It is well established that locusts of the migratory species exist in countries without giving rise to swarms, or causing any serious injuries; thus Pachytylus cinerascens—perhaps the most important of the migratory locusts—is always present in various localities in Belgium, and does not give rise to swarms. When migration of locusts does occur it is attended by remarkable manifestations of instinct. Although several generations may elapse without a migration, it is believed that the locusts when they migrate do so in the direction taken by predecessors. Their movements are to a large extent dependent on the wind, and it is said that they make trial flights to ascertain its direction. When on the wing probably very little muscular effort is necessary. Their bodies contain elastic air sacs in communication with the tracheae, and at the time of flight it may be presumed that the body is comparatively empty, food being wanting, and the internal organs of reproduction, which occupy a large space when in activity, yet undeveloped, hence the sacs have full room for expansion, as explained on p. [283]. Thus the Insects exert but little effort in their aerial movements, and are, it is believed, chiefly borne by the wind. Should this become unfavourable it is said that they alight and wait for a change.
The most obscure point in the natural history of the migratory locusts appears to be their disappearance from a spot they have invaded. A swarm will alight on a locality, deposit there a number of eggs, and then move on. But after a lapse of a season or two there will be few or none of the species present in the spot invaded. This appears to be partly due to the young locusts dying for want of food after hatching; but in other cases they again migrate after growth to the land of their ancestors. The latter fact is most remarkable, but it has been ascertained by the U.S. Entomological Commission that these return swarms do occur.
Fig. 176.—Portions of body of Caloptenus spretus to show some of the air-sacs. (Modified from Packard.) A, Dorsal aspect of anterior parts; B, lateral aspect of posterior parts of body; a, enlargements of tracheae in head; b, pair of large sacs in thorax; c, sacs on the tracheal trunks of abdomen; s, spiracles.
In South Africa it would appear that the movements of the migratory locusts are frequently made before the Insects have acquired their wings. Mrs. Barber, in an account of "Locusts and Locust-Birds in South Africa,"[[227]] has illustrated many points in the Natural History of these Insects. The South African species manifests the gregarious and migratory disposition when the individuals are quite young, so that they travel in flocks on foot, and are called by the Dutch "Voetgangers." After hatching, the various families of young amalgamate, so that enormous numbers come together. Having denuded the neighbourhood of all its food-supplies, they move off in search of fresh crops and pastures new. They take advantage of roads, and sometimes a good many miles will be traversed in a day; they proceed by means of short leaps, rapidly repeated. When the "Voetgangers" are thus returning northwards towards the lands in the interior from which their progenitors departed, no obstacles can stay their course. Forests or rivers may intervene, diverting them for a while from their line of march, but they succeed ultimately in continuing their journey to the interior.
The manner in which these wingless locusts occasionally cross broad rivers is interesting, as it has some bearing on the difficult question of the possibility of winged locusts crossing seas of considerable width. Mrs. Barber refers to an instance that took place on the Vaal River in the spring of the year 1871, shortly after the discovery of the Diamond-fields. The country was at that time swarming with young locusts; every blade of grass was cleared off by them. One day a vast swarm of the "Voetgangers" made their appearance on the banks of the Vaal River; they appeared to be in search of a spot for crossing, which they could not find, the river being somewhat swollen. For several days the locusts travelled up the stream; in the course of doing this they paused for some time at an abrupt bend in the river where a number of rocks were cropping out, as if in doubt whether to attempt a passage at this place. They, however, passed on, as if with the hope of finding a better ford; in this apparently they were disappointed, for three days afterwards they returned to the same bend of the river, and there plunged in vast multitudes into the stream, where, assisted by a favourable current and the sedges and water-plants which grew upon the projecting rocks, they managed to effect a crossing, though great numbers were drowned and carried away by the flooded river. Mrs. Barber adds that "Voetgangers" have been known to attempt the passage of the Orange River when it was several hundred yards in breadth, pouring their vast swarms into the flooded stream regardless of the consequences, until they became heaped upon each other in large bodies. As the living mass in the water accumulated, some portions of it were swept away by the strong current from the bank to which they were clinging, and as the living locusts tightly grasped each other and held together, they became floating islands, the individuals continually hopping and creeping over each other as they drifted away. Whether any of the locust-islands succeeded in reaching the opposite bank is unknown; probably some of them were drifted on land again. They are by no means rapid swimmers; they do not perish easily in the water when in masses, their habit of continually changing places and hopping and creeping round and round upon each other being very advantageous as a means of preservation. It is a common practice for the young locusts to form a bridge over a moderately broad stream by plunging indiscriminately into it and holding on to each other, grappling like drowning men at sticks or straws, or, in fact, anything that comes within their reach, and that will assist in floating them; meanwhile those from behind are eagerly pushing forward over the bodies of those that are already in the stream and hurrying on to the front, until at length by this process they reach the opposite bank of the river; thus a floating mass of living locusts is stretched across the stream, forming a bridge over which the whole swarm passes. In this manner few, comparatively speaking, are drowned, because the same individuals do not remain in the water during the whole of the time occupied by the swarm in crossing, the Insects continually changing places with each other; those that are beneath are endeavouring to reach the surface by climbing over others, whilst those above them are, in their turn, being forced below. Locusts are exceedingly tenacious of life, remaining under water for a considerable time without injury. An apparently drowned locust will revive beneath the warm rays of the sun, if by chance it reaches the bank or is cast on shore. Mrs. Barber relates an interesting case where the instinct of the "Voetgangers" was at fault, they plunging into a river from a steep sandy bank, only to find another similar sandy precipice on the other side. On this they could gain no footing, and all perished in the stream, where they putrefied, and caused the death of the fish, which floated likewise on the surface; so powerful were the effluvia produced that no one was able to approach the river.
Locusts are able to travel considerable distances, though how far is quite uncertain. Accounts vary as to their moving by night. It has, however, been recently proved that they do travel at night, but it is not ascertained how long they can remain in the air without descending. The ocean is undoubtedly a source of destruction to many swarms; nevertheless, they traverse seas of considerable width. They have been known to reach the Balearic Islands, and Scudder gives[[228]] a well-authenticated case of the occurrence of a swarm at sea. On the 2nd of November 1865 a ship on the voyage from Bordeaux to Boston, when 1200 miles from the nearest land, was invaded by a swarm of locusts, the air and the sails of the ship being filled with them for two days. The species proved to be Acridium (Schistocerca) peregrinum. This is an extraordinary case, for locusts do not fly with rapidity, being, indeed, as we have remarked, chiefly carried by the wind. Possibly some species may occasionally rest on the water at night, proceeding somewhat after the fashion of the "Voetgangers" when passing over rivers as described by Mrs. Barber. In Sir Hans Sloane's history of Jamaica an account of an occurrence of this kind is given on the authority of Colonel Needham, who states that in 1649 locusts devastated the island of Tenerife, that they were seen to come from Africa when the wind was blowing thence, that they flew as far as they could, then alighted on the water, one on the other, till they made a heap as big as the greatest ship, and that the next day, being refreshed by the sun, they took flight again and landed in clouds at Tenerife. De Saussure says[[229]] that the great oceans are, as a rule, impassable barriers, and that not a species of the tribe Oedipodides has passed from the Old World to the New. It is, however, possible that Acridium peregrinum, of the tribe Acridiides, may have originally been an inhabitant of America, and have passed from thence to the Old World.
Fig. 177.—European migratory locust, Pachytylus cinerascens ♀.
The species of Acridiidae that have been ascertained to be migratory are not numerous.[[230]] The most abundant and widely distributed of them is Pachytylus cinerascens (Fig. 177), which has invaded a large part of the Eastern hemisphere, extending from the Atlantic Ocean to China. It exists in numerous spots in the Oriental region and the Asiatic Archipelago, and even in New Zealand. It is the commoner of the locusts of Europe. Its congener, P. migratorius, is much less widely distributed, its migrations being, according to de Saussure, limited to Turkestan and Eastern Europe. A third species, P. migratorioides, inhabits Eastern Africa, and a variety of it is the "Yolala" or locust of Madagascar. Mr. Distant has informed the writer that this migratory locust is found in South Africa. P. (Oedaleus) marmoratus has almost as wide a distribution in the Eastern hemisphere as P. cinerascens, except that it is more exclusively tropical; it is thus excluded from New Zealand. P. (Oedaleus) nigrofasciatus has a more northern distribution than its congener, but has extended to Africa and the Asiatic Archipelago. This Insect is so variable that the distinctions of its races from other species of the same genus are not yet clear. All the above-mentioned locusts belong to the tribe Oedipodides. Acridium peregrinum, now more frequently called Schistocerca peregrina, belongs to the tribe Acridiides. It is a large locust (Fig. 84), and has a wide distribution. It is the chief species in North Africa, and is probably the locust of the plagues of Egypt mentioned in the book of Exodus. It is also, according to Cotes,[[231]] the chief locust of North-West India. In this latter country Pachytylus cinerascens and some other species also occur. With the exception of S. peregrina, the species of the genus Schistocerca are confined to the New World. In North America locusts are more usually called grasshoppers. Several species of the genus Caloptenus are injurious in that country, but the chief migratory species is C. spretus (Fig. 175). This genus belongs to Acridiides. A large locust, Schistocerca americana, is also migratory to a small extent in the United States. In South America other species of Schistocerca are migratory; it is not known how many there may be, and it is possible that one or more may prove to be the S. peregrina of the Old World. A Chilian species, according to Mr. E. C. Reed,[[232]] exhibits distinctions of colour similar to those that have been observed in S. peregrina in Algeria.