True Tales of the Insects

LOCUSTS AND GRASSHOPPERS (ACRIDIID.E). in

them away, and lie rotting in one huge mass for days. Fish die from the poisonous effects, and Hoat on the surface, adding to the already existing mortality, and so powerful the effluvia produced, no one dare venture near to gaze on the scene of desolation.

Locust swarms do not visit the places that are subject to their visitations every year, but, as a rule, only after intervals of a considerable number of years. It has been satisfactorily ascertained that both in Algeria and North America, the noted locust years occur usually only at considerable intervals. There is, however, no certainty in the migrations, no law of periodicity governing destructive flights, these only occurring at irregular intervals. Nevertheless, it may be pointed out, the history of the most noted locust years, both in North America and in Europe, shows a tendency to their recurrence about every eleven years. From the respective areas of their most abundant development, the European and Asiatic, the African, and the American species swarm in exceptional years, to ravage adjacent regions in which they are not found permanently.

These interims between migrations seem at first unexplainable, for it would be supposed that as locusts are capable of excessive increase, when once they were established in any spot in large numbers, there would be a constant development of superfluous individuals which

would have to migrate regularly, in order to procure food. The irregularity seems to depend on three things: that the multiplication of locusts is kept in check by parasitic insects; that their supposed to be eggs—which were comparatively easily affected by climatic influences—may lie hidden in the soil for years and yet hatch out in a favourable season; and that the disposition to migrate

is—though some locusts appear essentially migratory-only effective when immense numbers of individuals are produced. Every animal necessarily meets with checks of one kind or another to its undue multiplication, and the balance of power does not always lean to the side of the enemies of the locusts. During a year when the locusts are not numerous the abundance of the parasites may decline, and the bird destroyers of the locusts may greatly fall off in numbers; so that the locusts get on the rising side of the scale, as it were, and for a time may increase rapidly, while the enemies are much inferior to them in numbers. If there should come a year when very few of the locusts hatch, then the next year the parasites will be greatly reduced, and if then large numbers of locusts from eggs that have been in abeyance should hatch out, the parasites will not be present in sufficient numbers to keep them in check; and by the following year the increase in number of the locusts may be such as to give rise to a swarm.

ri; LOCUSTS AND GRASSHOPPERS {ACRIDIIDAi).

There arc Many Causes of Migration.

The explanation of migration to be gathered from the preceding remarks, is excessive multiplication. As a single explanation of the phenomenon no better can be offered; for this is evidently the immediate or special cause, and the others are mostly secondary, or but consequences of this one; for these movements cannot truly be ascribed to any single cause. Moreover, we come to recognize the fact that the several influences bearing on migration fall into two distinct categories, viz. immediate or special, and remote or general. That certain climatic conditions, those of heat and more than ordinary dryness, are in some way necessary, or most favourable to the excessive increase, and the origin of the migratory habit or instinct, may be maintained. Wherever locusts are largely and frequently developed, whether in the Eastern or Western Continent, we find either extensive deserts or vast treeless areas, and a climate arid or dry. These conditions are believed to develop irritating or uneasy sensations, which cause migration. But. plainly, there are other exciting causes than the impulse simply to fly. The immediate cause, increase, has been already considered. Annoyance from natural enemies probably often proves a valid cause, as no one who lias witnessed the

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1T4 TRUE TALES OF THE INSECTS.

excessive abundance in which some of these at times prevail will deny. Tachina- flies especially, will follow the locusts in dense crowds, so thick that not one can rise from the ground without being pursued by several; and there is no escape from persecution till the victim rises high in the air. When food is lacking, whether through excessive multiplication of species, or through the droughts that are not uncommon in the locusts' native habitat, there must needs be the strongest incentive to change of place. Such is the case, under the same circumstances, with other animals normally non-migratory. That hunger will cause locusts to move from place to place in search of food is undoubtedly true, yet it would be a violent presumption to say that a swarm starts on the lengthy journey that they often undertake, in search of food. There seems thus still another factor in the problem, viz. instinctive impulse. It is more than probable there is a certain instinctive prompting to that which is best for the preservation of the species. Fresh breeding-grounds, away from the location of birth, seem desired. The evident disposition often manifested to go in a given direction in spite of contrary winds, or other potent obstacles, is not, to the same extent, susceptible of any other explanation. We have likewise to deal with that most remarkable fact, the return migration of locusts bred in the Temporary region to

the land of their ancestors. Hunger or excessive multiplication, however important as causes of the migration from the Permanent breeding-grounds, seem to have httle to do with this return migration, because the insects all depart, whether few or many, and they pass over great stretches of luxuriant vegetation. Pre-vailing winds do not govern them, dangerous obstacles do not stay them. The uncongenial climate of the Temporary region doubtless prompts them to get back to their more congenial native home, and we must allow a certain amount of instinctive guidance akin to that possessed by migratory birds.

Remarkable Manifestations of Instinct attend Migration.

The manifestations of instinct that attend migration are indeed remarkable. It is believed that when the locusts migrate, they do so in the direction taken by their predecessors, although several generations may elapse without a migration. Their flight is to a large extent dependent on the wind, and it is said when the air is calm and warm, and they are ready to move, they have the habit of making short flights, circling upward, apparently for the purpose of ascertaining the condition or direction of the upper currents, if they are favourable to bear them away. They support themselves on the

ii6 TRUE TALES OF THE INSECTS.

wing probably with little muscular effort. Their body, w^e have seen, contains elastic air-sacs in connection with the tracheae, which, constantly filled and refilled, float them up in the air, and at the time of flight, it may be presumed, the internal balloons have room for perfect expansion, as previously explained. Thus the insects spend but little muscular force in their aerial movements, and, instead of really flying as does a bird, are borne along chiefly by the wind. Suppose a swarm has risen from the ground on a clear sunny morning, and is being wafted straight on their course by favouring winds. Opposing winds suddenly prevent them from moving on in the direction they desire to go. Instantly they drop, and wait for a change. With a return of the wind to its original quarter, if during the warm part of the day, often, with wonderful unaniniity, all again take wing, and fly off towards their intended destination. An apparently inexplicable point in the natural history of the migratory locusts is their disappearance from a spot they have invaded. A swarm wall arrive in a locality, deposit there a number of eggs, and then pass on. But after a season or two there wnll be few, or none of the species in the spot invaded. This appears to be partly owing to the want of food, and consequent mortality among the young after hatching; but in other cases they in turn migrate after growth— they disappear

on those odd return travels towards lands from which their progenitors came.

" Voetgangers; " Interesting Points in their Natural History.

In South Africa it would seem that the movements of the migratory locusts frequently take place before the insects have acquired their wings. Mrs. Barber, in an account of " Locusts and Locust-birds," mentions this among many interesting traits of the South African species. These locusts manifest the gregarious disposition at a very early age, for the small family parties in which they at first appear, sunning themselves in the vicinity of the holes whence they have escaped from the ova, rapidly amalgamate, so that enormous numbers come together. Roosting in company upon shrubs and grasses, they denude the neighbourhood of its foliage. Their name, " Voetgangers by the "—given early Dutch settlers, and retained ever since— denotes their habit of travelling in flocks on foot. No sooner have they made their appearance from the earth, and have obtained a slight degree of strength, than they will at once take up a northern course, always towards lands in the interior of the continent from which their progenitors departed. Farmers and agriculturists adopt

ii8 TRUE TALES OE TflE INSECTS.

methods all but in vain to save their crops and pasture-lands on their approach in countless multitudes, when swarm after swarm has to be contended with. They then fill up the whole country; their numbers are overwhelming, and cannot be driven away. They take advantage of paths and roads, and many miles will be traversed in a day; they proceed by means of short quick leaps or hoppings rapidly repeated. Nothing can stay the " Voetgangers' " progress northward. Occasionally they march through towns and villages, and have been seen attempting to scale a stone wall, rather than be thwarted in their advance. Mountain ranges, forests, rivers, may intervene, diverting them for a while from their course; nevertheless, they ultimately succeed in continuing their journey to the interior.

The manner in which these wineless locusts occasionally cross broad rivers is surprising, and has some bearing on the vexed question of the possibility of wincred locusts crossingf the ocean. Mrs. Barber refers to an instance on the Vaal River in the spring of 1871, shortly after the discovery of the Diamond-fields, The country at the time was swarming with " Voetgangers;" every blade of grass was cleared off by them. One day a vast swarm appeared on the banks of the river, evidently in search of a spot for crossing, which they could not find, as the river was slightly flooded. For several days the locusts travelled up the stream; in the

FIG. 22.—A very aberrant and beautiful Grasshopper (Pneumora scutellaris).

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course of doing this they paused for some time at a sudden bend in the river where a number of rocks were cropping out, as if uncertain whether to risk a passage at this place. However, they at length passed on, as if with the hope of finding a better ford; in which apparently they were disappointed, for three days later they returned to the same bend, and there plunged in vast multitudes into the stream, where, assisted by a favourable current and the water-plants which grew upon the projecting rocks, they succeeded in effecting a crossing, though great numbers were drowned and carried away by the Hooded river; but these casualties do not count when myriads are advancing.