A locust is a grasshopper that has undergone a dramatic physical and behavioral transformation in response to crowding. There is no separate taxonomic group called “locusts.” Certain grasshopper species carry the ability to shift between a solitary form, which looks and acts like any ordinary grasshopper, and a gregarious form that is darker, bulkier, and prone to forming enormous swarms. This shift, known as phase polyphenism, is what makes a locust a locust, and it changes nearly everything about the insect’s appearance, chemistry, and behavior.
A Grasshopper With a Switch
The word “locust” does not describe a fixed kind of insect the way “beetle” or “ant” does. It describes a state. Locust phase polyphenism is an extreme form of density-dependent plasticity in which solitary, camouflaged grasshoppers transform into gregarious, conspicuous locusts when local population density rises.1PubMed. Phylogeny of bird-grasshopper subfamily Cyrtacanthacridinae (Orthoptera: Acrididae) and the evolution of locust phase polyphenism Of the roughly 12,000 grasshopper species on Earth, only about two dozen have this ability. The most studied are the desert locust (Schistocerca gregaria), which devastates crops across Africa and the Middle East, the migratory locust (Locusta migratoria), which ranges across Africa, Asia, and parts of Europe, and the Australian plague locust (Chortoicetes terminifera). Each species has its own triggers and quirks, but they all share the core feature: an individual animal can look and behave like two completely different insects depending on how many neighbors it has.
What the Two Phases Look Like
In the solitary phase, a locust-capable grasshopper is cryptically colored, typically in greens and browns that blend into surrounding vegetation. Its body is relatively slender, its hind legs are proportionally long, and its wings may be shorter relative to its body. It avoids other grasshoppers and tends to be nocturnal or crepuscular, staying hidden during the day. If you picked one up in a field, you would have no reason to think it was anything other than an unremarkable grasshopper.
The gregarious phase is strikingly different. The insect’s body becomes darker, often turning shades of yellow, orange, or black. In juvenile stages (called nymphs or “hoppers”), gregarious desert locusts display bold black-and-yellow banding that serves as a warning signal. Adults in the gregarious phase tend to have a stockier build with a wider thorax, relatively shorter hind legs, and longer wings suited for sustained flight. Their overall proportions shift to favor movement and endurance over hiding. The color change is not cosmetic; it reflects real changes in the cuticle’s pigmentation driven by shifts in gene expression and hormonal signals.
Size varies by species but falls in a range familiar to anyone who has seen a large grasshopper. Adult desert locusts are typically around 6 to 8 centimeters long, with females slightly larger than males. Wing span can reach roughly 10 to 12 centimeters in the gregarious form, making them competent long-distance fliers.
What Triggers the Transformation
The switch from solitary to gregarious is not gradual in the way growth is gradual. It begins with a specific sensory event: physical contact with other grasshoppers. In desert locusts, the critical trigger is touch on the outer surface of the hind legs. Researchers found that repeatedly stimulating the outer face of a hind femur caused a significant shift from solitary to gregarious behavior, while touching ten other body regions did not produce the same effect.2PubMed. Gregarious behavior in desert locusts is evoked by touching their back legs The conclusion was straightforward: when environmental conditions push grasshoppers into close quarters, they inevitably bump into each other’s hind legs, and that repeated contact is the primary sensory input that seeds swarm formation.
Follow-up work clarified that the gregarizing signal is not purely about feeling something on the skin. It combines an external touch component with proprioceptive feedback from the leg’s own movement. Touching just a quarter of the outer surface of one hind femur was enough to produce full behavioral gregarization within four hours, but only if the locust could move its leg in response. Restrained locusts that received the same touch did not change phase.3PubMed. Mechanosensory-induced behavioural gregarization in the desert locust Schistocerca gregaria The insect’s nervous system seems to require both the external sensation and its own motor response to register the signal as “I’m in a crowd.”
This mechanism is not universal across locust species. In the Australian plague locust, the same behavioral shift is triggered by touch to the antennae, not the hind legs, and visual or chemical cues alone did not cause the switch.4PubMed. Behavioural phase change in the Australian plague locust, Chortoicetes terminifera, is triggered by tactile stimulation of the antennae The fact that two locust species evolved the same dramatic phase change but wired it through entirely different sensory inputs is a striking example of convergent evolution. The outcome is the same (crowd detection flips a behavioral switch), but the hardware that detects the crowd differs.
Brain Chemistry Behind the Switch
The sensory triggers described above set off cascading changes inside the insect’s brain. One of the more counterintuitive findings involves serotonin, a neurotransmitter most people associate with mood regulation in humans. In the migratory locust, serotonin does not drive the shift toward gregariousness. Instead, serotonin levels in the brain rise during isolation of gregarious individuals, and injecting serotonin into the brains of crowded gregarious nymphs had no effect on their behavior. However, injecting serotonin into gregarious nymphs that had already been isolated accelerated their return to the solitary state.5PubMed Central. Serotonin enhances solitariness in phase transition of the migratory locust In other words, serotonin appears to reinforce solitary behavior rather than trigger gregarious behavior in the migratory locust.
This is worth pausing on because earlier research in the desert locust suggested serotonin played a role in driving gregarization. The picture that has emerged is that the neurochemistry of phase change is species-specific. The migratory locust and the desert locust, despite producing similar outward transformations, may rely on different molecular pathways to get there. The broader point for anyone trying to understand locusts is that “locust” is less a single biological phenomenon than a suite of independently evolved solutions to the same ecological problem: how to exploit sudden increases in food and population density.
How Behavior Changes With Appearance
The physical transformation is only half the story. Solitary-phase grasshoppers actively avoid each other. When they encounter another individual, they tend to move away. Gregarious-phase locusts do the opposite: they are attracted to other locusts and actively seek proximity.6PubMed Central. Locust dynamics: behavioral phase change and swarming This behavioral reversal is what makes swarms possible. A billion insects that all want to be near each other will naturally coalesce into bands (on the ground) or clouds (in the air) that move as coordinated masses.
Chemical signaling helps sustain and amplify this gregariousness once it gets started. Migratory locusts in the gregarious phase release an aggregation pheromone called 4-vinylanisole (4VA). Exposure to this compound accelerates the behavioral shift from solitary to gregarious by increasing the insects’ tendency to approach and make body contact with each other.7PubMed Central. 4-Vinylanisole promotes conspecific interaction and acquisition of gregarious behavior in the migratory locust The result is a positive feedback loop: crowded locusts produce pheromones that attract more locusts, which produces more crowding and more pheromone. This is how small local aggregations can snowball into swarms covering hundreds of square kilometers.
Gregarious locusts are also bolder, more active during the day, and willing to eat a wider range of plants. Solitary individuals tend to be pickier eaters and more cautious about moving through open space. In hopper bands, where juvenile gregarious locusts march together on foot before they can fly, individuals align their movement and maintain consistent spacing, which gives the bands a strangely organized, flowing quality.
Where Locusts Live and Why Swarms Appear
Locust outbreaks tend to follow specific environmental patterns. For desert locusts, the cycle usually begins with unusually good rains in semi-arid regions. Rain triggers vegetation growth, which lets grasshopper populations boom. As vegetation dries out and patches shrink, the growing population is funneled into smaller and smaller areas of remaining greenery. This crowding is exactly the condition that triggers phase change. Within weeks, what was a scattered population of solitary grasshoppers becomes a dense, organized swarm of gregarious locusts.
Desert locusts have a breeding range that spans parts of North Africa, the Middle East, and South Asia, roughly 30 million square kilometers in total. When conditions align, swarms can travel thousands of kilometers, crossing oceans and national borders. A single large swarm can contain billions of individuals and consume enough vegetation in a day to feed tens of thousands of people. The economic and humanitarian damage from major outbreaks is enormous, particularly in East Africa and the Indian subcontinent where subsistence farming is widespread.
The migratory locust has an even broader range, found on every continent except Antarctica and the Americas. The Australian plague locust is confined to Australia but can cause severe agricultural damage there. Other species with locust-like phase change exist in South America, Central Asia, and southern Africa, though they tend to receive less international attention.
Monitoring Outbreaks From Above
Detecting locust outbreaks early enough to respond has always been one of the central challenges. Much of the breeding habitat for desert locusts is remote, politically unstable, or both, making ground surveys difficult and dangerous. Satellite-based remote sensing has become increasingly important for tracking the environmental conditions that precede outbreaks, such as rainfall patterns and vegetation greenness. Open-access satellite data archives and cloud computing have made this kind of monitoring more accessible and faster than it was even a decade ago, and unmanned aerial vehicles are opening up new possibilities for direct survey and targeted control.8PubMed Central. Application of Remote Sensing Data for Locust Research and Management-A Review
The UN’s Food and Agriculture Organization maintains a global Desert Locust Information Service that integrates satellite data with ground reports to issue forecasts and warnings. Early detection matters because intervening during the initial gregarization phase, when locust bands are still small and localized, is far cheaper and more effective than trying to suppress a full-blown swarm that has already taken flight.
Common Misconceptions
Several popular misunderstandings about locusts persist. The most common is that locusts are a separate species from grasshoppers. They are not. A locust is a grasshopper in a specific phase state, and the same individual can shift between phases during its lifetime. If conditions become less crowded, gregarious locusts can gradually revert to the solitary form over subsequent generations.
Another misconception is that all grasshoppers can become locusts. The vast majority cannot. Phase polyphenism is a specialized trait that has evolved independently in a handful of lineages within the grasshopper family Acrididae. Your garden grasshopper, no matter how many friends it has, will never form a swarm.
A third misunderstanding involves cicadas. In parts of North America, periodical cicadas that emerge in massive numbers every 13 or 17 years are sometimes called “locusts,” but they are not related to true locusts. Cicadas belong to an entirely different order of insects (Hemiptera, the “true bugs”), while locusts are orthopterans, closely related to crickets and katydids. The confusion dates back centuries and persists in casual speech, but the two groups have almost nothing in common biologically.
Locusts as Food
Humans have been eating locusts for millennia, which makes practical sense: they are large, abundant during outbreaks, and nutritionally dense. Roughly ten of the known locust species have been traditionally consumed by humans or used as animal feed in at least 65 countries.9Global Food Security. Global overview of locusts as food, feed and other uses Their nutritional profile is competitive with conventional meat, with protein content in grasshoppers and locusts reported across a wide range depending on species and preparation, and a fatty acid profile that can be rich in polyunsaturated fats beneficial for human health.10Annals of Animal Science. The Nutritional Value of Grasshoppers and Locusts – A Review
There is growing interest in harvesting locusts during outbreaks as both a food source and an alternative to blanket insecticide spraying, which is the current default response. The logic is compelling on paper: you turn an agricultural disaster into a protein harvest while reducing the environmental damage from pesticide use. In practice, the safety issues are real. Locusts collected after insecticide application can carry dangerous chemical residues, and even untreated locusts can harbor microbial contaminants or trigger allergic reactions in people sensitive to arthropod proteins.9Global Food Security. Global overview of locusts as food, feed and other uses Removing wings, legs, and antennae before consumption has been shown to reduce allergenicity.10Annals of Animal Science. The Nutritional Value of Grasshoppers and Locusts – A Review
Several countries are developing formal regulatory frameworks for edible insects that include locusts. For regions where locust outbreaks cause food insecurity, the ability to safely harvest and sell the insects could turn a crisis into an economic opportunity, though scaling that idea up requires infrastructure and food safety protocols that many affected regions currently lack.
Why the Phase Change Evolved at All
From an evolutionary perspective, the question of why some grasshoppers evolved this extraordinary ability while most did not is still debated. The leading idea is that phase polyphenism is an adaptation to boom-and-bust environments, particularly semi-arid regions where rainfall is unpredictable. When rain arrives, plant growth explodes, and grasshopper populations can surge. But the good times are short. As vegetation recedes, individuals are forced together, and the ability to shift into a gregarious, mobile form that can collectively migrate to new food sources becomes a survival advantage. Solitary grasshoppers in the same shrinking habitat simply die in place. Gregarious locusts move.
The phase change has evolved independently multiple times across different grasshopper lineages, which suggests the selection pressure is strong and the underlying genetic architecture is achievable through multiple routes.1PubMed. Phylogeny of bird-grasshopper subfamily Cyrtacanthacridinae (Orthoptera: Acrididae) and the evolution of locust phase polyphenism The fact that different species use different sensory triggers (hind legs in desert locusts, antennae in Australian plague locusts), different neurochemical pathways, and slightly different physical transformations, yet all arrive at functionally similar swarming behavior, makes the locust one of the more remarkable examples of convergent evolution in the insect world.