A failure to confirm the rule, the exception often advances science. An article published on March 7 in the Proceedings of the American Academy of Sciences (PNAS) provides a good illustration of this. The heroine of this story is called Anelosimus eximius A spider almost like any other, judging by the photos. However, the attentive eye will be surprised to never see her alone in the pictures. Surprising, for an order reputed to be solitary. Except that among the approximately 50,000 species of spiders listed, about twenty defy the norm and display social behavior. At the house of A. superb, it is collectively that the females take care of the brood. Collectively also that hundreds, sometimes thousands of individuals construct their immense web. Collectively, finally, they move their eight legs to catch prey (flies, horseflies, butterflies but also crickets or grasshoppers…), weighing up to 700 times their weight, fallen into their silky trap.
The finding is not new. Darwin (1809-1882) already, at the end of his trip to South America, was surprised that species “so bloodthirsty” can cooperate. A few decades later, the French arachnologist and ornithologist Eugène Simon (1848-1924) described more precisely the behavior of this Guyanese spider. He revealed the existence of these huge sheets of silk, woven over several tens of meters, marrying the luxuriant vegetation of the forest, surmounted by vertical threads. By hitting these, the prey falls into the web. “This canvas is not sticky. To immobilize their prey, spiders shoot sticky silk at it. But first they must locate her and get to her before she breaks free.”summarizes Raphaël Jeanson, research director at the CNRS and ecologist at the Center for Research on Animal Cognition (CRCA-CBI, University of Toulouse).
Synchronization between individuals
In the article published in PNAS, the Toulouse researcher and his team describe in 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 web was drowned out by the noise of its conspecifics”, says Alain Pasquet, one of the authors of the article at the time. And that is precisely what the new research confirms and demonstrates.
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