Locusts migrate in swarms containing thousands of individuals. Despite swarm size, collision rates between locusts are generally low. The process of detecting movement is ubiquitous amongst most animals. Movements are converted into electrical signals by the eye, sent through a chain of neurons, and read by the brain. If a locust, however, detected each of the thousands of its cohorts, its brain would be over-stimulated. To remedy this, the locust has evolved to only recognize movements that will interfere with its flight path. Therefore, it only recognizes objects moving directly toward it, rather than things moving around it.
How do locusts do this? Locusts convert the movement of each object into a corresponding electrical voltage. An object coming straight toward a locust’s head has a higher voltage than an object moving parallel to it. As the locust flies, moving stimuli are converted into a voltage, and travel up the optic nerve until they hit a neuron called the lobula giant movement detector (LGMD).
At the end of the LGMD, stimuli must cross a neural synapse, connecting the LGMD to a neuron called the descending contralateral movement detector (DCMD). A synapse is like a river between two neurons, or “land-masses,” that the electrical signals need to jump across. Only certain signals are strong enough to jump across the river and make it to the brain. The LGMD-DCMD synapse is particularly wide (think Mississippi River) and stimuli must have exceptionally large voltages to jump across. In locusts, only stimuli caused by objects moving directly toward it have that voltage. All other stimuli are dismissed or sent to other areas of the brain.
This mechanism essentially eliminates visual “white noise,” allowing the locust to focus only on those objects that will interfere directly with its flight path.
This summary was contributed by Allie Miller.Edit Summary
“Locusts, which can consume their own weight in food each day, have a large neuron called the locust giant movement detector (LGMD) located behind their eyes. The LGMD releases bursts of energy whenever a locust is on a collision course with another locust or a predatory bird…The [European scientist] team found that the LGMD releases more energy when something is coming directly at the locust.
“These spikes of energy, called action potentials, prompt the locusts to take evasive action. The entire process from motion detection to reaction takes about 45 milliseconds—or 45 thousandths of a second.
“‘Locusts, like most insects, can see many more images per second than we do. This means they can react in time to things that are approaching very rapidly and so make their escape before collision,’ Rind [Claire Rind, a biologist at the University of Newcastle upon Tyne in England] said.
“And because the insects only detect things that are on a collision course with them, the locusts are ignorant of all other movements. It’s a particularly useful trait, as the locusts travel in dense swarms akin to rush hour traffic.” (Roach 2004)