474 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



To the north of the limits which have just been indicated, the Pachytylus migratorius 

 has not the power of undergoing its whole cycle of metamorphoses, neither, conse- 

 quently, to reproduce itself. This does not prevent its occasional appearance in swarms 

 even in countries very northern ; thus, it was observed in England (1693 and 1748), 

 and even at the latter date near Edinburgh ; in Sweden (as far as Ostrogoth), 

 at latitude 67° to 58° north, in 1748 and 1844, and finally on the Duna, near Dunabourg, 

 and at Polozk, in 1545. But these troops of voyagers did not hatch out in the same 

 places where they were observed, nor did they leave any progeny in subsequent years. 

 The only known example of an exception to this rule is the discovery made once by 

 Boheman, in September, in the middle of Sweden, of a Pachytylus migratorius in the 

 proper state. Evidently this is an exception wholly accidental, which does not prove 

 anything against the rule. The more we advance toward the north, the less are large 

 swarms of locusts observed, and we end by meeting only isolated individuals, as have 

 been seen several times at St. Petersburg, and even near Wasa in Finland (latitude 

 63° north). 



The want of facts prevents our extending these studies to the southern boundary of 

 the area of distributioii of Pachytylus migratorius. However, we can remark that in 

 New Zealand, the extreme southern point of this distribution, the mean temperature 

 of the warmer months is, according to Schmid (Lehrbuch der Metecrologie, p. 363), at 

 15°.5 R. (about 66° Fahrenheit), which does not differ much from the corresponding 

 temperature of the northern limit of the area in Europe. 



The localities out of Europe where the Pachytylus migratorius has been observed are 

 as follows : Madeira, Algeria, Tunis, Egypt, Chartoum, Asia Minor, Syria, Arabia, Per- 

 sia, India, Siam, China, Japan, Java, Lugon, Fidschi, New Caledonia, New Zealand, 

 Northern Australia, and finally Mauritius Island ; but this last locality indicated by 

 Serville needs confirmation. In Central Asia the sijecies has been observed near Lake 

 Aral, on the borders of Syr-Darja, on the upper side of Ischim and of Irtisch, and 

 finally toward the lakes KurgaMschin, Nor-Saisan, and Balchaasch. 



According to M. Koppen, the great chains of mountains are a powerful obstacle to 

 the diffusion of Pachytylus migratorius. The Alps especially play a large part in its dis- 

 tribution in Europe, and it is without doubt to them that we should attribute its rela- 

 tive rarity in the countries of the southwest of Europe and northwest of Africa, where 

 it is almost completely replaced by other speciesof the same group, i. e., the Caloptenus 

 italicus in Spain, Italy, and in the middle of France ; the Acrydium peregrinum in Algeria. 



It should be observed that this species, and in general all the Aerydiidce, shun mount- 

 ainous and wooded countries. They are most fond of the plains,, of regions quite dry, 

 and it is also a circumstance which influences necessarily their geographical distribu- 

 tion. 



" The development of the organs of flight of the migratory locust," continues M. Kop- 

 pen, '' determines the facility and the amplitude of its flight, and conseqilently favors 

 its migrations. They are evidently the cause of this colossal geographical distribution 

 of the species. They remind us of the remark of Darwin, that species rich in individu- 

 als and with a wide habitat, which, owing to their organization, have had in their 

 country the pre-eminence over many surrounding species, are those which, in the case 

 of emigrations out of their area, should have the greater chances of overrunning new 

 territories." 



Koppen examines successively the causes which may determine the migrations of 

 this orthopter in armies more or less numerous, and then the observed direction of 

 these movements. It is said that they fly more often from east to west, but M. Koppen 

 thinks that it is not necessary to attribute this circumstance, as has been done, to the 

 predominance of the east winds at times when the sterility of the country that they 

 inhabit, increased still by the prevalence of these same winds, forces them to seek 

 places which can furnish them a more abundant pasturage. Numerous facts appear, 

 he says, to contradict this explanation. In reality, the movements of these hordes is 

 rather centrifugal, as M. Koppen establishes from observations made especially in the 

 plains of Eastern Europe ; that is to say, that all the migrations appear to radiate 



