The Social Spider as Alien Invader
by Robert E. Furey
Meet the Spiders
It was twenty years ago that I first saw a spider city. Scattered sundrops from the canopy dappled the understory plants and set translucent web-works to glow. The structure appeared to soar upward with curving support cables of as I approached the enormous spider city, talking into the tape recorder fixed to an epaulet on my left shoulder.
Standing near the nests I noted observations into the recorder. As I spoke the spiders turned to face me and advance in short bursts of speed toward where I stood. For them I was a moth, a buzzing fly, a frantic katydid tumbled to the spiders’ killing fields. They searched for where I should have been entangled in the webbing until I stopped my speaking and they returned to whatever duties they had been performing.
The bulk of the structure does not reach the ground, perhaps a way to keep the hordes of dangerous predators from the forest floor at bay. A central area of the city was open, surrounded by a dense torus of inhabited webbing. I entered the city on my belly, crawling supine through the narrow gap between thick webbing and forest litter. Just above my head the older, deeper parts of the city were long abandoned by spiders. In those older chambers, dark and humid, denizens of facultative and obligate symbiosis let nothing go to waste. Ants and terrestrial isopods live out their lives in those abandoned yet welcoming sectors of the city. Vipers sleep between meals where their own predators would be too slow to tread through the dangers of spiders or resident ants. Even higher mammals, a small bat species, is found almost exclusively to roost inside these nests in what could be an obligate relationship, domesticated by a long-term association with the spider society. Kerivoula woolly bats roosted in the more open middens of the deep colony, abandoned as the colony’s volume swelled.
Once inside the torus I could once stand again. There was a central courtyard encircled by thick walls of opaque webbing scattered with retreat funnels and patrolling spiders. Scaffolding cables arched up and out, supporting the living area of the arcology like an organo-gothic segue between city and surrounds. The resilience of these structures in areas as dynamic as a West African rainforest is testament to the evolutionary genius of social spider adaptation. Winds, rain, falling fruits and branches: the spider cities survive it all.
I located the particular arcology described above in the Ogooué-Ivindo rainforest, in the West African country of Gabon. Built by many thousands of spiders over generations of inhabitants, this extraordinary structure began about a meter from the ground and hovered in the understory like a donut the size of a two car garage. It’s difficult to estimate the numbers of spiders in a certain volume of nest—the value has wide error bars—but this particular colony must have housed tens of thousands of individual spiders.
Think about Evolution
Half a billion years ago arachnids were the first larger animals to leave the ocean and crawl up the sand. They found themselves the pioneers and kings of a barren land. Yet colonize they did. With a UV-scorched surface and a poisonous atmosphere, arachnids breached the waters’ surface and established a beachhead. Preadaptations in arachnid morphology allowed this extraordinary feat. Hard exoskeletons served to protect them from the hellish conditions away from ecologically mediating seawaters. They defeated unbridled solar radiation, eked out oxygen on imperfect book lungs and lifted themselves against gravity due to the physics of external skeletal support. Although probably a long time passed before arachnids left the protected flotsam accumulated in littoral zones, they were the first to leave footprints on an alien shore 400 million years ago.
So it will not be unprecedented for these types of creatures to be first among the first again. Given the stochastic nature of providence and the little discussed importance of “luck” in the evolutionary process, a world, perhaps a world other than Earth, giving rise to “arachniforms” evolved as top predators for almost half a billion years is almost inevitable. Driven by a foundation of simple compulsions, our star spiders should venture forth from their comfortable home worlds—in fact, they will be driven to. They will not be alone in space. They will find us.
Spiders are generally highly aggressive predators, territorial and even cannibalistic. Several stages of a spider’s life take place within a communal egg sac where competition begins between siblings. Some of these siblings will never emerge, whereas their well-fed brothers and sisters will be fat and happy when they eventually disperse. Even with the dubious inclusion of trophic individuals—those destined to be eaten by others—all spiders are creatures of sometimes complex social interaction. Social spiders are relatively rare, and thus an exception. But this makes them interesting. How did they come about?
Evolution is conservative. It’s conservative in multiple ways. And while it may seem odd for a conservative system, evolution is supremely adapted for an intrinsically dynamic environment like the world around us and has done a remarkable job in producing a myriad of organisms with a relatively limited palate of options.
Starting from the obvious four-letter alphabet that encodes our DNA, the conservative nature of evolution is easily documented in the gross anatomy of disparate organism solving similar ecological problems. Convergent evolution has given us penguins, ichthyosaurs, tuna, and dolphins, which all evolved sleek, hydrodynamic body shapes to speed through a relatively viscous medium. Homologous body structures use the same anatomical building blocks to perform different functions, such as vertebrate forelimbs called variously hands, paws, flippers or wings; while analogous structures provide the same function but use different body parts to achieve it, such as flight in birds, bats, butterflies or pterosaurs. Evolution stumbles onto rapid swimming, high flying, and anatomic frugality, and retains them for the incredible advantages they bring.
Since the trait of sociality in spiders is persistent, nature has found advantage in retaining it for what is otherwise a ferocious predator. This should give us pause.
Evolution has worked in a consistent manner across time. It should work the same way across space.
Parallel evolution has shown us time and time again the persistency of certain niches, filled by eerily recognizable organisms that are only distantly related. Marsupial-rich ecosystems produce simulacra of mice, moles, cats and dogs with behaviors and physical adaptations solving ecological problems with the same solutions. Ichthyosaurs and cetaceans, velociraptors and wolves, pteranodons, birds, and bats, evolution never needs to restructure a successful solution too much. The evidence suggests that Earth species can give us a glimpse into a catalog of other planets, other ecologies and other taxonomies. Evolution likes certain solutions and remixes winning traits. Somewhere out among the stars they’re there. The spiders are coming.
Think about Spiders
Consider that most celebrated weaver, Arachne. Her skills were unparalleled among mortal women. She even scorned the skills of the gods. The goddess Minerva, annoyed by Arachne’s boasting (and by the stories revealed within the weavings of her tapestries), transformed the young woman into a spider. Yet as a spider she retained her great skills as a weaver.
Spiders have been present on the Earth since the Devonian period. All of the approximately 37,000 species of extant spiders are predators and the group has evolved a close relationship with insects as their main prey. The great commonality between species is the use of silk. Silk is not just employed to catch unwary insects. It is not limited to cobwebs or spectacular orbs outside kitchen windows. Spiders are much more clever than that.
Not all spiders do all things with their webbing, of course. And not all spiders solve the same problems in the same manner. For trapping insects there are webs covered with drops of glue and there are webs that are mechanically “sticky” with tiny loops of webbing along a strand that entrap insect feet in velcro-like lariats. The iconic image of a web-wrapped prey item subdued and hung up for later consumption is an incomplete view of what a spider can do with silk.
In addition to prey capture, webbing is used for prey immobilization and storage, reproductive behavior, dispersal routes as both recruiting threads that entice spiders to follow in the path of others, or flight, nest building, sensory or body extensions, guide lines, drop lines and anchor lines, and pheromone trails. With the defining trait of web production and the emergent behavioral aspects of all its possible uses, spiders have carved out their own access to sociality.
Sociality in the spiders is both rare and ephemeral over evolutionary time for any given species, yet it is persistent for the larger taxon. Of the above mentioned 37,000 extant species only a small handful of perhaps fifteen or twenty are permanently social with persistent colonies. A high level of social behavior in the spiders often comes with heavy costs; inbreeding and weakness of any one individual are two common ones. Generations of spiders that remain in a central site breed with brothers and sisters over time until the levels of genetic diversity reduce a colony’s ability to combat disease or other threats. The ephemeral nature of sociality in the spiders is largely due to the genetic costs that build over time and eventually drive a colony, and a species, to extinction. While any given social species is expected to weaken and disappear, sociality in the spiders is such an attractive trait that another species will readily develop it. Social behavior has risen many times in the history of spiders and from this we can assume it will likely be maintained in the taxa and reused by evolutionary cobbling many times over.
And then there is the alien spider city itself. A soaring arcology of gracefully arching structures, with their support cables stretching into surrounding trees to segue from artifact to forest, and the elevated silken killing fields where the city denizens hunt for their meals. Narrow funnels scattered over the surface dive to interior warrens where the city’s residents sleep, rear their young or seek safely. Deep inside, in chambers abandoned by spiders, other species, both social and solitary, live in relative harmony as they maintain the collective. Araneoidean domestication had been inevitable for some, given the final dependence on living within the confines of a spider city. Species in close association with social spiders that have gone from convenience to obligate symbiosis have become the domesticated, client species to the social spiders. Bats, vipers, terrestrial isopods and ants are among the more obvious species that have taken residence inside the spider nests. In the end, the complexity permitted growth and stability and survivability for conspecific and client individuals that otherwise would have been outcompeted without the social structure of the colony.
A colony of spiders is a problematic thing to measure even if easy to define. Like many things in ecology there is a difficulty and certain messiness to limits or boundaries. In this case, the boundary we are interested in defining is where a colony ends. For our working definition, delimiting a colony’s boundaries is strictly defined by connectivity. Any concentrated area of spider activity, or nest, which is physically connected by spider silk bridges, no matter how tenuously with another concentrated area of spider activity, are considered the same colony. Of course in the tempestuous, wind-blown rainforests of West Africa the connections can come and go. But once established, a new nest or colony can persist for generations of spiders. These characteristics associated with colonization and expansion seem ripe as preadaptations for a greater fate.
Quite close to the earliest signs of life on Earth, evolution produced something interesting: sociality. Social behavior, as opposed to asocial behavior, is generally defined as interaction between organisms. Given this definition, social behavior can be the highly specialized organization and cooperation found in leaf cutter ants, or bloody face-offs between tyrannosaurs defending the adjacent edges of their territories. For the purposes of this essay we will use the amicably cooperative end of the social continuum where organisms function for a collective good, benefitting a group rather than an individual.
The earliest signs of life on Earth were simple organisms and we find the fossil evidence of single celled bacteria in early strata of the geological record. Among the early bacterial denizens of the Earth, cyanobacteria found success by adopting a group behavior. Colonial cyanobacteria cement grains of sedimentary material around themselves into short stacks of stony pancakes called stromatolites. There is strong evidence that stromatolites have been present for almost three and a half billion years and living examples are still found today. Although the sociality exhibited by these organisms is primitive, their success is cataloged by their constant presence in the fossil record for these past several billion plus years. Stromatolites surpass the success of even such metazoan champions of longevity as the trilobites, which clock in for a short 270 million year run, or the dinosaurs, box office hits but with only 160 million years from first rise to rock attack.
What do the stromatolites have? My guess is sociality. Strength in numbers, another trait that evolution has used again and again, weaving benefits into taxonomic hierarchy at all levels since the beginning. Grouped together, organisms can do what individuals cannot. While not all taxonomic groups have produced social members, the advantages this behavior brings is undeniable. It’s to defend; it’s easier to attack en masse; it’s easier to find a mate. There are obvious benefits to social groups; social organisms are more efficient.
Like the convergent evolution through analogous and homologous structures, the evolutionary precursors of social behavior reconstituted time and again to better exploit the niche of the organism that is given the trait. Preadaptations, or precursors, might be a predilection to limited dispersal or skewed sex ratios or genetic systems like haplodiploidy, some organisms find themselves particularly well suited for the incorporation of sociality. For example, ants, bees and wasps (the hymenopterans) or our own family, the hominids, are two taxonomic groups that have been especially driven by social behavior and organization, while other groups like gastropods or bears might seem less so.
Think Like a Spider
Order in spider society is based on equanimous anarchy and emergent decision making. There is no centralized control in a spider colony. Dispersed, collective decision making has proven to be a successful strategy for many organisms. Social spider building activity responds to resources and structure, wind and exposure to the elements. Stochastic or rare events dictate behavioral responses, not through choice but rather a dogged adherence to simple rule sets: such as the periodic and probabilistic attachment of drag lines to substrate, or the orientation of individuals to a common stimulus through shared web vibrations. These simple behaviors result in the defense of a colony from attack or for overwhelming larger prey items for reduction to usable resources. Both of these conditions pit the unpredictable aspect of other species’ behaviors against the spiders’ own battery of genetically programmed responses.
Individual spiders are constantly laying silk, take a few steps, attach. This simple behavior results in scaffolding to knock down flying prey, sheets of thick webbing, tunnels and support cables. All of this silk laying comes together as a complicated structure of interconnected nests. Individual spiders will sometimes send out floating exploratory strands of silk. Eventually coming in contact with something, the silk will adhere to structures in the surrounding area.
Once a strand has been established, traffic along the thread will strengthen and thicken the silk. If there is pressure of resource constriction, a new nest may be constructed at the far end of the silk thread. Recruitment along the strand will draw individuals from the more populous, established nests to the new ones. New nests then become either self-sufficient or are abandoned.
A successful nest must attract and retain a minimum number of individuals. Since the great advantage of social living remains the emergent abilities of a group itself, a minimum group size is required to maintain a nest. Collective behaviors dealing with construction and maintenance, prey capture and defense, and communal brood care, all figure into this requirement. A characteristic of social spiders is a drive to collect themselves together. A rich and attractive location at the far end of an exploratory thread draws and retains individuals as they form a satellite nest to expand the colony at large.
Since one of the advantages of being a social spider is the ability to take larger prey, hunting spider groups must be able to coordinate their efforts. When a prey item finds itself in distress in a social spider web the hunting spiders orient and close in on the unlucky insect from all sides, taking the prey in a rain of venom-filled bites. But how do they orient?
In spite of having a number of eyes, spider vision is relatively poor. Instead spiders rely on vibratory cues for many forms of communication. One of these cues is how they catch prey. When a prey item entraps itself in the webbing, the spiders orient toward the disturbance. They approach the prey from all sides and then do something remarkable. The spiders approach the prey in short, synchronized bursts of movement. Without a centralized decision maker, spiders move in such a way as to both close in on a prey item and not interfere with others trying to locate the same prey. These simple rules dictate emergent behaviors without conscious decision making.
Think Like Spiders with Starships
So if evolution so often reuses these successful strategies, might we expect something out there like social spiders? We might find a remix of traits accompanied by a modicum of technology discovered and maintained through simple behaviors. Remember that our own brains function on bundled nerves with a binary response, yet collectively those nerves think about building enormous particle accelerators. Spiders are well known for problem solving abilities, and coming to Earth would be a laudable problem.
First they have to find us. Or at least they have to find suitable sites to establish new nests, presumably rich, wet worlds like our own Earth. The drive for spiders to expand territory results in exploratory threads sent out at random. If strands stick, spiders investigate.
Imagine a technology that mimics what spiders know from biology. Von Neumann probes scattered to space might carry explorers who evaluate new nodes, new planets. Each planetfall expands a web work across space of increasing complexity and shifting connectivity. Communication vibrates along the connection points like a hum across the galaxy, with the spiders focusing on attractive worlds. The fact is, with expansion following their ancient ethological roots, they might eventually visit every world in the Milky Way.
Those species that do not fight the spiders’ invasion, instead remaining quiet and discovering some way to blend with the spiders, get swept up into the advance across the galaxy. Subservient species of overrun worlds become tolerated, allowed to live with their spider overlords in coexistence, if not dominated and eventually domesticated through dependence. If a few individuals are harvested during difficult periods, that would be a small price to circumvent annihilation and participate on the great march across the galaxy.
Space travel might be a solution to one handicap suffered by social spiders. Dangerous levels of genetic homozygosity could be avoided by increased levels of mutation suffered in radiation-ridden space. Since these mutations are largely harmful, and given the likelihood of genetic damage due to spaceflight, many individual spiders would die from the crossing. But since individual spiders killed by accumulated mutations from cosmic radiation are meaningless to the collective, sending more spiders would pose no obstacle to Araneoidean public opinion. Since a few will both survive and have a beneficial mutation, reconnecting colonies will reduce dangerous levels of homogeneity and genetic weakening. Space flight will only make spiders tougher.
When they arrive here, they will not care about individuals. They will seem pitiless in the way they advance. There will be no opportunity for prisoner exchange. No quarter given or expected, indeed no conception of such a thing. We will likely not understand them and they can’t even try to understand us. We will see a campaign of conquest, but should realize the intellectual emptiness of evolution’s moral compass.
As a hierarchical species capable of independent thought at different levels, our advantage on the battlefield will play greatly to our advantage. While initially we may be awed and cowed by the technology they wield, the uncanny way they coordinate their ground troops, or their pitiless methods of advance, human instincts should launch our own problem solving abilities to defend our world and defeat the menace. We, who killed mastodon with fire-hardened sticks. We, who took the dens of cave bears as our own. We, who crossed mountains with nothing but furs wrapped about our bodies. We, who must now repel the seemingly implacable alien menace from our midst.
The tactics and strategy of the spiders following ancient biological algorithms would manifest in rapid adaptations on a battle field. Fluid with emergent properties, spider troops react almost instantaneously to our positions and movements with their own age old mechanisms honed over almost a half billion years longer as a social species. But their thinking is not heuristic, it is only reactionary. And this is their weakness. Our individuality will outwit programming. We did it once in our own trial by evolution and we will prevail in our meeting with alien intelligences that have developed from older forms than our own. We always kill the cave bear.
Our niches overlap as technological, spacefaring creatures. And when niches overlap there must be competition. What we will have that they will not is a richer ability to recognize the parameters of a threat and to respond to the complex problems involved in squelching that threat. And we will have no choice but to vanquish the spiders immediately. Because, above all else, we cannot allow any of them to go home only to return with a horde. Our response must be swift and cold and merciless. Given our history this may not be the case. But mankind’s ability to adapt through conscious intellect will trump the blind aggressive advancement of emergent behavior. We will beat them because we can rapidly adapt and conceive what is at stake, that being extinction or enslavement of our species and homeworld. Or perhaps victory will come from our ability to value our fellows trumping the spiders’ complete disregard for individuals in their own species. Whatever the case, the spiders must go quickly. While our emotion and imaginative facets may initially betray us, in the end these are the very traits that will deliver us our eventual triumph over the invasion.
There will be plenty of time to mourn the spiders after the war is done.
Rob E. Furey is Professor of Integrative Sciences at Harrisburg University in Pennsylvania. Furey earned his doctorate at the University of Tennessee, Knoxville where his work was centered around social aspects of spider behavior, but his interests have broadened to include areas of astronomy, physics, geology and forensics.
Copyright © 2012 by Robert Furey