Failing to confirm the rule, the exception often advances science. An article Posted on March 7 in the reports of the American Sciences Academy (PNA) offers a beautiful illustration. The heroin of this story is named Anelosimus Eximius. A spider almost like the others, if we judge by the photos. The attentive eye will be surprised, however, never to see it alone on the clichés. Astonishing, for a solitary renowned order. Except that among the approximately 50,000 species of spiders listed, twenty challenges the norm and has social behavior. At A. Eximius, it is collectively that females bring care to the brood. Collectively also that hundreds, sometimes thousands of individuals build their immense canvas. Collectively, finally, that they move their eight legs to catch the prey (flies, taons, butterflies but also locusts or grasshoppers …), weighing up to 700 times their weight, fallen in their silky trap.
The observation does not date from yesterday. Darwin (1809-1882) already, at the end of his trip to South America, astonished that “as bloodthirsty” species can cooperate. A few decades later, French arachnologist and ornithologist Eugène Simon (1848-1924) described more precisely the behavior of this Guyanese spider. He revealed the existence of these huge tasks of silk, woven over several tens of meters, marrying the lush vegetation of the forest, surmounted by vertical threads. By hitting the latter, prey falls into the web. “This canvas is not sticky. To immobilize their prey, the spiders project on it a glutinous silk. But first they must locate it and join it before it is released,” says Raphaël Jeanson, Director of Research At the CNRS and ecologist at the Animal Cognition Research Center (CRCA-CBI, University of Toulouse).
Synchronization between individuals
In the article published in PNAS, the Toulouse researcher and his team describe by detail the curious strategy adopted by the “pack”. An alternation of rapid movements and synchronized stops. This ballet had already been observed in 1992 by researchers from the University of Lorraine, who had even proposed an explanation model. “Our hypothesis was that each spider stopped moving when the signal of the prey on the canvas was covered by the sound of its congeners,” says Alain Pasquet, one of the authors of the article of the time. And that’s exactly what the new research confirms and demonstrates.
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