Grass-hoppers or locusts?

Water and earth analogy

There is no taxonomic difference between locust and grasshopper species, but, in English, the term “locust” is used for notorious species that change morphologically and behaviourally on crowding, to form swarms or bands.

The word “locust” is derived from the Vulgar Latin “locusta”, which was originally used to refer to various types of crustaceans and insects. Spanish has mostly preserved the original Latin usage, since the cognate term “langosta” can be used to refer both to a variety of lobster-like crustaceans and to the swarming grasshopper, while semantic confusion is avoided by employing qualifiers such as “de tierra” (of the land) when referring to grasshoppers, “de mar” and “del rio” (of the sea/of the river) when referring to lobsters and crayfish respectively. French presents a similar case: during the 16th century, the word “sauterelle” (literally “little hopper”) could mean either grasshopper or lobster (“sauterelle de mer”).

In short, there was an ancient analogy between the gregarious grass-hopper and the lobster, i.e., between the earth locust and the water locust.

 

Two phases of the same species

Solitary and gregarious locusts had been previously thought of as separate species, till being known they were phases of the same species. Sir Boris Petrovich Uvarov, who studied the desert locust, identified for the first time its phase polymorphism.

The species’ two phases – one solitary and the other gregarious – can be distinguished by differences in coloration, form, physiology, and behaviour.

A solitary phase nymph, for example, adjusts its coloration to match that of its surroundings, does not collect in groups, has low metabolic and oxygen-intake rates, and is sluggish. A gregarious phase nymph, on the other hand, has black and yellow or orange coloration in a fixed pattern, gathers in large groups, has high metabolic and oxygen intake rates, and is active and nervous.

Adult locusts differ more in form than in colour. The solitary phase has shorter wings, longer legs, and a narrower pronotum, or dorsal sclerite (with higher crest and larger head), than gregarious phase individuals. The adult of this phase has a more saddle-shaped pronotum, broader shoulders, and long wings.

When a nymph of a solitary phase locust matures in the presence of many other locusts, it undergoes a physiological change and produces offspring of the gregarious type. If crowding is sufficiently dense and of long enough duration, the majority of a local population will shift to the gregarious migratory phase.

The young of a gregarious phase locust, on the other hand, will produce offspring that revert to the solitary phase if it matures in isolation.

 

Gregarious plague occurs in unfavourable environment

Normally, grass-hoppers remain in what is called a “solitary” phase. The solitary phase is the normal state of the species, with the gregarious phase being a physiological response to violent fluctuations in the environment.

The swarming or plague behaviour results from overcrowding. If favourable breeding conditions cause more young to hatch than usual, resulting in overcrowding and scarcity of food, the locusts migrate to regions where the habitat is unsuitable and the insects will go through a “phase change”. Migratory swarms do not form in regions favourable for the growth of a species. Instead they form in regions in which suitable habitats are scarce.

The largest swarms can cover hundreds of square miles and consist of many billions of locusts. The desert locust (Schistocerca gregaria) inhabits can fly upward to about 1.500 metres in huge towers of individuals. In 1869, desert locust swarms reached England, probably from West Africa, and a flight across the Red Sea, in 1889, was estimated to be about 5.000 square km (2.000 square miles) in size. The long-distance dispersal of these swarms is usually associated with either frontal winds of storm systems or high-level jet-stream winds. The acridids typically fly almost straight up into these fast-moving winds and then are carried with the winds until they slow to the point where gravity overcomes wind speed, causing them to drop from the sky.

A gregarious phase locust is restless and irritable, and it flies spontaneously on warm, dry days, when its body temperature is high. The muscular activity of flight, tickling their thoraxes and increasing the tactile stimulation of the hind legs, further raises its temperature.

The transformation of the locust to the swarming variety is induced by several contacts per minute over a relatively long period (four hours). When desert locusts meet up in concentrated numbers, exposed to the sight and smell of others, their nervous systems triggers the release of serotonin, which causes them to become even more agitated and mutually attracted, a prerequisite for swarming. This leads to the second state, called the “gregarious” phase, when locusts gather and finally evolve into a single migrating swarm, or plague. Serotonin converts normally solitary desert locusts into sociable, swarming insects, capable of mass destruction of crops.Serotonin is a neurotransmitter that regulates gastro-intestinal movements and is involved in multiple body functions including aggression, pain, sleep, appetite, anxiety, depression, migraine, and vomiting; it is also used as an anti-depressant drug. High levels of this brain chemical causes serotonin syndrome.

If we can find an equivalent to that human syndrome in locusts, “drugged”, let us say, by serotonin, it causes them to change color, eat much more, and breed much more easily.  

But, how to explain the sporadic appearance and disappearance of locust swarms?

A succession of favourable seasons enables a population to expand in numbers so that individuals are forced into marginal areas.

When unfavourable environmental conditions occur in the marginal regions, individuals are forced to return to smaller, permanently habitable areas, resulting in crowding and triggering the physiological shift to the gregarious form.

A swarm ceases flying only when environmental conditions change; e.g., rain falls, temperature decreases, or darkness occurs.

Once a plague of locusts is formed, it is very hard to control. “Usually what ends them is 11.000 litres of insecticide”, says Stephen Rogers, a biologist at the University of Cambridge, UK.