White crab spider

Appearance

"T. spectabilis" undergoes a unique color transformation from yellow to white. The color change helps them not only hide from predators, but stalk prey in similarly colored daisies as well. This transformation allows them to switch between appearing conspicuous and cryptic to their prey. The colorful bodies allow spiders to reflect UV-light in a manner that attracts bees to their flower. When the spider changes to a specific color, the color is displayed uniformly across its head, legs, and abdomen. The legs of this spider span up to 30 mm. They have small black eyes that are organized into a white band across its head resembling a mask. The abdomen of the spider has a pentagon shape and two small bumps across it. They have stout legs arranged similarly to a crab. The spider has pinching fangs, but no claw tufts, so it's not able to climb across smooth surfaces.

Naming

Australian crab spiders are very similar to other crab spiders across the world, such as the European, Alaskan, or Canadian crab spider. A similarity in these spiders is that rather than building webs, they hide from their prey and ambush them with their forelimbs.

Crab spiders are in the family Thomisidae. This family has four lineages/clades: "Borboropactus" clade, "Epidius" clade, "Stephanopis" clade and the "Thomisus" clade. The Australian crab spider belongs to the "Thomisus" clade. There is very minimal genetic divergence in this clade. The ability of "Thomisus spectabilis" to change color is shared by "Misumena", "Diaea", and "Runcinia" spiders which are also members of the Thomisidae family and have high genetic relatedness with the Australian Crab spider. "Misumena vatia" is a close relative of "T. spectabilis" but instead has a holarctic distribution. Thomisids fall within the larger Dionycha clade, and this clade is defined by the loss of an unpaired tarsal claw for the animal. Genetic sequencing of Thomisids found that the 16S gene was 430 nucleotide base pairs long, H3 gene was 328 base pairs, and COI gene was 557 base pairs. All of these basepair numbers are unique to the Thomisids within the Dionycha clade.

There are three distinct features that define a spider from the Thomisidae family: leg 3 and leg 4 are shorter and weaker than leg 1 and leg 2, lateral eyes that are larger than median eyes, and presence of a group of setae. The "Thomisus" clade of the Australian crab spider has the following morphological similarities: circular scopula hairs, subequal bulbus, disc shaped tegulum, a sperm duct with a spherical, peripheral course, no conductor, and no median apophysis.

Distribution

They are spread throughout Australia, but are primarily located in Eastern Australia. It is mainly a suburban spider. In Brisbane, they are normally found in backyards, bushes, and gardens.

Behavior

"Thomisus spectabilis" with larger body sizes are better able to capture prey. Honeybees are more likely to land on flowers that have larger spiders than smaller spiders. Thus, larger Australian crab spiders do not have to use their UV-reflective property as much as smaller spiders to attract bees. For smaller spiders, using UV-reflectance results in less hunting success than larger spiders using UV-reflectance; spider size is a larger factor than UV-reflectance for predicting bee attraction and hunting success. Further, levels of UV-reflectance increase for larger spiders suggesting coevolution of size and UV-reflectance traits.

Movement of "Thomisus spectabilis" has large impacts on the behavior of honeybees. If they move before honeybees approach, the bees are much more likely to stay away from the flower. This effect is more pronounced when the spiders are waiting below the inflorescence of the flower rather than above it. Below the inflorescence, the spiders remaining still makes them 70% more likely to attract a spider to land, but above the inflorescence they are 50% more likely to attract a spider to land by remaining still.

Movement of crab spiders alerts bees to their presence. This forces the bees to make a decision weighing the risk of the spiders and the reward of the nectar. These decisions are also influenced by the susceptibility of the bee. Highly susceptible bees mostly visit safe flowers, even if the resources are poor in those areas. Other bees often must choose riskier patches of flowers, because those flowers often contain the highest amount of nectar. Since the flowers with hiding spiders often become damaged due to the ambush of the bees, they actually have evolved to increase nectar production so that they can continue to attract bees despite the threat of predation. Honeybees are adept at finding the best flower patches to maximize nectar reward and minimize predation risk due to their impressive communication system. Bees are able to recruit bee mates to flower locations that they have scouted as high nectar locations, and they are also able to tell each other when there is a patch with hiding spiders.

Habitat

The Australian crab spiders choose habitats that increase their chances of catching prey. Since this involves creating contrast between itself and the flower, it will pick flower colors and flower positions that maximize the contrast. Due to the importance of contrast with flowers, these spiders cannot simply choose habitats with large numbers of its prey. Instead, they must think about both flower type and prey number while choosing habitats in a way that maximizes capture. "T. spectabilis" is mostly attracted to staying in flowers that are newer, and is drawn to them through olfactory cues. It is most commonly found in tropical or subtropical areas, but some also prefer white clothing lines.

Food

These spiders are a predatory species, and they feed mainly on insects. They mainly eat live or recently killed insects. Some examples of these insects are crickets, drosophila flies, and pollinators such as honey bees and butterflies. The spiders are an important form of pest control as they protect the flowers they inhabit from attack by insects. The energy obtained from consuming these insects allows the adult female Australian crab spider to produce a clutch of eggs.

Evolution

The Australian crab spider eats a variety of bees, but the two main varieties are honeybees and Australian native bees. While the spider's array of prey catching techniques are effective on both types of bees, the honeybee is specifically susceptible due to a lack of coevolution with the Australian crab spider. When the spider creates color contrasts with different flowers in order to trick the bees, the honeybee immediately is drawn to land on the flower in which the spider is waiting. On the other hand, the Australian native bee has coevolved with the Australian crab spider, and thus can discriminate between flowers that have spiders and those that do not. In order for this discrimination to work, the bees must approach the flower closely. They land on the flower that is unoccupied by the bee. This results in many more honeybees being captured by the crab spider than Australian native bees. It is also possible that the Australian native bee has evolved to detect the odor of the crab spiders to help them avoid the spider laden flowers.

The crab spider has coevolved as well to better match the bee varieties they are surrounded by. Different pollinating species are attracted to different levels of UV contrasts, so "Thomisus spectabilis" has evolved to be able to adjust the UV contrast that they create with the flowers through reflecting varied amounts of UV light. Since honeybees do not have the ability to discriminate between flowers with and without the crab spiders, "Thomisus spectabilis" will more often create the UV contrast that attracts honeybees than create the UV contrast that attracts the Australian native bee. The UV contrast is also influenced by the predators of "Thomisus spectabilis" that are in the area. If there are a high number of predators nearby, the crab spider will use low UV reflectance to attract the least amount of attention, and vice versa for times during which there are very few predators.

The non-cryptic nature of Australian crab spiders influences the type of bee that they can capture. The deceit that "Thomisus spectabilis" use to attract prey actually makes them more conspicuous to the prey. Bees have varying reactions to conspicuousness. For example, in "Austroplebeia australis" bees there is an aversion to conspicuousness, so the non-cryptic nature of "Thomisus spectabilis" makes them less likely to capture those types of bees. In contrast, "Trigona carbonaria" bees do not change their behavior based on the conspicuousness of spiders, so the Australian crab spider has a better chance of capturing them.