The Acridiidae are considered to be exclusively vegetable feeders, each individual consuming a very large quantity of food. The mode in which the female deposits her eggs has been described by Riley,[[222]] and is now widely known, his figures having been frequently reproduced. The female has no elongate ovipositor, but possesses instead some hard gonapophyses suitable for digging purposes; with these she excavates a hole in the ground, and then deposits the eggs, together with a quantity of fluid, in the hole. She prefers hard and compact soil to that which is loose, and when the operation is completed but little trace is left of it. The fluid deposited with the eggs hardens and forms a protection to them, corresponding to the more definite capsules of the cursorial Orthoptera.
The details of the process of oviposition and of the escape of the young from their imprisonment are of much interest. According to Künckel d'Herculais[[223]] the young Stauronotus maroccanus escapes from the capsule by putting into action an ampulla formed by the membrane between the head and the thorax; this ampulla is supposed to be dilated by fluid from the body cavity, and is maintained in the swollen condition by the Insect accumulating air in the crop beneath it. In order to dislodge the lid of the capsule, six or seven of the young ones inside combine their efforts to push it off by means of their ampullae. The ampulla subsequently serves as a sort of reservoir, by the aid of which the Insect can diminish other parts of the body, and after emergence from the capsule, penetrate cracks in the earth so as to reach the surface. Immediately after doing this the young Stauronotus moults, the skin it casts being called by Künckel an amnios. The cervical ampulla reappears at subsequent moults, and enables the Insect to burst its skin and emerge from it.
The process is apparently different in Caloptenus spretus, which, according to Riley, ruptures the egg-shell and works its way out by the action of the spines at the apex of the tibiae. This latter Insect when it emerges moults a pellicle, which Riley considers to be part of the embryonic membranes.
Riley states that a female of Caloptenus spretus makes several egg-masses. Its period of ovipositing extends over about 62 days, the number of egg-masses being four and the total number of eggs deposited about 100. The French naturalists have recently observed a similar fact in Algeria, and have ascertained that one of the migratory locusts—Schistocerca peregrina—may make a deposit of eggs at more than one of the places it may alight on during its migration.
It has been ascertained that the eggs of Acridiidae are very nutritious and afford sustenance to a number of Insects, some of which indeed appear to find in them their sole means of subsistence. Beetles of the family Cantharidae frequent the localities where the eggs are laid and deposit their eggs in the egg-masses of the Orthoptera, which may thus be entirely devoured. Two-winged flies of the family Bombyliidae also avail themselves of these eggs for food, and a mite is said to be very destructive to them in North America. Besides being thus destroyed in enormous quantities by Insects, they are eaten by various birds and by some mammals.
Most of the Insects called locusts in popular language are members of the family Acridiidae, of which there are in different parts of the world very many species, probably 2000 being already known. To only a few of these can the term Locust be correctly applied. A locust is a species of grasshopper that occasionally increases greatly in number, and that moves about in swarms to seek fresh food. There are many Orthoptera that occasionally greatly increase in numbers, and that then extend their usual area more or less; and some Acridiidae multiply locally to a great extent—very often for one or two seasons only,—and are then called locusts. The true migratory locusts are species that have gregarious habits strongly developed, and that move over considerable distances in swarms. Of these there are but few species, although we hear of their swarms in many parts of the world.
The migratory locusts do much more damage than the endemic species. In countries that are liable to their visitations they have a great influence on the prosperity of the inhabitants, for they appear suddenly on a spot in huge swarms, which, in the space of a few hours, clear off all the vegetable food that can be eaten, leaving no green thing for beast or man. It is difficult for those who have not witnessed a serious invasion to realise the magnitude of the event. Large swarms consist of an almost incalculable number of individuals. A writer in Nature[[224]] states that a flight of locusts that passed over the Red Sea in November 1889 was 2000 square miles in extent, and he estimated its weight at 42,850 millions of tons, each locust weighing 1⁄16 of an ounce. A second similar, perhaps even larger, flight was seen passing in the same direction the next day. That such an estimate may be no exaggeration is rendered probable by other testimony. From official accounts of locusts in Cyprus we find that in 1881,[[225]] up to the end of October, 1,600,000,000 egg-cases had been that season collected and destroyed, each case containing a considerable number of eggs. By the end of the season the weight of the eggs collected and made away with amounted to over 1300 tons, and, notwithstanding this, no less than 5,076,000,000 egg-cases were, it is believed, deposited in the island in 1883.
When we realise the enormous number of individuals of which a large swarm of locusts may consist we can see that famine is only a too probable sequence, and that pestilence may follow—as it often has done—from the decomposition of the bodies of the dead Insects. This latter result is said to have occurred on some occasions from locusts flying in a mass into the sea, and their dead bodies being afterwards washed ashore.
Locust swarms do not visit the districts that are subject to their invasions every year, but, as a rule, only after intervals of a considerable number of years. It has been satisfactorily ascertained that in both Algeria and North America large swarms occur usually only at considerable intervals. In North America Riley thought[[226]] the average period was about eleven years. In Algeria the first invasion that occurred after the occupation of the country by the French was in 1845, the second in 1864, the third in 1866, since which 1874 and 1891 have been years of invasion. These breaks seem at first strange, for it would be supposed that as locusts have great powers of increase, when once they were established in any spot in large numbers, there would be a constant production of superfluous individuals which would have to migrate as regularly as is the case with swarms of bees. The irregularity seems to depend on three facts: viz. that the increase of locusts is kept in check by parasitic Insects; that the eggs may remain more than one year in the ground and yet hatch out when a favourable season occurs; and that the migratory instinct is only effective when great numbers of superfluous individuals are produced.
It is not known that the parasites have any power of remaining in abeyance as the locust eggs may do; and the bird destroyers of the locusts may greatly diminish in numbers during a year when the Insects are not numerous; so that a disproportion of numbers between the locusts and their destroyers may arise, and for a time the locusts may increase rapidly, while the parasites are much inferior to them in numbers. If there should come a year when very few of the locusts hatch, then the next year there will be very few parasites, and if there should then be a large hatching of locusts from eggs that have remained in abeyance, the parasites will not be present in sufficient quantity to keep the destructive Insects in check; consequently the next year the increase in number of the locusts may be so great as to give rise to a swarm.