The fruit fly, long the organism of choice for scientists studying genetics and basic biological processes, still harbors some secrets of its own.
A fruit fly (Drosophila melanogaster) feeding off a banana. Image credit: Sanjay Acharya via Wikimedia, CC-BY-SA-4.0
One of these is the fly’s so-called “head-heart.” This tiny pumping station supplies hemolymph—the insect equivalent of blood—to the fly’s antennae. While biologists have long known about the existence of the fly head-heart, no one had explored how it worked in much detail.
In a new study, biologists at the University of Iowa give a full accounting how the fly’s head-heart functions. The researchers discovered the fly’s head-heart beats much like a human’s: Its rhythmic pulses are automatically generated by a tiny muscle that runs through the middle of the brain, from front to back of the fly’s head, and it’s this muscle’s contraction and relaxation that drives the supply of hemolymph into the antennae.
“What we showed is that the rhythmic contraction of the head-heart is not controlled by the fly’s brain but the muscle itself,” says Alan Kay, professor in the Department of Biology at Iowa. “It’s like its own pacemaker.”
The human heart also is controlled by specialized muscle cells located within the heart. That main muscle in our chests is a wondrously synchronized metronome, taking in blood when it relaxes and forcefully pushing out blood when it contracts. That blood is then conveyed throughout our bodies by a vast, intricate network of tubes, from main branches like arteries to tiny channels called capillaries.
Insects operate differently. There is a main heart in the abdomen that supplies hemolymph, but that pump’s action doesn’t reach the extremities. To compensate, insects instead have secondary hearts, which make sure hemolymph reaches vital outer areas, such as the antennae, where smell and hearing take place.
One of these auxiliary hearts is in the fruit fly’s head. A critical part of that head-heart, the Iowa biologists found, is an elastic-walled chamber called an ampulla. When the muscle in the head contracts, it stretches this ampullar membrane, opening valves in the wall and allowing the ambient hemolymph—which had been sent to the head by the fly’s main heart—to flow into the 50-micrometer-wide ampullary chamber. When the head-heart muscle relaxes, the valves close, and the hemolymph is pushed through tubes that connect to the antennae by the tension in the elastic membrane.