16 THE entomologist's RECORD. 



less indirectly evolved from these older forms of insect life, the migra- 

 tion of the latter in search of food gives us at least a clue whence 

 other insects obtained their migrating instinct. 



Swarms of locusts do not invade the same country year after year, 

 but, as a rule, successive invasions take phice only after a long 

 interval. It has been calculated by Riley that invasions of locusts 

 only take place in North America, on an average, once in about eleven 

 years — 1845, 1864, 1866, 1874, and 1891 are given as years in which 

 swarms took place. This irregularity in the period of invasion 

 appears to be due to the circumstance that an invasion can only take 

 place in a year when meteorological conditions favour the production 

 of the locusts to such an extent that a vast superfluity of individuals 

 is produced in those districts in which the insects are normally 

 sedentary and comparatively harmless. The abnormality of the 

 excess, giving rise to large requirements in the direction of food- 

 supply, seems to put in motion the migratory instinct. It must not 

 be altogether overlooked that possibly some unfavourable checks on 

 the usual parasites of these insects may also lead to an increase in 

 their number, for it is perfectly well-established that the migratory 

 species of locusts do exist in countries without giving rise to swarms, 

 and without doing much harm, their natural enemies keeping them 

 well in check. Thus, one of the most important of the migratory 

 locusts, Paclujtiilm vincraaccm, is always present in certain parts of 

 Belgium and Central Europe without giving rise to swarms. 



It is remarkable that, although many years may elapse between two 

 swarms of locusts going off from a given district, yet the swarm 

 usually takes a fixed direction and follows that taken by its prede- 

 cessors. It would appear probable, too, that their movements are to 

 some extent dependent on the wind, and it has been reported that they 

 make trial flights to ascertain whether or not its direction be favour- 

 able. It is also probable that, once they are on the wing, little muscular 

 exertion is necessary to keep up the pace at which they fly, for their 

 bodies contain elastic air-sacs in communication with the tracheae. 

 It is further possible that, at the time of Hight, the body is, to a great 

 extent, devoid of food, and the internal space in which the air-sacs 

 may increase in size is further enlarged by the ill-developed condition 

 of the ovaries in the early life of the perfect insect. 



As a rule, it is, of course, the adult locust that migrates, for locusts 

 do not attain their wings until the final ecdysis. In spite of this, 

 however, Sydon, Donzingk, Techemewsky, Finot, and others describe 

 the migrations of the larval forms, and some of the South African 

 species appear to carry out extensive migrations before they have 

 reached the winged stage. These migrate on foot, and are called by 

 the Dutch, " Voetgangers." Having stripped a district of everything 

 green, they move on in search of new feeding-grounds. The direction 

 they take is always to the interior, strangely enough, towards the lands 

 from which their winged progenitors departed. What have been 

 assumed to be return swarms of adults have been noticed in America, 

 and the facts connected therewith were ascertained by a United States 

 commission appointed to enquire into this subject. 



Mrs. Barber describes the remarkable manner in which the wing- 

 less locusts of South Africa occasionally cross broad rivers. The 

 observation recorded was made on a swarm of young locusts that 



