Locust

LOCUST, the name applied to certain insects belonging to the family Acridiidae of the order Orthoptera (q.v.). The idea of a destructive insect is universally associated with this term, and in its strict usage it is confined to those species of Acridiidae which at times greatly increase in numbers and migrate considerable dis tances in large swarms. In Europe the expression locust is asso ciated with insects of large size, while the smaller members of the Acridiidae are known as grasshoppers (q.v.). In North America the latter insects are very commonly termed locusts, although in most species migratory habits are undeveloped. Unfortunately the term locust has also been applied to the periodical cicada which belongs to the order Hemiptera.

One of the chief Old World locusts is Locusta (Pachytylus) migratoria and, according to the theory of Uvarov, this insect ex ists in two phases—a destructive migratory phase and a less harm ful, solitary one. In temperate regions one of its chief breeding grounds is in the vast reed-beds in the deltas of the rivers flowing into the Caspian and Aral seas and Lake Balkash. From these regions the insect, in its migratory phase (fig. 1), issues in huge swarms and the resulting progeny develop into the solitary phase; the latter, given suitable conditions, gives rise to the migratory phase over again. In its migratory phase L. migratoria is rare in western Europe but extends eastwards to the Philippines. Its solitary phase is often regarded as a distinct species and is termed by Uvarov the form danica; it ranges from Belgium to Japan and New Zealand, while occasional stragglers have occurred in Eng land, and it is common in southern Europe. In South Africa the common locust is Locusta pardalina, which likewise is found in two phases, but we know little of its permanent breeding grounds.

Among other Old World locusts Schistocerca gregaria (pere grina) (fig. 2) is essentially an African insect, but also extends into western Asia; its permanent breeding grounds are unknown. Pa tanga (Acridium) succincta is the Bombay locust, prevalent in India and ranging through southern Asia to the Malay archipel ago. Calliptamus. italicus (fig. 3) is a smaller insect, but often very destructive in southern Europe and northern Africa.

In North America the majority of the so-called locusts inhabit and breed in the same general area throughout the year, but several species are migratory locusts. These latter breed on the grassy slopes of the Rocky, Cascade and Sierra mountains and leave their breeding grounds in vast swarms when the grass dries up in the summer. The extension of cultivation in their breeding areas, however, has resulted in locust swarms becoming far less frequent than formerly. The Rocky mountain locust (Melanoplus spretus)

has been in the past one of the greatest insect scourges of the United States, but to-day true migrations are practically unknown. Although a small insect, not so large as many English grasshop pers, its destructiveness has procured for it great notoriety. The lesser migratory locust (Melanoplus atlantis) is widely spread in North America and is sometimes a serious pest. This species and Cananula pellucida are at times exceedingly destructive in the prairie provinces of western Canada. Among other species Me lanoplus devastator is the most serious locust in California and the large Schistocerca americana occurs in the southern States where it sometimes assumes migratory habits.

The eggs of locusts are deposited in long, cylindrical masses; each mass is enveloped in a glutinous secretion and placed in a hole in the earth excavated by the female (fig. 4). The young nymphs after hatching soon commence their destructive career, which continues throughout the life of the insect. After a series of gradual growth changes, ac companied by periodical moults, the winged stage is attained. In this condition their capacity for destruction is commonly accom panied by the development of gregarious migratory instincts. The factors governing migration are very imperfectly under stood and have been much dis I cussed. In the case of Locusta migratoria the best conditions for the insect are afforded by moist reed-grown areas in a rather hot, damp climate. Under these conditions the migratory phase develops, and when it is at its height large swarms are formed. Such swarms periodically emigrate, and the causes of migration are believed to be internal physiological changes in the insects themselves and not to lack of food resources ; such locusts feed little during migration and mostly exist at the expense of their fat bodies. The cessation of flight does not appear to be dependent upon the discovery of suit able territory, and it has also been explained as being the result of physiological changes, including the approaching maturity of the reproductive organs. Once a swarm has settled and commenced breeding, vegetation is rapidly destroyed and the resulting progeny are of the solitary or danica phase. It thus follows that if the per manent breeding grounds are altered by cultivation, conditions are rendered unsuitable for the development of the migratory phase and swarms will no longer occur. The migrations of Ameri can locusts appear to follow a definite scarcity of food.