Stegodyphus sarasinorum

Stegodyphus sarasinorum, like most Eresids, are short, stout spiders lined with several fine hairs. These fine hairs give the spiders an ash-colored appearance. The dorsal side of the abdomen is marked with three longitudinal white stripes and six pairs of dots, which are part of the spider's patterning. There are two irregular surfaces containing the six spinnerets encased in a structure called the cribellum. Unlike most spiders, S. sarasinorum does not have a well defined cephalic groove. The arrangement of the eight eyes consisted of two rows of four and two in front and the other rows facing the side to allow for the wider range of vision. Their modified mandibles, known as falces, are black in color and the claws are curved for defense on the sides of the body.[5]

Within the genus Stegodyphus, 20 species were assigned according to social status, distribution range, and phenotypic expression of male and female reproduction characteristics. Three of the social spider species are Stegodyphus mimosarum, Stegodyphus africanus, and Stegodyphus sarasinosum. S. sarasinorum is a social spider, hence the populations have high levels of inbreeding and relatedness. The low rates of dispersal and high turnover rates result in low gene flow and lack of speciation. Accordingly, social spiders are thought to likely be evolutionary dead ends especially with their practice of obligate group living.[6]

The colonies are highly dependent on females, and they have highly skewed sex ratios in favor of females. For every female, there are 0.15 to 0.28 males.[7]

Stegodyphus sarasinorum is native to south and southeast Asia, predominantly in the countries of India, Sri Lanka, Nepal, and Myanmar. One of the preferred places of S. sarasinorum is the prickly pear bush, because it serves as a strong support for their silken dwellings and webs. In some cases, the bushes are covered by these spiders’ dwellings to the extent that the leaves are not visible. These spiders are more abundant on bushes, trees, and fences where they can easily construct their webs on the sturdy surface. S. sarasinorum likes building nests in shaded areas which is an advantage for catching prey.[5] They also live along rivers, especially when prey abundance is considerably higher near sources of water. S. sarasinorum prefers to build their nests over thick continuous spreads of vegetation, using a fence or bridge for support.

Stegodyphus sarasinorum is known to strategically position their webs in mostly shady areas. Consequently, they almost always have an ample food source as they are able to lure in a variety of insects. Sensing the vibrations of struggle from caught insects, the spiders scurry over to pull and drag the prey back to their nest. At times, disentangling prey can damage the webs, and following capture, female workers get back to filling in any gaps with new threads. Once the prey is brought to the nest, the spiders hold onto some part of the prey pulling it towards themselves, exhibiting selfish behavior. Spiders have been observed to grab onto the prey's leg and move away into an area when none of the spiders can see it eating first. Otherwise, all the individuals in the nest eat together, breaking away different portions to jointly divide the meal. Communal feeding occurs even with spiders who may not have taken part in the prey capture. Since these are social spiders, and therefore not very skilled in carefully securing prey without disturbing the web, they typically follow the drag, pull, bite technique in obtaining prey.[5]

Stegodyphus sarasinorum eats many types of prey such as bees, mosquitoes, crickets, beetles, butterflies, and moths that get caught in the webs. Some of the common orders of insects consumed by this spider are Coleoptera, Orthoptera, Hymenoptera, Hemiptera, and Isoptera, and Coleoptera and Orthoptera tend to be the most preferred. This species focuses more on consuming all available prey and is not especially choosy about specific prey types. S. sarasinorum is known to capture and eat prey up to 50 mm in size, much larger than their own 7.5 mm bodies. As spiderlings, they consume the liquid regurgitated by their mothers.[13]

The web of S. sarasinorum is made up of longitudinal and zig-zag lines and the thickness and use of threads depending on the purpose of that section. The foundational line is jointly made by six or seven spiders moving back and forth on the threads in order to thicken that region. When the warp lines are laid, there are bundles of few threads that appear to be thin yet are strong enough for their purpose. In the weaving process, multiple spiders come together and start spinning threads in different regions of the web and lay down sticky connecting threads. S. sarasinorum uses the hind pair of legs, rubbing against spinnerets and moving quickly across the web to lay out the threads in an efficient fashion. The sticky lines are very elastic and this allows the spiders to create large and effective webs. As the main goal is to create a large net, not much precision or symmetry is required. S. sarasinorum is well known for the collaborative nature of web construction with little idleness and each spider moves to an incomplete section to finish the task. Usually webs are built within two to three hours in the evening and following that, spiders return to their next location and rest till their active hours in the night.[5] When energy needs increase, S. sarasinorum either focuses more on capturing large sized insects or they more quickly immobilize their prey to improve their efficiency. For example, it is better for them to invest energy in capturing grasshoppers over beetles since they provide more food for the larger community in the nest.[13] The web silk of S. sarasinorum has been found to have a significantly high non-enzymatic antioxidant potential.[14]

Breeding season occurs in the spring between January and April for S. sarasinorum. The courtship rituals may involve the male chasing after the female even through narrow passages to attract the female. When the female resents the approach, she steps aside or runs away, but does not respond with aggression like many other spider species do. If she allows the male to be close, there is a mating period of usually 3–4 minutes when the male rubs his pedipalps against the female's genital pore.[5]

Gravid females are much larger and slower than normal females, and they do not take part in web building. Since they are not active enough to capture prey by themselves, other members of the colony drag prey into the nest for the gravid females to feed. They use their hind legs to groom the dorsal part of their abdomen and groom their appendages against each other in a motion called toilet movements. Their main tasks are construction of 2 to 4 brood chambers for their young during breeding season and initiating new colony formation when they disperse to create new breeding nests nearby. Gravid females also guard the egg sac, mostly because they rely on only one breeding season in their lifetime. The 10 stages of cocoon spinning are: construction of the platform, making the receiving valve, rest, oviposition, concealing the egg mass, covering valve, removal of some supporting threads, cribellar silk, separation of the cocoon, and transportation.[15]

Stegodyphus sarasinorum is a semelparous species and females invest all their time and resources into a single reproduction event. Eggs are typically laid in a lenticular cocoon packed in silk, with the maximum number of cocoons being made in the month of February. A singular egg sac can contain between 350 and 550 eggs, and webs enclosing them are built in a more compact and sturdy fashion. The 6 mm cocoons are white colored and the females attach them to the side walls of the nest. 13 to 15 days later, the juveniles tear open the cocoon walls and emerge, moving to settle on their mother's back. The young are generally round and pink, and they do not intake food until after 2-3 molts when they begin to get involved in web construction. As the young develop more, adults in the nest tend to desert the nest else they starve to death with the selfish feeding of the juveniles.[5]

Around 4 to 5 weeks after the females lay their eggs, they open the egg sac and the spiderlings stay within the maternal webs. Unlike most spiders, females do not capture prey for their young, nor do the juveniles catch or eat prey prior to their mother's death. Mothers feed their juveniles through regurgitation which involves liquefaction of her internal organs to create material that will increase the fitness of her young. Once her ability to liquefy internal organs is exhausted, the young climb onto her body and consume fluid from her body in a practice called matriphagy and within 3 to 4 hours, the mother's exoskeleton can be extracted from the webs. After spending a few more weeks in the maternal web, spiderlings feed on trapped insects and later migrate to build a new web.

Stegodyphus sarasinorum is a social spider, so web building and feeding are all communal practices, depending on each other for survival. An interesting aspect of S. sarasinorum behavior is antagonism. The dwarfed males thrive only when they stay agile and cunning and even between sexes there is lots of selfish behavior. Females attack the males and even bite off their legs in the process of courtship to reduce their fitness levels. Due to this evolutionary struggle, in related families like Araneidae, this antagonism has resulted in mental and physical shortcomings like dwarfism, dull color, and deformities.[5]

The main protection for S. sarasinorum is the silken nest that they jointly construct to shelter themselves and reduce the risk of predation. It is more difficult for predators to detect these spiders when they are within their nest especially when their retreats are covered with leaves and the swarming tendency of these spiders benefits them as well. These nests protect them to some extent from wind and rain, but it does not help them when temperatures are low.

The nests constructed by the Stegodyphus genus are the primary means of protection from predators and environmental stressors such as wind and fire as well. Because the nests are so dense and compact, they serve as shields to protect from predators and maintain moisture within the living area. Leaving the nests is highly risky because they become vulnerable to predator attack and they face fitness costs from solar radiation and rains during solitary hunting. [10]

Uloborus ferokus has been found to be a commensal of Stegodyphus sarasinorum and has never been observed without association with its host. The sticky webs of S. sarasinorum serve as a site where the primary orbs of U. ferokus can attach. Another advantage for the commensals is that staying in the web provides them with access to the small insects that get lodged in the threads. The webs also prevent Idris, an egg parasite, from damaging the cocoons of U. ferokus.[17]

Stegodyphus sarasinorum has not been observed to cause any harm to humans or vertebrate animals.[5]

• List of Eresidae species

• Population Structure in an Indian Cooperative Spider, Stegodyphus sarasinorum Karsch (Eresidae)
• Some observations on the ecology of social spider Stegodyphus sarasinorum Karsch (Araneae : Eresidae) from India
• Social Spider, Stegodyphus sarasinorum, are different from most spiders as they are social colonizing spiders.