By Scott Solomon, Rice University
In 2019, crop plants in Yemen began being decimated. By the end of the year, the damage had spread from the Arabian Peninsula to East Africa and as far East as Iran, India, and Pakistan. Entire fields of crops were destroyed, as well as pasture used to raise livestock like cattle, sheep, and goats. The culprit in all places was the same—the desert locust.
What Are Desert Locusts?
The desert locust has probably been a pest for people since the dawn of agriculture in the Fertile Crescent about 10,000 years ago. Plagues of locusts are mentioned in the Qur’an and the Iliad.
In the Old Testament of the Bible, locusts were one of 10 plagues that befell the Egyptians and ultimately convinced the pharaoh to release Moses and his people from slavery. Depictions of locusts can even be found in ancient Egyptian tombs.
Classifying Insects
Locusts are a type of grasshopper, which are classified in the order Orthoptera. In the system of classification used by scientists to categorize all living things based on shared features, an order is a subset of a class, which is a subset of a phylum, which is a subset of a kingdom.
Insects belong to the kingdom Animalia, the phylum Arthropoda, and the class Hexapoda. Within the class Hexapoda, which means ‘six legs’, scientists recognize about 29 different insect orders.
The vast majority of described insect species—about 93%—fall into one of six orders. The order Orthoptera are the grasshoppers, crickets, and katydids, which have about 20,000 described species.
Each order is divided into families. Within a family are the different genera (the word genera is the plural of genus). Locusts are classified in the family Acrididae. When we talk about a particular species, we always use both the genus and the species name; the desert locust is Schistocerca gregaria.
This article comes directly from content in the video series Why Insects Matter: Earth’s Most Essential Species. Watch it now, on Wondrium.
Exoskeleton: The Hard Body Wall
Locusts and other large grasshoppers in the order Orthoptera are a helpful starting point if we want to learn about the basic body plan of insects. Let’s start with the exoskeleton, the hard body wall that all insects share, even those that are soft and squishy like caterpillars.
The exoskeleton is more complex than it appears, with several layers much like our skin. The outermost layer, the cuticle, is the hardest part and is made of a substance called chitin that gives it its strength. Chitin is a polysaccharide, a long chain of carbohydrates that also contains nitrogen.
Having an exoskeleton with a tough cuticle helps insects avoid excess water loss, protects them from predators, and gives their muscles something rigid to attach to.
Below the cuticle is a layer of cells called the epidermis that secretes the chitin and other proteins that form the cuticle. In some places, like on some parts of the desert locust’s legs, the epidermis and cuticle are folded into sharp, rigid spines that serve as a defense against predators.
Insect Mouths: One of the Keys to Their Success
If we look carefully at a locust, we can see lines, or sutures, separating different sections of the exoskeleton, which are known as sclerites. Groups of sclerites form the larger body regions of an insect. Like all insects, the desert locust’s body has three main segments: the head, thorax, and abdomen.
The head has the majority of the sensory organs, including two compound eyes, a pair of antennae, and the mouthparts. Insect mouths can vary greatly in form and function and is one of the keys to their success.
The desert locust, like all orthopterans, have mouthparts specialized for chewing. A pair of mandibles with serrated edges provide a powerful cutting apparatus, allowing the locust to quickly slice through vegetation.
Behind the mandibles are a second set of paired structures called maxillae that have pointed tips and help the locust to hold its food and move it toward the mouth. Above the mandibles is the labrum, sort of like an upper lip that extends over and protects the mouth and prevents food from falling out. Below the mouth is the labium, which acts like a bottom lip and performs a similar function as the labrum.
Surrounding the mouth are two pairs of jointed appendages, the labial palps, and maxillary palps. Both pairs of palps look like miniature antennae and are used to feel and taste food. When you watch a locust eat you can see how each of these mouthparts can be moved independently yet in a coordinated fashion that makes these insects very efficient eating machines.
But how do they locate the vegetation that they eat?
The Search for Food
Locusts and other orthopterans have large compound eyes that consist of hundreds of individual eye facets, or ommatidia. Each ommatidium is like a miniature eye—each has its own lens and photoreceptors, with nerves that connect to the brain. While each ommatidium can only capture light from a small area, having hundreds of ommatidia allows the insect to form an image of its surroundings with a wide field of view.
The compound eyes of desert locusts have an additional function. Some of the ommatidia in the dorsal rim area have specialized photoreceptor cells that allow them to detect differences in the polarity of light. Desert locusts are thought to use the plane of polarized light as a navigation tool as they fly from one location to another searching for new sources of food.
Common Questions about Desert Locusts
Plagues of locusts are mentioned in the Qur’an, in the Iliad, in the Old Testament of the Bible, and even in ancient Egyptian tombs.
In the system of classification used by scientists to categorize all living things based on shared features, an order is a subset of a class, which is a subset of a phylum, which is a subset of a kingdom.
The outermost layer of the exoskeleton, the cuticle, is the hardest part and is made of a substance called chitin that gives it its strength. An exoskeleton with a tough cuticle helps insects avoid excess water loss, protects them from predators, and gives their muscles something rigid to attach to.
