"There's a division of labor in who digests what," Brennan said.
The finished saliva has several times more carbohydrates and amino acids (the building blocks of proteins) than traditional insect meals, such as blood or plants.
According to Brennan, adult paper wasps likely evolved the vibratory signal as a threat to elicit more saliva. The implicit message to larvae: Produce little or no saliva, and the adult may turn to cannibalism. For the larvae, the signal helps identify which individuals in the nest are legitimate feeders.
"There's a whole suite of potential recipients of this saliva, which are unintended or not desirable from the larvae's point of view," Brennan said. For example, parasites and other insect competitors might invade the wasps' nest and try to help themselves to a snack.
Using video cameras and equipment that records vibrations, Brennan learned that adult wasps produce the signal by scraping their abdomens across the nest. This information allowed him to reproduce the sound and study its significance.
When Brennan removed young paper wasps from their nests, resident adults immediately stopped their wagging. The signaling began again when larvae were returned.
Brennan then fluctuated the adult's food supply to prove the wagging signal was a call for a meal. Wagging was greatest when the wasps were hungriest.
Most surprisingly, Brennan found that larvae control how much saliva they produce, and they produce more when they receive a vibration.
The larvae learn how much saliva to give based on prior experience and adjust the amount accordingly, Brennan said. For example, when they interact with an invading insect, which takes saliva without signaling, the larvae learn to reduce the amount they give to non-signalers.
Cocroft, the University of Missouri biologist, noted this finding is intriguing "because people often think of insects as so much simpler than vertebrates. Findings of insects making adaptive but subtle distinctions like this show that, while they are very different from vertebrates, their behavior isn't necessarily simpler."
Brennan hopes to focus next on whether paper wasp larvae vary how much saliva they give to certain adult wasps within the nest. After all, the more nutrients the larvae keep for themselves, the stronger and more competitive they'll be when they're grown.
"Inherent in the transaction is some kind of repercussion for not pulling your end of the deal," he said. "It would be fascinating to learn how much the larvae take that into account when they decide how much to give versus receive."
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