MIGRATION AND DISPERSAL OF INSECTS : ORTHOPTERA. 15 



only a few of these can the name " locust" be move ov less properly 

 applied. The term "locust," as properly used, may he considered aa 

 referring to particular species of grasshoppers, which occasionally 

 increase greatly in numbers, and which then move from place to place 

 in search of food. The true migratory locusts are species with 

 gregarious habits, which move about in vast swarms, sometimes for 

 considerable distances. Of these there are but few species, although 

 they occur in many parts of the world. 



The endemic species of locusts usually do but little harm, but the 

 migratory kinds inflict great damage on the crops of the countries 

 they visit. A large swarm will often, in the space of a few hours, 

 clear off all the vegetable food that can be eaten and leave nothing 

 green over a vast area, and it is practically impossible for any one who 

 has not seen such a swarm to realise the injury inflicted. Carruthers 

 describes a flight of locusts that passed over the Red Sea in November, 

 1889. He estimated it at 2,000 square miles in extent and the weight of 

 the locusts at 42,850,000,000 tons, assuming each locust to weigh only 

 one-sixteenth of an ounce ; whilst he farther states that a second similar, 

 and probably larger, flight was seen travelling in the same direction the 

 next day. That these estimates are possibly not at all exaggerated is 

 very probable, judging from other accounts of similar flights. Sharp 

 says that from official accounts referring to the locusts in Cyprus we 

 find that in 1881, up to the end of October, 1,000,000,000 egg-cases 

 had been that season collected and destroyed, each case contain- 

 ing a considerable number of eggs. By the end of the season 

 the weight of the eggs collected and made away with amounted to 

 over 1,500 tons, and, notwithstanding this, no less than 5,070,000,000 

 egg-cases were, it is believed, deposited in the Island in l88;-5. 



The consideration of the migration of locusts becomes very interest- 

 ing from the fact that it is based on one well-known necessity — the 

 want of food. From this fact the movements of these insects are less 

 complicated and uncertain, and more distinctly traceable to a given 

 cause than those of any other order of insects. Inhabitants of those 

 fertile countries which travellers in the Tropics have made known to 

 us, the migratory locusts are sometimes produced in such marvellous 

 numbers that they devastate the countries they traverse, changing 

 the most fertile spots into a wilderness, and then, obliged to seek fresh 

 pastures, renew their journey in order to obtain the food necessary for 

 their sustenance. 



To obtain this food immense journeys are undertaken by them and 

 almost incredible distances covered. Practically polyphagous, scarcely 

 anything comes amiss to them as food, and, hence, when a swarm 

 settles upon cultivated ground, the damage it does is incalculable. 

 Certain plants, among others Lcniiininusae and Citcitrliitaci'ac, are said, 

 however, to have practical immunity. Every continent has, more or 

 less, its own particular locust pests, Europe being, perhaps, of all the 

 great land divisions, the least subject to their visits. 



The order Orthoptera, to which, as we have already mentioned, 

 locusts belong, is supposed to be one of the oldest orders of insects 

 known. Remains of these insects, diftering but little from some that 

 now exist, have been found in rocks of Carboniferous and Silurian 

 age, and since the more recently evolved insects — butterflies, moths, 

 bees, ants, and two-winged flies — are supposed to have been more or 



