Desert Locust

Schistocerca gregaria is the most economically devastating locust species on Earth. The species exhibits 'phase polyphenism' — a dramatic shape, color, and behavior change triggered by population density. In its solitarious phase the animal is a calm green grasshopper that lives a normal solitary life. When density rises beyond a critical threshold, frequent contact between individuals stimulates serotonin release in the nervous system; over hours to days, the locust changes color (green to yellow with black markings), grows shorter wings adapted for sustained flight, develops a faster metabolism, and switches behavior from solitary avoidance to gregarious aggregation. A small group becomes a band; a band becomes a swarm. Mature swarms can reach 80 million locusts per square kilometer and 1,200 km² in extent — a Manhattan-sized swarm of locusts. Such a swarm consumes the food equivalent of what 35,000 people eat in a single day. Swarms travel up to 150 km daily on prevailing winds and have devastated agricultural regions from biblical Egypt (the eighth plague) to the 1915 Palestine outbreak to the 2019–2021 East African plague. The 2020 outbreak threatened the food security of 25 million people across Kenya, Ethiopia, and Somalia. The phase transition is fully reversible: as density falls, individuals revert to solitary phase within a few molts.