Metepeira incrassata

Metepeira incrassata, also known as the colonial orb-weaving spider, belongs to the spider family Araneidae and genus Metepeira. They are most famous for their social organization and group living behavior. They are generally found in tropical rainforest and agricultural sites in Mexico, and their habitats tend to be highly productive (high generation rate of biomass).[1] Their group sizes are relatively larger than other colonial spiders, typically ranging from hundreds to thousands of individuals.[2] 99% of the females are observed to participate in colonial living, generally with at least two other individuals. Because most M. incrassata females are communal, the colonies are often dominated by larger males.[1] There is minimal sexual dimorphism observed in M. incrassata. Unlike other orb-weaver spiders, M. incrassata builds a colonial web by connecting each spider’s individual webs together through semi-permanent framelines. These colonial webs of M. incrassata are prone to invasion by kleptoparasitic (prey theft) and araneophagic spiders such as the Theridiidae family.[3] The reproductive cycle of M. incrassata occurs throughout the entire year, with multiple generations sharing the same time period. Within their colonies, M. incrassata is seen to change locations. Larger, fertile females with egg sacs prefer to reside in the central area of the group for increased protection from predators, while the younger spiders are mostly found in peripheral positions.[4] Larger adult M. incrassata are also known to finish web-building earlier than smaller ones, gaining an advantage in strategically positioning themselves.[5]

Spider

Metepeira incrassata
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Infraorder: Araneomorphae
Family: Araneidae
Genus: Metepeira
Species:
M. incrassata
Binomial name
Metepeira incrassata

F. O. Pickard-Cambridge, 1903

. . . Metepeira incrassata . . .

Unlike the rest of Araneidae, the spiders of the American genus Metepeira are noted for their unique light eye region, white median line on their sternum, relative length of the leg segments, small male palpus (appendages of chelicerates), and the weakly sclerotized (hardened) epigynum in females.[6]

Metepeira incrassata possess a distinctive internal organ called coxal glands, a structure known to be present in select arthropods for the collection and excretion of urine. The coxal glands in the genus M. incrassata show relatively poor development compared to other Araneidae. Though the coxal glands resemble a sensory organ called the “pharyngeal taste organ,” the coxal glands are not sensory in their function.[6]

Unlike many other spider species, M. incrassata shows little sexual dimorphism. Dimorphism is typically more prevalent in spiders who have to search for widely distributed females and consequently face a higher male mortality rate. Because M. incrassata engages in group living in large colonies where females tend to be much more accessible, M. incrassata males are not forced to travel large distances in search of females, and thus show relatively low male mortality and sexual dimorphism.[1]

Metepeira incrassata can be found in mid-elevation tropical rainforest and/or agricultural sites, most notably in Mexico. Specifically, it has been located in Fortin de las Flores, Veracruz, where there is a plethora of insects and pleasant weather all year round, with the temperature only fluctuating between 20-32ºC daily.[7] The habitat of other Metepeira spiders, such as M. atascadero, includes desert and grassland sites. In contrast, the habitat for M. incrassata tends to be moist tropical forest vegetation with banana and coffee plantations, with high rainfall ranging from 170-220cm per year and high humidity (68-99%).[7] The tendency of M. incrassata to reside in warmer tropical habitats contributes to the multiple occurrences of the spider throughout the year.[2]

Metepeira incrassata can also be observed to live in highly productive habitats, which refers to the rate of generation of biomass in an ecosystem.[1] Its high productivity compared to its relatives may be attributed to most females being communal.[1]

M. incrassata reproduces continuously throughout the year, unlike other Metepeira spiders who reproduce only once a year.[1] All M. incrassata colonies, with the exception of the very new ones, contain some females with egg sacs.[8] The responsibility of guarding these egg sacs is a huge risk for M. incrassata, and the egg-sac predators such as Arachnidomyia lindae serve as a significant cause of mortality of M. incrassata.[9]

One of the most notable behaviors observed in M. incrassata is group living. Unlike other Metepeira spiders, M. incrassata is known to form larger colonies, ranging from few hundreds to thousands of individual spiders.[2] Rather than a single generation dominating each year, multiple generations of M. incrassata overlap throughout the year due to its multiple reproductive cycles.[2] Other species of orb-weavers can be observed to coexist within and on the edges of the M. incrassta colonies, such as Micrathena gracilis (Araneidae), Gasteracantha cancriformis (Araneidae), Leucauge sp. (Tetragnathidae), Mecynogea sp. (Araneidae), and Nephila clavipes.[10]

Within each group, each spider is strategically positioned, with fertile adult females residing in the center for maximal protection. Younger spiders that are at a lower capture risk from the predators typically outline the periphery of the group.[4] In other words, the large adult females who are responsible for reproduction are prioritized in terms of level of protection within the colony. The smaller individuals in the periphery are better equipped to forage and receive less protection from the predators.[5] Interestingly, however, they tend to be at a lower risk from the predators overall.[5] Larger group size is associated with greater rates of parasitism, increasing the cost per egg loss.[9] However, this cost is often offset by higher foraging success found in bigger groups, as well as enhanced level of protection per individual.[9]

M. incrassata tends to engage in sedentary living in relatively fixed locations.[7] It uses vibration to detect prey and communicate, and the average reaction distance of M. incrassata is seen to increase with greater colony size.[7] Most importantly, M. incrassata uses the ‘early warning system’ that detects the web-borne vibrations from the predators and the evasive behavior of other spiders to perceive danger.[7] This antipredator defense mechanism is one of the greatest benefits of the colonial living style of M. incrassata.

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