Last time in sinister evolutionary leaps: catfish eating pigeons. This time, it's two different spider species working together for mysterious purposes.

A team led by Lena Grinsted of Aarhus University in Denmark...[was] studying a social species of spider called Chikunia nigra, living near Beratan Lake in Bali. Later, as they looked in more detail at their specimens, they realised its genes and genitalia revealed that it was actually two species, according to their findings just published in Naturwissenschaften [a science journal].

The two species appear to live and work cooperatively. While mutually beneficial social interaction within a particular species of spider is not unique, inter-species reciprocity has yet to be observed. "Altruism," the Economist notes, "is not a concept often associated with spiders." Yet.

Exactly what the spiders get out of being social is not clear. They do not hunt together. One explanation may be that the colony is acting like a giant [nursery].

Female spiders who were tending to eggs or young hatchlings were "surprisingly tolerant" when spiders from a different species within the colony ("what would, in most spider species, be a serious threat") was brought into their environment.

These same female spiders were also just attentive and protective of the eggs belonging to the other species as they were of their own. "Your clutch of curled, egg-bound spider babies is my clutch of curled, egg-bound spider babies," they seemed to say cheerily to one another before devouring the world.

No word on whether lead scientist Grinsted has been plagued by strange dreams since the experiments ended - a buzzing in her ears and a thirst in her throat she can't explain, and skittering visions of many-limbed sisters waiting for her whenever she closes her eyes.

"It is as though anthropologists had discovered villages populated both by human beings and chimpanzees," the Economist writes; please submit your spider-themed Planet of the Apes spec script to me by noon tomorrow.

[Picture via Getty Images]