THE LOCUSTS OF THE OLD WORLD. 473' 



■which, like Southern Russia, may present, sometimes favorable seasons, sometimes 

 years, or even simply seasons, unfavorable to the multiplication of Pachytylus. Thus, 

 according to M. Koppen, the persistent prolongation of dry heat duriog a part of the 

 autumn will exert an influence on the quantity of eggs laid in favorable jilaces ; and, 

 on the other hand, a temperature less than 14° R^aumer [63|° Fahr.], prolonged for 

 several days toward the end of May, will be indispensable to the hatching of the larva. 

 There would result from the more or less perfect realization of these conditions, and 

 their succession or their interruption during several years, those differences observed 

 in the northern limit of the species, which alternately increase or diminish the area of 

 distribution. 



M. Koppen has distinguished and traced quite completely on the map for Europe 

 and Siberia three different limits of the geographical area of Pacliytylus migratorius : 1. 

 The limits of its permanent distribution. 2. The limit of its temporary existence in all 

 stages of development, a little more to the north. Finally, 3. The limits of its presence 

 in the condition of bauds of winged insects of a stated age, out of the regions where 

 the species may live and propagate. It will be necessary still to establish the limits of 

 accidental individual appearances, but that would be of questionable importance. The 

 northern limit of the permanent geographical distribution of FacTiytylus migratorius 

 begins in Western Europe, from the coast of Portugal, near 40° latitude north, and ex- 

 tends from there toward the northeast as far as the mouth of the Bidassoa, thus leaving 

 out all the northwest portion of Spain ; it continues to rise obliquely in France up as 

 far north as the lake of Geneva, and extends east, following more or less the forty- 

 eighth degree of latitude, and embracing Valois, all of the north of Italy, Carinthia, 

 and Hungary, it passes into Southern Russia, where it attains nearly the fiftieth de- 

 gree, passes likewise across the middle of Siberia, whence it passes over the north of 

 China, to end in Japan, at a latitude a little inferior to that of its point of departure 

 in Portugal, leaving out the island of Niphon. M. Koppen remarks that all this limit 

 does not deviate much from the isothermal of 16- R. [68° Fahr.] for the month of June. 

 To farther circumscribe the area, so extensive, of this species, the line goes from Japan 

 to the islands of Fidochi, to New Zealand and Australia, of which it only embraces 

 the northern parts, passes from there to the island of Mauritius, then rises to the 

 north, crosses Africa up to Madeira. But in this last part of the passage the limits are 

 more hypothetical, from want of an exact knowledge of the existence of the species 

 in the interior of Africa. 



When, in a country comprised in this area, as has been frequently observed in 

 Southern Russia, the locusts develop in a certain abundance, the want of food obliges 

 them to migrate in part in different directions, and to break over their limits. If cir- 

 cumstances permit these emigrants to multiply for a certain period beyond their nor- 

 mal area, there results a temporary extension of this area, and occasionally new mi- 

 grations to the north, until only a single spring, colder or more humid, comes to put an 

 end to their invasion and to oblige them to go back to their natural limits. Tempo- 

 rary extensions like this of the area of distribution of Pachyfylus migratorius took 

 place in 1746 to 1749, and in 1822 to 1828 ; at these periods they appeared in Germany, 

 and have multiplied, themselves during several successive years. The northern limit 

 of these temporary extensions may be also n^arked on the chart by a line which, tak- 

 ing its point of departure in ihe bouthwestern portion of Bavaria (where the Fachyty- 

 lus migratorius has been observed from 1333 to 1339, and from 1748 to 1749), rises to 

 the northeast by Jena and Halle toward Juterbogk and Berlin, when it takes a nearly 

 eastern course, following more or less the parallel of 52^° of latitude, near Muncheberg, 

 Kiistrin, Birnbaum, and Posen (regions which the species was known to have visited in 

 1730, 1752, and from 1827 to 1828) ; then the line passes across Southern Poland, at the 

 fifty-second parallel, through the southern part of the government of the Mohilew, in- 

 clining gradually toward the south, and extending so as to reach the Wolga and the 

 Ural. It is apparently to the humidity of the climate, injurious to the locust, likewise 

 to the state of the eggs during the winter, that we should attribute the less extension 

 of this limit toward the north in Western Europe. 



