The term "locusts" refers to a morphologically and behaviorally distinct swarming phase that exists in a few a species of short-horned grasshopper which are normally solitary. 

"Swarming behaviour is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin.[13] This causes the locust to change colour, eat much more, and breed much more easily. The transformation of the locust to the swarming form is induced by several contacts per minute over a four-hour period."

https://en.wikipedia.org/wiki/Locust


haggadah Section: -- Ten Plagues