The terms grasshopper and locust are frequently used interchangeably, often leading to confusion. Their relationship is a complex biological one, tied to a dramatic shift in behavior and physiology. While all locusts are a specific type of grasshopper, not all grasshoppers can become a locust. The difference is not one of species, but of a specific life-phase change that transforms certain species into the swarming pests known as locusts.
Understanding the Taxonomy
Grasshoppers and locusts both belong to the insect order Orthoptera, which includes crickets and katydids. They are classified within the family Acrididae, often referred to as the short-horned grasshoppers due to their short, stout antennae. Structurally, there is no separate taxonomic category for “locusts”; they are simply certain species of grasshoppers.
The family Acrididae contains thousands of species, but only a small fraction possess the ability to change from a solitary existence to a gregarious, swarming one. These are the species referred to as locusts, such as the Desert Locust (Schistocerca gregaria) or the Migratory Locust (Locusta migratoria). Early scientists mistakenly classified the two phases of the same species as entirely different insects because the physical differences between a solitary grasshopper and a swarming locust are so pronounced. The distinction is found in the insect’s ability to undergo a dramatic behavioral and physical transformation, not in the species name itself.
The Mechanism of Transformation
The defining characteristic of a locust is its ability to switch between two very different forms, a process known as phase polymorphism. This is a specialized type of density-dependent phenotypic plasticity. The two phases are the solitary phase, which resembles a typical grasshopper, and the gregarious phase, which is the destructive locust. This switch is triggered by environmental conditions that cause a rapid increase in population density, such as drought followed by heavy rainfall, leading to abundant vegetation and successful breeding.
The transformation begins with physical contact, which is the primary stimulus for the change. When large numbers of nymphs are forced into a shrinking area of vegetation, they begin to touch each other frequently, particularly on the hind legs. This mechanical stimulation causes a rapid increase in the neurotransmitter serotonin in the insect’s nervous system, initiating a cascade of changes. Within just a few hours, the insect’s behavior begins to shift from being repelled by other individuals to being mutually attracted, marking the onset of the gregarious phase.
The behavioral shift is quickly followed by significant physical changes. Solitary nymphs are typically green or brown for camouflage, but gregarious nymphs change to a striking black and yellow pattern. Gregarious adults develop stronger muscles for sustained flight and become more active. This collective attraction leads to the formation of dense, coordinated groups known as bands (for nymphs) or swarms (for adults), fueled by pheromones that further reinforce the aggregation.
The Behavior of Plagues
The result of this phase transformation is the formation of a “plague,” defined by massive, coordinated movement and feeding. The gregarious behavior transforms a relatively harmless, solitary insect into a highly mobile and destructive force. Once a swarm forms, it is driven by a collective movement that allows it to migrate vast distances, sometimes over 150 kilometers in a single day, riding the wind currents.
Locust swarms are notable for their staggering density and size. Large swarms historically cover hundreds of square kilometers and contain up to 80 million individuals per square kilometer. A single adult locust can consume roughly its own weight in vegetation daily. This means a large swarm can consume the amount of food necessary to feed tens of thousands of people in that same timeframe. This dense, relentless feeding pattern is what makes locust plagues agriculturally devastating and historically feared, as they strip entire fields and natural habitats bare, leading to widespread food shortages.