life science
What Makes Them Tick: The Fruit Fly’s Internal Gyroscope By Harris Davis
Illustration by Hannah Kennedy
There’s something special about flying. Birds, reptiles, mammals, and insects have been flying for millions of years, and yet until very recently in our evolution, the physics of flying have eluded human beings. Animal flight mechanisms have fascinated biologists since Darwin, and have been studied in a diverse range of species. For the first time, neuroscientific research at UNC is revealing the role of otherwise poorly understood mechanisms of flight control in fruit flies. The fruit fly (D. melanogaster) has long been of particular interest to researchers for a unique and deceptively simple aspect of its flight. A few centuries ago, a pair of tiny structures that twitch and vibrate during flight were discovered on the fly’s thorax. It was later found that the removal of these structures completely compromises the fly’s
Figure 1: (A) Map of haltere muscles, where hDVM is the power muscle. Steering muscles include basalares (hB1and hB2) and axillaries (hI1–hIII3). (B) Experimental setup of LED arena, which tracks wing motion and images muscle activity. (C) amplitude and muscle activity in axillaries and basalares when visual field simulates rotation to the left (red) and right (blue).
ability to get off the ground.1 And unlike many insects—which have two sets of wings—flies have only one, along with a pair of these appendages that came to be known as halteres. Despite these observations, the function of halteres in flies has largely remained a mystery. Approximately 80 years ago, however, it was proposed that halteres may function as a kind of biological gyroscope, maintaining the fly’s balance midair.3 This is one of the hypotheses that Dr. Brad Dickerson and his team have spent the last year exploring at UNC. When they began their work, the lab didn’t just want to verify the halteres’ function as a gyroscope; they wanted to know exactly how sensory input causes changes in muscle motion during flight. Furthermore, they wanted to investigate the halteres’ potential role in the timing of wing motion.2 Because of the high speed at which the fruit fly beats its wings—200 to 250 times per second, the team knew that motor circuits commonly involved in timing the gaits of other animals were unlikely to control fruit fly wing strokes. Thus, the fly must have a faster, more reflexive way to control motor activity. First, Dr. Dickerson’s team explored the relationship between visual inputs that trigger neural signals to the haltere muscles. They then looked at how those signals in turn provide immediate feedback to the wing muscles to determine if neural information from the halteres to the wing muscles plays a significant role in controlling wing motion. Figuring out which inputs trigger feedback is key to interpreting the function of any biological structure. To understand how halteres regulate flight, it is first necessary to consider the nerves that make them tick. Each of the halteres has one “power muscle” for generating force, and a group of “control muscles” for fine tuning the direction of that force.1,2
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Brad Dickerson Ph.D., Assistant Professor in UNC Biology Department and Kenan Honors Fellow
Each muscle is controlled by a single motor neuron. A “firing” of that neuron triggers a contraction of the corresponding muscle. These firings are triggered by physical input from sensors, which line the fly’s wings and halteres, and from visual input transmitted from the eyes to the brain.1 While this may seem relatively straightforward, the key to completely understanding how motor changes occur in response to stimuli has remained hidden in the difficult-to-analyze neural feedback mechanisms between halteres and wings. Past analyses of the motion of haltere muscles in response to stimuli have largely failed because the physical movement of the structures is so finely
Figure 2: Diagram of halteres on D. melanogaster.
