|Female with prey silver-spotted skipper (Hesperia comma)|
|Male is much smaller than female|
Misumena vatia is a species of crab spider with holarctic distribution. In North America, it is called the goldenrod crab spider or flower (crab) spider. It is commonly found hunting in goldenrod sprays in the autumn. Males are quite small, but females can grow up to 10 mm (0.4 in) (excluding legs); males reach 5 mm (0.2 in) at most. These spiders are yellow or white, and have the ability to change between these two colors based on their surroundings.
M. vatia feed on common insects and face threats due mostly to parasites. The primary sex ratio is biased toward females. Females of this species are stationary and choose a flower to settle on while males cover great distances searching for mates. After mating, females will guard their nests until they die, soon after their young have hatched.
These spiders may be yellow or white. This ultimately depends on the flower in which they are hunting (active camouflage). Younger females especially, which may hunt on a variety of flowers such as daisies and sunflowers, have a strong tendency to adapt to the color of the surrounding flower. However, the color-changing process is not instant and can require up to 25 days to complete. Older females require large amounts of relatively large prey to produce the best possible clutch of eggs. They are therefore, in North America, most commonly found in goldenrods, bright yellow flowers which attract large numbers of insects, particularly in autumn. It is often very hard even for a searching human to spot this spider on a yellow flower. These spiders are sometimes called 'banana spiders' because of their striking yellow color. They are known to prey on insects such as bumblebees (bombus ternarius) and, in Central America, Trigona fulviventris (a stingless bee).
Females have light complexions, either white or yellow with darker sides. They may have some markings on the abdomen that can be brown or red. These markings are genetically determined and not affected by a background color change. Males are darker than females, with red or brown outer shells. They have a characteristic white spot in the middle that continues through the area around the eyes. Males specifically have two sets of red and white bands both dorsally and laterally.
Other physical characteristics
This species has a wide, flat body that is short and crab-like. It can walk sideways in addition to being able to move forward and backward. Of its eight legs, the first two pairs are the largest. These sets of legs are usually held open, as the spider uses them to capture its prey. The female's legs are white or yellow, while the male's first and second legs are brown or red and the third and fourth are yellow. Females are larger than males and tend to fall between 6-9mm in length while males are only 3-4mm long.
Mesumena vatia have two rows of eyes. The anterior row are equally spaced and curved backward while the second row vary in appearance from animal to animal, and can be curved more or less than the first. The area around the eyes is narrower in the front than the back. The spider's hair is erect and can be "either filiform or rod shaped." The legs do not have spines except under the tibia and metatarsal bones of the first two sets of legs. The appearance of the clypeus and the structure of the cephalothorax can be used to distinguish the genus Misumena within its subfamily.
Population structure, speciation, and phylogeny
Close relatives include Mecaphesa asperata (formerly known as Misumeops asperatus), which is similar in size and shape but with different coloration. It is light-gray to brown and has pink stripes on the abdomen and cephalothorax. It is also coated with hairs that are short and stiff.
Habitat and distribution
The species prefers a temperate climate and generally inhabits forest biomes. M. vatia are terrestrial spiders and can be found on many plants and flowers such as milkweed, trillium, white fleabane, white flower, and goldenrod.
Females of this species do not travel more than a few meters from where they typically feed. They are attracted by the fragrance of flowers. Other visual and tactile clues also help them choose a territory. Their survival depends on their ability to choose a small area home to flowering plants which will attract prey. The males are highly motile and may disperse great distances as they search for mates. Additionally, spiderlings may travel great distances by ballooning if they find the area around their nest to be lacking in resources, however this is risky as there is no guarantee that the search for a new territory will be successful.
Immature M. vatia commonly feed on smaller-sized prey such as thrips, aphids in the family Aphididae, and dance flies the in family Empididae. They may also use nectar and pollen as food sources when prey is scarce.
Bumblebees (Bombus appositus) provide the spider with the most biomass, but small syrphid flies (Toxomerus marginatus) are the prey captured most frequently. Other frequently captured prey include honeybees (Apis mellifera) and moths.
M. vatia are primarily dependent on vision to hunt, so they find and capture food during the day. Adult females will hunt, "in the upper stratum of the field vegetation, primarily on the flowers of ox-eye daisy (Chrysanthemum leucanthemum), red clover (Trifolium pratense) and buttercups (Ranunculus acris)." Adult males, in pursuit of potential mates, will search these areas for females. The spider can hunt bugs and insects larger than itself because it has the ability to use venom to immobilize its prey. M. vatia will wait, camouflaging itself on a flowering plant or on the ground, for prey to pass by, and then grab the prey with its forelegs. They immobilize prey by injecting venom with their fangs. Unlike many spiders which wrap their prey in silk, M. vatia use their fangs to suck the liquid out of prey until satiated. This is a form of external digestion, in which spiders inject digestive enzymes into prey. As a result, prey size is not a limiting factor for consumption.
Though M. vatia are most often found hunting during the daytime, there is evidence that the spider will be driven to hunt nocturnally due to an increase in nocturnal prey activity. This behavior occurs most commonly in response to increased night-time activity by moths in early September.
Cannibalism is uncommon among M. vatia. However, it has been observed in individuals in roughly 1% of broods. In these broods, cannibalism tended to occur among spiderlings. Cannibalistic individuals can be up to three times larger than those who are non-cannibalistic.
M. vatia have the ability to retain their excretions for at least 50 days and will not excrete when confined to small spaces or near their hunting sites. Excretion may alert predators to the spider's whereabouts.
Reproduction and life cycle
Sex ratios among M. vatia vary from a primary sex ratio of 1.5 females to 1 male to a ratio of 2.5-5.1 females to 1 male by the time they reach adulthood. Since males must spend considerable time searching for females, they face danger from the environment, reducing their numbers. Males cannot mate multiple times in quick succession but require a two day interval between matings.
Female M. vatia prefer common milkweed (Asclepias syriaca) over spreading dogbane (Apocynum androsaemifolium), pasture rose (Rosa carolina), and chokecherry (Prunus virginiana) for nest construction. Females who lay eggs on milkweed have higher nesting success, which correlates with early survival of clutches.
In nature, M. vatia produces a single brood. However, females are capable of producing another brood if artificially induced.
A minority of males will guard pre-reproductive females. About 10% of males will guard penultimate females as they molt into the adult stage. Almost all males who guard pre-reproductive females will mate with them after they have molted. Low levels of guarding are related to the female leaning sex ratio expressed by M. vatia. Males of this species tend to guard less frequently and exhibit less aggression than other closely related species, such as Misumenoides formosipes, that do not have a female-biased sex ratio.
In May and early June, males molt into adulthood, with the number of adult males peaking between June 5 to July 15. Females do not molt into adults until mid to late June, with numbers of adult females peaking around June 25. After males molt, their body mass does not increase, remaining at about 4 mg. Males, however, do undergo body changes as they enter the adult stage. The front legs of the male lengthens while the abdomen shrinks. Females may live an average of two years and spend most of this time guarding their eggs sacs and the territory (flower) on which they hunt. Males have shorter lifespans by about a month. Only near the end of the female's second year of life will she let males onto her territory to mate. Females usually die very soon after their eggs have hatched, during their second winter.
The much smaller males scamper from flower to flower in search of females and are often seen missing one or more of their legs. This may be due either to near misses by predators such as birds or to fighting with other males. The methods by which males locate female crab spiders have been difficult for researchers to determine. Males exhibit a random pattern of searching for mates, which only becomes a more systematic approach when a male discovers a female dragline. Females leave these draglines behind them as they search for prey. Males follow the draglines in search of potential mates. The lines, unlike many spider species, do not contain pheromones deposited by females, however. Males follow the lines mechanically rather than chemically. The tendency for a male to follow a line is highly influenced by its life stage and the stage of the female of interest, which is a key detail that led researches to believe that line following is related to mate detection. Adult males preferentially follow adult female and juvenile female draglines, while penultimate males do not display a specific preference. Male M. vatia are also more likely to follow lines laid by their own species than those of a related species.
Since encountering a female is relatively rare, in the case that two males are interested in the same female a fight will ensue. Fights may include light touching, chasing, foreleg lashing, grappling, and biting between males. In the case that the female is being guarded by an existing male, the guarding male may either fend off the challenger or be replaced. In the case that one male arrives at the female of interest first, he has an advantage in the ensuing contest. In this species, unlike many other species of spider, older and younger males are generally the same size. Younger males win significantly more contests than do older males. Older males have also been shown to initiate contests more frequently than younger males, but the nature of their attacks are less likely to include extensive bodily contact. After a contest between males, the winner immediately mates with the female while the loser retreats.
When a male finds a female, he climbs over her head, over her opisthosoma and onto her underside, where he inserts his pedipalps to inseminate her. The male may wrap the female loosely with silk during copulation. Females have a pair of gonopores which the male pedipalp may enter for copulation. When the male inserts his pedipalp into the female's gonopore he will make rhythmic, vibratory movements that can last from 1–2 seconds. Males are most likely to enter both gonopores of a virgin female, while he may only enter one gonopore of a previously-mated female. Gonopore contacts of less than 30 seconds will result in unfertilized eggs and a failed copulation.
Matings will last an average of four minutes.
Males of this species can accurately identify the reproductive condition of potential mates, and they prefer to mate with virgin females over previously mated ones. Males will mate longer with virgin females and have produce more pedipalp movements than during copulation with previously-mated females. Females have a low probability of mating with a second male, and have a higher probability of mating with a second male than they do a third one.
When preparing to lay eggs, the pregnant female M. vatia first identifies a suitable location for her brood. She will descend the plant stalk down to the leaf that she chooses and then will roll up the end of a leaf of her desired plant, usually a milkweed leaf. She will secure the leaf by spreading silk, creating a cocoon-like structure, and lay her eggs inside the nest she has created. The M. vatia tends to lay its eggs at night.
Because M. vatia employs camouflaging, it can focus more energy on growth and reproduction rather than finding food and escaping from predators. As in many Thomisidae species, there is a positive correlation between female weight and egg clutch size, or fecundity. Selection for larger female body size thus increases reproductive success.
This species exhibits a high first-male sperm precedence, so providing a virgin female with a large sperm quantity is advantageous. Consequentially, there is a very limited quantity of virgin females available at a given time in this species, so there is strong selective pressure that favors males that will provide large sperm quantities. The need to produce a large sperm quantity for each copulation prevents males from remating quickly. Additionally, females are likely to deny subsequent males after their first mating as further reproduction will interfere too strongly with the female's foraging success. More matings are also unfavorable because they increase the risk of infection with sexually transmitted parasites or diseases.
In some unusual cases, males can tell whether a female has mated previously from a distance. This may be due the female's heightened aggression that manifests after she has mated once. It is more common, however, that in order to assess the reproductive history of a given female the male must first mount the female, which is dangerous for the male as there is a chance the female may attack, capture, and kill the male. Since females are difficult to locate and the cost of searching for them is so high, the risk is usually worthwhile to the male. In addition, since locating females is difficult, there is a low chance that another male has previously inseminated a given female so it is therefore beneficial for males to provide large quantities of sperm. This adaptation is also beneficial in the case that a female loses her first brood. This is because when a female has been inseminated with a large quantity of sperm she may have enough to fertilize a second brood to replace the lost first brood.
Like many other arachnids and insects, M. vatia may express sexual cannibalism, however it is not routinely practiced and is only considered moderately common. In cases of precopulatory sexual cannibalism, older males are more likely than younger males to be targets of attack and are more likely to suffer death or injury as a result of an attack from a female, especially during the latter half of the mating season. This could be a result of a decreased ability among older males to evade attack from females. Older males do not tend to display riskier mating behavior than younger males. The size of the male does not influence its likelihood of being cannibalized during copulation. Females increase cannibalistic attacks as the mating season progresses. More males tend to be cannibalized after mid July, which could be a result of male aging but is more likely a result of increased female aggression during this time.
Like many other species, M. vatia guards nests to protect vulnerable eggs from attack. Guarding the nest has increases the overall reproductive success of the spiders by protecting against predation from ichneumonid and dipteran egg predators. These spiders are usually observed guarding the nest by standing on its underside, the most vulnerable face of the nest. Most guarding spiders will remain by the nest until the young have begun to emerge from their eggs, with some variance in guarding times. The average guarding time for a female M. vatia is about three weeks. A minority of spiders may abandon their nests before spiderlings have hatched, while some may remain until all young have hatched or longer. M. vatia usually die within a few days of the hatching of their young.
These spiders change color based on visual cues. Depending on the color of flower they see around them, they will generate a liquid yellow pigment and then secrete it into the body's outer cell layer. The baseline color of the spider is white. In its white state, the yellow pigment is sequestered beneath the outer cell layer so that inner glands which are filled with white guanine are visible. These spiders have the unusual ability to match the color of the yellow or white flower with great accuracy. They are able to match with greater accuracy to white flowers, the Chaerophyllum temulum (the rough chervil) in particular, compared to yellow flowers based on the spectral reflectance functions. While the spider is residing on a white plant, it will tend to excrete the yellow pigment. In order to change back to yellow, the spider will first have to produce enough of the yellow pigment. For this reason it takes these spiders much longer to turn from white to yellow than it does for them to go from yellow to white. It was first noted that the color changes are induced by visual cues when an experiment showed that spiders with their eyes painted shut had lost this ability.
Diet-induced color change
M. vatia can also change color as a result of prey consumption. Once consumed, colorful prey can show through the thin, transparent epidermis of the abdomen, affecting opisthosomal coloration. Ingestion of red-eyed fruit flies will cause the abdomen to turn pink. Coloration changes caused by prey consumption will revert to normal white or yellow 4–6 days after ingestion. Color change intensity is positively correlated with the amount of colorful prey consumed. Color change intensity also decreases with the spider's age. These spiders have been observed to have pink, orange, yellow, brown, green, or white opisthosomas depending on the prey consumed.
Parasitization by the ichneumonid wasp, Trychosis cyperia, an egg predator, is common. The wasp attacks the nest and feeds the eggs. One attack can destroy the nest completely. M. vatia experience strong selection to minimize attack from wasps, which is why egg guarding by the female is important for reproductive success. Wasps tend to feed on small egg masses guarded by small spiders, as small spiders cannot defend their nests as effectively. When defending the nest from an approaching predator, females will typically raise their front legs in a display otherwise observed when the spider is attacking prey. Other known predators include ants (Formicidae), other spiders (Araneae), birds (Aves), lizards (Squamata), and shrews (Soricidae).
Bites to humans and animals
M. vatia are harmless to humans, as their fangs are not powerful enough to penetrate human skin and its venom is too weak to harm larger animals.
Vision plays a role in the spider's substrate color matching. The existence of two types of photoreceptors in M. vatia is supported by electrophysiological evidence.
These spiders have two rows of four eyes each for a total of eight eyes. The antero-lateral (75 μm diameter) and postero-lateral (65 μm diameter) eyes are the larger in size of the four sets of eyes. The antero-median (59 μm diameter) are considered the principal eyes and along with the postero-median (55 μm diameter) constitute the smaller of the sets. All of the eyes other than the principal eyes are considered the secondary eyes. The antero-median eyes appear the clearest, while the other sets of eyes appear darker.
The four pairs of eyes are similar in structure, all containing a retina, a dioptric apparatus, and a cellular vitreous body. The outermost layer of the eye is the lens. The columnar cells of the vitreous body stand between the lens and the retina, and their nuclei rest next to the retina. Three layers of pigment cells surround the vitreous body. The epidermis is the outer layer, and it contains "electron-dense granules and electron-lucent inclusions of micro-crystals." The middle layer contains dark, pigment granules. The innermost layer contains larger, dark, pigment granules inside glial cells. These layers prevent light entering through a nearby transparent cuticle from reaching the retina, keeping each eye isolated. The retina contains photoreceptor cells and other supporting cells.
The postero-median eyes look directly upward, and their field of view overlaps somewhat with that of the postero-lateral eyes. The antero-lateral and postero-lateral eyes also share a slight overlap in their visual fields. The antero-lateral eyes give these spiders a region of binocular vision. The organization of the antero-lateral, postero-median, and postero-lateral eyes allows these spiders to see nearly their entire upper visual environment.
M. vatia have the necessary physiological machinery to see color, and are most sensitive to wavelengths of light between 340-520nm.
The principle eyes have a complex and unique organization. They have three different photoreceptive segments. The periphery contains a half-circle of one type of rhabdomere, while the center is pigmented and contains two types of rhabdomeres. These spiders also have a "giant rhabdom" in the lowest layer of the center of the retina. Only the light entering along its optic axis stimulates this giant rhabdom, so the visual information comes in the shape of a dot. M. vatia can control the trajectory of the giant rhabdom by moving their eye muscles, which means these single points of visual information are integrated to generate the spider's visual field.
The principle eyes M. vatia have tiered retinas, with four layers containing different types of photoreceptors. These spiders have been proven to have green and UV photoreceptors, and likely have many other types which allow them to see a full range of colors. The secondary eyes are dichromal, meaning that they have two types of photoreceptors.
Significance for color change
Since M. vatia use their visual systems to inform color changes, they must be able to see color in their environment and on their own bodies. The visual field of the antero-lateral and antero-medial eyes allow the spider to see its legs, while the postero-lateral eyes see the opisthoma. Since the visual fields are so wide, these spiders see their own bodies and the color of their surroundings, which supports the idea that color matching is facilitated by the visual system.
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