The frequent appearance of large insect populations, whether through historical accounts of devastating plagues or recent mass emergences, often leads to confusion between two very different creatures: the locust and the cicada. Both insects can appear in overwhelming numbers and generate significant noise, causing many people to mistakenly use their names interchangeably. Acknowledging the long-standing misnomer, especially the term “17-year locust,” this comparison provides a straightforward look at the fundamental biological and behavioral differences that separate these two insect groups. Understanding their distinct characteristics clarifies why they look, act, and sound so differently.
Taxonomic Identity and Classification
The fundamental difference between these two insects lies in their scientific classification, as they belong to entirely separate insect orders. Locusts are specific species of short-horned grasshoppers, placing them within the order Orthoptera, a group that includes crickets and katydids. These insects are characterized by chewing mouthparts and powerful hind legs adapted for jumping.
Cicadas, conversely, are classified in the order Hemiptera, commonly known as the “true bugs,” a group that also contains aphids and leafhoppers. The distance between Orthoptera and Hemiptera means a locust is no more closely related to a cicada than a butterfly is to a beetle.
Physical Differences in Morphology
Locusts possess a body structure optimized for mobility and consumption, presenting as slim and elongated, typical of a grasshopper. They have robust hind legs designed for powerful leaping, which is a primary mode of movement. Their antennae are short, and their mouthparts are mandibles built for chewing and quickly consuming large quantities of plant foliage.
Cicadas have a much stouter, bulkier body shape, often described as rounded or barrel-like. They feature broad heads with large, compound eyes set wide apart. Their legs are not specialized for jumping; instead, their forelimbs as nymphs are adapted for digging through soil. The adult cicada has a sharp, needle-like proboscis, called a rostrum, which it uses to pierce plant tissue and suck up xylem fluid instead of chewing.
Life Cycle and Behavior
The behavioral and life cycle differences are the most significant distinction, particularly in how each insect interacts with its environment. Locusts exhibit phase polymorphism, allowing them to switch between a solitary phase and a gregarious phase based on population density. Overcrowding triggers a neurological change, mediated by serotonin, causing them to transform in color, morphology, and behavior.
The gregarious phase results in the formation of highly mobile and destructive “swarms” that migrate vast distances, rapidly consuming all available vegetation. Conversely, the cicada life cycle centers on an extended nymph stage spent underground, feeding on fluid from tree roots. Periodical cicadas (Magicicada) remain subterranean for prime-numbered cycles of 13 or 17 years before emerging almost simultaneously.
This synchronized, massive appearance is an “emergence,” not a true swarm, as the adults do not migrate or destroy crops like locusts. The primary damage caused by cicadas is limited to the female laying eggs in small slits cut into tree twigs, which can cause the twig tips to die, a process known as “flagging.” Upon emergence, adult cicadas live for only a few weeks, focusing solely on mating.
Sound Production and Communication
The sounds produced by these two groups are created through entirely different mechanical processes, resulting in distinguishable acoustic signatures. Male locusts generate sound using stridulation, which involves rubbing two body parts together. They create noise by scraping a row of pegs (a file) located on the inner surface of the hind femur against a hardened vein (a scraper) on the forewing.
This friction produces a chirping or ticking sound, often used for communication. Cicadas, however, are known for their much louder, continuous buzzing or whining songs, produced by specialized organs called tymbals. Tymbals are a pair of ribbed, drum-like membranes located on the male’s abdomen.
Muscles rapidly contract and relax, causing these membranes to buckle inward and snap back out, creating rapid clicks that blend into a continuous sound. The cicada’s abdomen is often hollow, acting as a resonant chamber to amplify the sound. Some species reach volumes over 100 decibels, making them among the loudest insects in the world.