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An Insect Flees Danger. Suddenly, It Picks Up a Tiny Hitchhiker.

Category: Science,Science & Tech

A spring-green aphid clambers over a clot of soil, busily making its way to the shelter of a forest of plants in the distance. The insect’s long legs help it lever itself over the uneven ground at surprising speed, but if you look closely at its back, you’ll see that it has a passenger: A tiny juvenile aphid, or nymph, is riding the adult cowboy-style.

This behavior, which scientists described for the first time Wednesday in Frontiers in Zoology, results in the young one reaching the safety of a host plant much faster than it could on its own small legs. But the tactic is unpopular with the adults, who do not appreciate carrying a hitchhiker.

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Aphids generally prefer to stay in their airy roosts among the foliage of their host plants, said ecologists Moshe Gish and Moshe Inbar of the University of Haifa in Israel, the paper’s authors.

The insects drop to the ground mainly when they sense serious, unavoidable danger, heralded by the plant’s trembling and the warm breath of a grazing mammal intent on devouring their home.

Once they hit the ground, they are vulnerable to other predators, and so they rush to make it to a new host. While studying this behavior, the researchers noticed the peculiar sight of the young riding their elders.

To study the strategy more closely, they arranged a bare patch of earth surrounded by a ring of plants. Then, they held a potted plant bearing a thriving colony of aphids over the arena, shook it gently, and exhaled on it, prompting the insects to drop. With cameras rolling and the clock ticking, the researchers watched the aphids begin their march to shelter.

Many nymphs headed straight for adults and tried to climb aboard. When they clung to the adults’ legs or lower back, the adults shook them off, or tried to. The cowboy riding position, with the nymph centered on the adult’s upper back, seemed to be a sort of compromise.

“That was actually the least irritating position for the adult,” said Dr. Gish.

The senior aphids wasted time trying to get the young off as the nymphs crawled on their antennas and limbs, but once a nymph got all the way up into that spot, an adult was likely to cut its losses and just start moving toward the plants.

Rider nymphs reached safety about four times faster than nymphs walking on their own, suggesting that the behavior aids in their survival. Adults bearing a nymph, once they actually started walking, were no slower than adults without passengers.

“We would expect that this added burden on the adult would slow it down, but actually it’s not that much,” said Dr. Gish.

You might think that the adults, despite their irritation, are carrying the young out of some instinct to save their families. But experiments with groups of aphids whose members were closely related and groups with a variety of different genetic backgrounds showed no difference in the behavior.

Even if it is not pleasant for the adults, however, the behavior probably does help the group survive in the long run, said Dr. Gish.

“For the nymphs, it’s life or death,” he said. “For the adult, even if it does slow them down a little bit, it’s probably not that significant.”

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