The secret lives of velvet spinners

A ghoulish guava tree grows along the path to the cottage from Navilu Kaadu’s main gate. The boughs of the little tree are perennially sheathed in web and tangled in fibrous clumps. A custard apple tree near the cottage, too, is similarly entangled. My younger one pointed out that both trees were playing hosts to enormous arachnid metropolises! These were the Indian cooperative spiders or social spiders (Stegodyphus sarasinorum) from the family Erasidae, also called Velvet Spiders, for the downy hair on their rotund, squat forms. The females measure between 7 and 8 mm. The males are smaller.

Spiders of the genus Stegodyphus are said to be the only arboreal or tree-dwelling species among Erasids or Velvet Spiders. While the solitary signature spider (featured in an earlier column) cannibalises siblings and mates, these social spiders throw clichés of lonesome, hostile, mate-eating spiders to the wind!  They thrive in large, extended families, build communal nests, care for the colony’s young, snare prey and feast on them together.   

There are approximately 20 social spider species among more than 53,000 spider species that inhabit our planet.

One marvels at colonies of ants, bees and wasps where the hierarchy and caste decide every inhabitant’s life purpose. Each member of the colony performs one designated role all its life, either rearing the young, maintaining the nest, foraging, guarding or reproducing.

Social spider colonies, on the other hand, are egalitarian societies — they are free of hierarchies. No role is ironclad. All female social spiders procreate when food is abundant and nesting conditions are favourable, unlike in most social insect colonies, where procreation is the preserve of the queen.

An Indian social spider colony can hold tens and hundreds of individuals. Females make up most of the social spider colony. Males are a minority. The ‘spider women’ run the colony — together, they build, maintain and repair nests, care for the colony’s young, and take down prey. The colony’s males are Casanovas who do little else other than seduce females! Indian social spiders breed between January and April. The females are semelparous, which means they reproduce just once in their lifetime, and death is a planned strategy to nourish their progeny. The female lays up to 550 eggs and cossets them in a silken cocoon or egg sac. Spiderlings emerge in about two weeks. The devoted mother pours herself into their care, feeding them regurgitated formula made from her own liquified innards. Once she exhausts this, the spiderlings climb onto the mum and draw fluids directly from her body, until she is reduced to a lifeless exoskeleton, in a behaviour called ‘matriphagy.’

On reaching maturity, the young spiders either continue to live within the colony or may choose to balloon to other social spider colonies. Social spiders fashion nests from twigs, dry leaves, bits of prey parts and their own exoskeletons, all held together with dense spider silk. A larger web span and communal living also mean social spiders are able to deploy pack power to snare larger quarry, sometimes even birds and bats, and the spoils are shared. The inhabitants of the colony emerge at dusk to orchestrate a coordinated attack on any down-on-its-luck mantis, beetle, grasshopper, moth, mosquito, butterfly or dragonfly trapped in their vast capture webs. Scientists have observed that when they are unable to trap enough prey, these smart spiders weave extra tangly webs with a special woolly fibre called cribellate silk that they secrete from a specialised spinning organ called the cribellum. (Ellendula, S, Tresa, C, & Uma, D, 2021)

The Indian social spider colonies at Navilu Kaadu do a swell job trapping pests on the farm, in exchange for real estate on a pair of guava and custard apple trees. I’d say this is a solid bargain as far as ecosystem services go!

Rooting For Nature is a monthly column on an off-kilter urban family’s trysts with nature on a natural farm. The author runs Green Goobé, a sustainable venture. Reach her at [e-mail] or @ramyacoushik on Instagram.