Spiders: Webs, Behavior, and Evolution
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View image of This video is no longer available. Most spiders are indeed lone wolves, but a scant handful have evolved a level of sociality to rival ant colonies, bee hives, or even primate societies. Anelosimus eximius , the species I encountered in the rainforest, is not the only kind of social spider in the world, but it does construct the biggest webs.
Some can reach more than 25ft 7. A web that size could contain as many as 50, individual spiders. That is a lot of legs, eyes and fangs. More social spiders have been discovered since. Sociality shows up in at least seven spider families. In all we know of around 25 social species among the 45, described spider species on the planet. Sociality evolved at least a dozen separate times among them. While the details vary from species to species among the social spiders, many of their features are similar. For example, an A. Communal spiders work together to build, maintain, and clean their webs.
They cooperate in capturing prey, and dine together when they snare a large feast. The females feed their offspring by vomiting up food for them, just like mother birds. They even regurgitate food for juveniles other than their own. In other words, they work together to care for the youngest in the colony. It is a sort of spider day-care. Sometimes, a large web is broken in half, perhaps by heavy rainfall or falling branches, or even a falling monkey. In its place, two smaller colonies are left behind.
Other times, individual females will strike out on their own, setting up a new web. Eventually other dispersing females will join the group; perhaps ones who tried to set up their own webs but failed. Survival is more likely in groups, after all. These two processes together help to explain why A. Gregarious arachnids are not much studied — perhaps because so few of them exist in the first place.
Social behaviour in otherwise solitary animals is cool. As I watched the social spiders go about their day deep in the jungle, a bee found itself stuck in the giant web. Suddenly, dozens of the eight-legged predators descended en masse onto the struggling black-and-yellow striped insect. It was impossible to see through the writhing mass of tiny spider legs, but one thing was certain: that bee was done for.
If death by one spider seems bad enough, it must be nothing compared to an attack by a whole swarm of them. Why was the bee subject to such an awful fate? Or, to put it another way, how and why did a few spiders become social when the overwhelming majority of their fellow species are solitary? View image of This bee got stuck in an A. The first spider, the proto-spider, was probably solitary. So the spiders that gave rise to today's truly social species must have been too.
But then they did something remarkable: they turned sub-social: they learned to tolerate each other, at least for a time — even if they did not exactly enjoy hanging out in groups. That early tolerance could have come about as a result of parental care.
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In some spiders alive today, that is just defending an egg sac, but in others — the ones that are poised to evolve sociality — it looks more familiar to our mammal brains. In those species, females provide food and protection for their developing offspring, and occasionally even to other juveniles. In some species, mothers even make the ultimate sacrifice , allowing themselves in their last moments to become a meal for their brood.
Sub-social spiders can still be cannibals, after all. In other words, juvenile tolerance alone is not enough to turn spiders fully social. The environment in which the spiders live also has to be right. Researchers have discovered three ecological elements that often lead to cooperative living among arachnids. Solitary spiders living in places where it is difficult to subdue large or more profitable prey alone may eventually figure out that it is in their interest to work together.
It is a smaller step from tolerance to cooperation than from aggression to cooperation. Another common feature is heavy rain.
Meet the spiders that have formed armies 50,000 strong
Rain does not have to be frequent, but when it is really intense, it has the potential to seriously damage spider webs. When a tempest takes out the web and threatens a spider's survival, the ones with just a glimmer of social behaviour might fare just a little better than their isolationist peers.
As a result, some have hypothesised that rough weather favours cooperation, since in those conditions it is the spiders that share the task of maintaining and repairing their webs that fare best. Then there is the web itself.
There are only a couple of instances in which spiders that do not build webs have evolved to be sub-social. These web-less social spiders, living in places like the Australian deserts or in African scrub habitat, have to cope with unusual circumstances — like enormous prey or particularly aggressive raiding ants. Instead, many orb spiders depend upon rare, but very large insects for the bulk of their calories 47 , Dissipating the high kinetic energy of these insects is a formidable challenge for orb webs Our investigation shows that Fecenia MA silk lacks many aspects of mechanical performance that are essential for energy dissipation by true orb webs, therefore suggesting that Fecenia may have reduced ability to capture large or fast-moving prey.
Furthermore, the absence of MaSp2 proteins likely limited the evolution of Fecenia's MA silk properties during the origin of the pseudo orb. Thus, our study adds to growing evidence that the evolution of innovations in silk production, such as the origin of novel silk proteins like MaSp2, plays a critical role in determining patterns of spider diversification 5 , 6 , Vouchers are deposited in the invertebrate collection of the University of Vermont.
We added these sequences to the preferred alignment of the orbicularian phylogenetic analysis of Blackledge et al. We used jModeltest v0. This resulted in a total of 10 data partitions. We also collected silk from the pseudo-radii of Fecenia webs and characterized its properties using a Nano Bionix testing system. We then constructed the broadest data set of material properties for naturally produced spider dragline silks to compare Fecenia silk to both its relatives and true orb spiders.
Small leaves were placed in the cages for the construction of retreats. We collected silk from the pseudo-radii of Fecenia webs using standard protocols that are well-established for orb spiders In total, 65 samples of silk from 14 webs spun by a total of nine different spiders were obtained from regions of webs lacking capture spiral onto cardboard mounts and secured with cyanoacrylate adhesive. We compared the data to two existing data sets on the material properties of MA silk from spiders.
Swanson et al.
Most of the silk in this study was collected from naturally spun silk structures, such as draglines and webs. However, silk from the orb spider taxa were collected not from webs, but rather through forcible silking of restrained spiders, which alters the alignment of silk proteins such that the resulting fibers are unusually stiff compared to naturally spun silk in webs.
Therefore, we substituted data from Sensenig et al. Silk in both of these studies was collected using techniques similar to our own and was tested using a Nano Bionix under similar conditions. For each species, we used individual spiders or webs as the primary sampling unit. Finally, we analyzed the amino acid composition of the MA silk produced by Fecenia to test for proline content.
Because proline is largely confined to the MaSp2 locus in Orbiculariae 26 , the percentage of proline in MA silk is a strong indicator of the presence of MaSp2 and its proportion relative to MaSp1 The vapor phase hydrolization and amino acid analysis were performed using an established protocol described by Smith The derivatized amino-acids were separated by the 3.
Chromatographs were analyzed and peak areas integrated using Unicorn 5. Darwin, C. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. John murray, Schluter, D. The ecology of adaptive radiation. Oxford University Press, Losos, J. Contingency and determinism in replicated adaptive radiations of island lizards.
Science , — Ecological character displacement and speciation in sticklebacks. Bond, J.fdy.madeline.dev3.develag.com/546.php
Webs hold clues about evolution : UA News
Testing adaptive radiation and key innovation hypotheses in spiders. Evolution 52 , — Blackledge, T. The form and function of spider orb webs: evolution from silk to ecosystems. Advances in Insect Physiology 41 , — Platnick, N. The world spider catalog, version Reconstructing web evolution and spider diversification in the molecular era. Coddington, J. Shear — Stanford University Press, Griswold, C. Phylogeny of the orb-web building spiders Araneae, Orbiculariae : Deinopoidea, Araneoidea.
Dimitrov, D. Tangled in a sparse spider web: single origin of orb weavers and their spinning work unravelled by denser taxonomic sampling. Cranford, S. Nonlinear material behaviour of spider silk yields robust webs. Nature , 72—76 Sensenig, A.
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Spider orb webs rely on radial threads to absorb prey energy. Interface 9 , — Gatesy, J. Combined support for wholesale taxic atavism in gavialine crocodylians.