in color and do not exhibit color changes. [16] Female aggressive mimicry provides camouflage from predators and works to fool insect prey, usually pollinators of flowers on which spiders reside. Spiders are capable of mimicking chromatic contrast of different flower species, allowing them to be cryptic in the color-vision systems of both avian predators and hymenopteran prey. More specifically, they are able to mimic flower color in four cone types corresponding to UV-blue-green-red for birds and three cone types, UV-blue-green, for insects. When aiming to detect smaller targets and/or see over larger distances, birds and bees preferentially use achromatic vision (brightness) over color contrast. As such, T. onustus mimicry also applies to achromatic vision as they are able to modulate both their achromatic and chromatic contrast [17]

Mutualism with plants

Female Thomisus onustus sharing its flower with velvet mites (cf. Eutrombidium rostratus). Spider and mites May 2008-1.jpg
Female Thomisus onustus sharing its flower with velvet mites (cf. Eutrombidium rostratus).

T. onustus can deter certain pollinators, such as bees, and have had key impacts on floral trait evolution. Following florivore attack, plants are adapted to release floral volatile emissions that attract T. onustus, which consume and/or deter florivores. The compound β-ocimene, produced by plants in both floral and leaf tissues, acts as an attractive signal for both T. onustus and pollinators. This leads to overlapping floral preferences for both the spiders and their prey, providing a strong selective mechanism for sit-and-wait tactics of prey capture. Furthermore, β-ocimene is produced by 71% of plant families, explaining the broad range of flowers in which T. onustus might stay in. While T. onustus can harm plant fitness in the absence of florivores, they provide an overall benefit to plants threatened by florivores. As such, plants attract T. onustus only when florivores are present by inducing β-ocimene emission, making infested flowers more attractive to spiders. This mechanism generates strong selection pressure on plants to develop a mutualistic relationship with T. onustus and suggests a key role for the spiders in the evolution of floral traits. [18]

As Thomisus onustus act as ambush predators on flowers, they likely influence both the reproductive success of flower species but may also interfere with pollen flow within the immediate community due to their deterrence or consumption of pollinators, such as hoverflies and honeybees. However, reproductive success of plants will also depend on the phenotype, not only of the plant itself, but also that of surrounding plant species. [11] Although T. onustus resides on a broad range of flower species, several host species include Erigeron annuus , Bellis perennis , [13] Glebionis segetum , Malva sylvestris , and Chrysanthemum segetum. [12] [11]

Physiology

Coloration

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 Thomisus Onustus in Behbahan, Iran.jpg
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]

See also

Related Research Articles

Thomisidae Family of spiders

The Thomisidae are a family of spiders, including about 175 genera and over 2,100 species. The common name crab spider is often linked to species in this family, but is also applied loosely to many other families of spiders. Many members of this family are also known as flower spiders or flower crab spiders.

<i>Parasteatoda tepidariorum</i> Species of spider

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<i>Nephila</i> Genus of spiders

Nephila is a genus of araneomorph spiders noted for the impressive webs they weave. Nephila consists of numerous species found in warmer regions around the world. They are commonly called golden silk orb-weavers, golden orb-weavers, giant wood spiders, or banana spiders.

<i>Theridion grallator</i> Species of spider in the family Theridiidae

Theridion grallator, also known as the Hawaiian happy-face spider, is a spider in the family Theridiidae that resides on the Hawaiian Islands. T. grallator obtains its vernacular name of "Hawaiian happy-face spider" from the unique patterns superimposed on its abdomen, specifically those that resemble a human smiling face. T. grallator is particularly notable because of its wide range of polymorphisms that may be studied to allow a better understanding of evolutionary mechanisms. In addition to the variety of color polymorphisms present, T. grallator demonstrates the interesting quality of diet-induced color change, in which its appearance temporarily changes as it metabolizes various food items.

<i>Misumena vatia</i> Species of spider

Misumena vatia is a species of crab spider with a holarctic distribution. In North America, it is called the goldenrod crab spider or flower (crab) spider, as it is commonly found hunting in goldenrod sprays and milkweed plants. They are called crab spiders because of their unique ability to walk sideways as well as forwards and backwards. Both males and females of this species progress through several molts before reaching their adult sizes, though females must molt more to reach their larger size. Females can grow up to 10 mm (0.39 in) while males are quite small, reaching 5 mm (0.20 in) at most. Misumena vatia are usually yellow or white or a pattern of these two colours. They may also present with pale green or pink instead of yellow, again, in a pattern with white. They have the ability to change between these colors based on their surroundings with these color changes occur through the molting process. They have a complex visual system, with eight eyes, that they rely on for prey capture and for their color-changing abilities. Sometimes, if Misumena vatia consume colored prey, the spider itself will take on that color.

<i>Micrommata virescens</i> Species of spider

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<i>Nephila pilipes</i> Species of spider

Nephila pilipes is a species of golden orb-web spider. It resides all over countries in East and Southeast Asia as well as Oceania. It is commonly found in primary and secondary forests and gardens. Females are large and grow to a body size of 30–50 mm, with males growing to 5–6 mm. It is the second largest of the orb-weaving spiders apart from the recently discovered Nephila komaci. The first, second, and fourth pairs of legs of juvenile females have dense hairy brushes, but these brushes disappear as the spider matures.

<i>Argiope argentata</i> Species of spider

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<i>Amaurobius ferox</i> Species of spider

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Chemical mimicry

Chemical mimicry is a type of biological mimicry, involving the use of chemicals to dupe an operator. A chemical mimic dupes an operator by showing an adaptive chemical resemblance to an object of its environment and as a consequence receives selective advantage. In all cases of chemical mimicry it has been found that the mimicking species is the only species to benefit from the reaction with either costs or no effect on the duped species. This is by adapting to produce chemicals that will cause a desirable behavioural reaction in the species being deceived and a selective advantage to the mimic. Chemical mimicry exists within many of the different forms of mimicry such as aggressive, protective, Batesian, and Müllerian mimicry and can involve a number of different senses. Mimicking semiochemicals, which cannot be seen, make up some of the most widely used forms of chemical mimicry and is therefore less apparent than more visual forms. As a result of this, this topic has been relatively neglected in research and literature. Two examples of organisms displaying chemical mimicry include the mimicking of Noctuid pheromones by bolas spiders in order to draw prey to the spider’s location and the duping of insects within their own nests by mimicking their odours in order to enter and hide within the nest undetected. It is important to note that in all forms of mimicry the mimicking organism is not conscious of the deceit used and does not act intentionally to trick other organisms.

<i>Misumenoides formosipes</i> Species of spider

Misumenoides formosipes is a species of crab spiders (Thomisidae), belonging to the genus Misumenoides. The species' unofficial common name is white banded crab spider, which refers to a white line that runs through the plane of their eyes. This species is a sit-and-wait predator that captures pollinators as they visit the inflorescences on which the spider sits. The spider has strong front legs which are used to seize prey. The female spider is much larger than the male. The pattern of markings on females is variable and the overall color of the body can change between white and yellow dependent on the color of their surroundings. The color pattern for males, which does not change in their lifetime, differs from females in that the four front legs of males are darker and the abdomen is gold. The spider can be found throughout the United States. Males search for sedentary females within a heterogeneous habitat and guard them until they are sexually mature to reproduce.

<i>Phidippus clarus</i> Species of spider

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<i>Pisaurina mira</i> Species of spider

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<i>Holocnemus pluchei</i> Species of spider

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<i>Tigrosa helluo</i> Species of spider

Tigrosa helluo is a species of spider belonging to the family Lycosidae, also known as wolf spiders. T. helluo was formerly known as Hogna helluo before differences between dorsal color patterns, habitat preferences, body structures, etc. were discovered. The species is native to the United States, Canada and Mexico. They can be found across the eastern half of the United States, primarily in the northeast and New England, and as far west as Nebraska and Kansas. T. helluo can be found in diverse habitats including woods, marshes, fields, and riparian areas. Typically, members of this species prefer to live in wetter areas as opposed to dry environments. Males tend to live for around a year and females will live for close to two years.

<i>Cyclosa argenteoalba</i> Species of spider

Cyclosa argenteoalba, in the trashline orbweavers genus, is a species of orb weaver in the spider family Araneidae. It is found in East Asia in the countries of China, Japan, and Korea. C. argenteoalba are diurnal, which means they are active during the day. Each individual has a unique appearance due to their differences in the ratio of black to silver coloring on their abdomen. Spiders with less silver coloring are better at catching prey, since the silver is bright and warns their prey. They catch their prey by waiting in the hub of their web until their prey is close enough to catch. Parasitic larvae are often found attached to C. argenteoalba, and the larvae are able to manipulate the spider's behavior. Females are on average 2 mm longer in size than males. During mating, female genital mutilation is common in order to increase the fitness of the male. On their webs, they often attach silk “decorations” that are thought to deter predators. Relocating to a different place to build a new web occurs frequently until they find a location with a significant amount of prey.

<i>Philodromus cespitum</i> Species of spider

Philodromus cespitum is a species of running crab spider in the family Philodromidae. It is found in North America, Europe, North Africa, and parts of the Middle East and Asia. P. cespitum is a foliage-dweller, and is the most abundant species found in European fruit orchards. It acts as a biological control by preying on orchard pests. P. cespitum is a diurnal ambush hunter and preys on aphids, insects, and occasionally competitor spider species. Males court females by tapping on the females’ bodies with their forelegs. They then insert a genital plug into the female during copulation. Unlike in many other spider species, subsequent males can mate with plugged females by removing part of the plug prior to copulation. Males discriminate among females based on virginity and plug size, and can determine these factors using the females’ draglines and plug samples.

<i>Mecaphesa celer</i> Species of spider

Mecaphesa celer, known generally as the swift crab spider, is a species of crab spider in the family Thomisidae. Its range is quite large, and it is found throughout much of North and Central America.

<i>Thomisus spectabilis</i> Species of spider

Thomisus spectabilis, also known as the white crab spider or Australian crab spider, is a small spider found in Australia and far east Asia.

<i>Australomisidia cruentata</i> Species of spider

Australomisidia cruentata, one of the crab spiders, is a small spider found in Australia. The body length of the female is up to 5 mm, the male 3 mm. An ambush predator, often seen on flowers in the Pultenaea group of egg and bacon plants, belonging to the pea family. The egg sac is also laid on the flowers. Petals being fastened with silk in a chamber. The spider stays with the eggs, probably still hunting from the entrance of the retreat, with the egg sac nearby. Prey is small flying insects. The genus Australomisidia was created in 2014, the word being a combination of Australia and Thomisidae, the crab spiders.

References

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