Further experiments showed that the flies are extremely discerning about differences in the wasps. They preferred to lay their eggs in alcohol when female wasps were present, but not if only male wasps were in the cage.
Theorizing that the flies were reacting to pheromones, the researchers conducted experiments using two groups of mutated fruit flies. One group lacked the ability to smell, and another group lacked sight. The flies unable to smell, however, still preferred to lay their eggs in alcohol when female wasps were present. The blind flies did not make the distinction, choosing the non-alcohol food for their offspring, even in the presence of female wasps.
"This result was a surprise to me," Schlenke says. "I thought the flies were probably using olfaction to sense the female wasps. The small, compound eyes of flies are believed to be more geared to detecting motion than high-resolution images."
The only obvious visual differences between the female and male wasps, he adds, is that the males have longer antennae, slightly smaller bodies, and lack an ovipositor.
Further experimentation showed that the fruit flies can distinguish different species of wasps, and will only choose the alcohol food in response to wasp species that infect larvae, not fly pupae. "Fly larvae usually leave the food before they pupate," Schlenke explains, "so there is likely little benefit to laying eggs at alcoholic sites when pupal parasites are present."
The researchers also connected the exposure to female parasitic wasps to changes in a fruit fly neuropeptide.
Stress, and the resulting reduced level of neuropeptide F, or NPF, has previously been associated with alcohol-seeking behavior in fruit flies. Similarly, the level of a homologous neuropeptide in humans, NPY, is associated with alcoholism.
"We found that when a fruit fly is exposed to female parasitic
|Contact: Beverly Clark|
Emory Health Sciences