25.

　　B

　　(2017河北邯郸调研)

　　While studying social insect behaviour,researchers from the Agricultural University of Krakow,Poland,discovered a remarkable example of self-sacrifice in a species of ants found in Brazil.They found that the ants sacrifice themselves in order to avoid a possible threat.

　　When the sun sets,the ants seal up the entrance to their nest,but in doing so a few remain outside to kick sand over the entrance hole until it becomes invisible.These ants then die due to the cold,or get blown away in the wind.This door-sealing activity of these ants represents the first recorded case of insects sacrificing themselves through a preventive procedure.The ants sacrifice their lives every evening even if the nest isn’t under attack by enemies.The task takes about 50 minutes and the technique the ants use to kick sand backward over the hole with their hind legs is unique to the chore,strengthening the evidence that this is a premeditated activity instead of casual action.Once their task is completed,they usually quickly walk away.

　　In order to learn of the fate of these ants,the researchers placed sheets of thin plywood (胶合板) containing false openings over some real nest entrances.The ants carried on working until they had finished and were then left to wait for the dangers of the night.The researchers found that out of 23 ants working at night,only six survived until morning.The bodies of 10 were recovered,and the other seven were missing,most likely blown away and probably dead.The researchers said that the ants are probably older workers approaching death who are more unimportant than their younger nest mates.

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