61^6 REPORT UNITED STATES GEOLOGICAL SURVEY. 



very broad, of tempornry visitations, AvLich is to tbe area properly so called what 

 the peunnibra is to tlio light, within the zone, of which the exterior lin)it is niiieh more 

 easy to trace than the inner; this last is subject to continual oscillations, with some 

 undulatory niovenients, dependent on the centrifiifial or expansive tendency of the 

 species, and from the resistance which opposes it, and external circumstances, and evi- 

 dently also the tendency of other species to spread ont, with which it carries on a 

 8trnf;glc for existence in endeavoring to maintain itself on an earth where the chances 

 are divided, and even vary from year to year. M. Koppen has thus been enabled to 

 figure on his chart three lines, as I may for the present call them, and the intermediate 

 line rei>resents the exterior actual limit of these oscillations of the true frontier line 

 of raclnjtiilus iniyratorluH ; their amplitude may vary from two to four degrees." 



The last thesis of M. Kiippen that I shall draw attention to at this time, namely, 

 that the absence of rachijtjilits migratorius iu America should prove that the species 

 exists only as a species, since the separation of the two continents toward the north 

 pole, seems to me scarcely necessary. A mere glance at the map which represents the 

 area of distribution of this locust allows us to affirm without hesitation that that view 

 is impossible. It is evidently not one of those species which we may call circumboreal 

 anfcfilaciaJ, because their juesence in two forms (races, varieties, or species) on each 

 continent indicates that they have had a common origin, a single area at that epoch, 

 anterior to the glacial period, when the two continents were reunited iu the Arctic 

 zone by a bridge, so to speak, that is, a continuity of land, iu conditions of climate 

 which should allow the existence at that latitude of a fauna which only at present 

 exists much farther south. The source of those species dispersed by the glacial period 

 does not now ])robably exist iu its integrity ; but t he two races confined, one in America, 

 the other iu the Old World, having undergone slow modilications each on its part, are 

 to-day very analogous species, but as distinct by their external characters as by their 

 separate geographical area. 



Nothing like this applies to PachyUiIus viigralorius ; it is one of those species which 

 may be called equatorial postyladal ; its expansion toward the north has been posterior 

 to the glacial period, which would then have ojtposed it ; and it can have no aftinitiea 

 in the New World, but degrees of consanguinity much farther removed than those 

 unite the circumboreal species of the temperate zone. Thus, if, as some think, the 

 northern hemisphere tends actually to retrograde towaid a new period of cold, the 

 PachiiiijJus migraiorius is destined to see its area also retrograde toward the equator, 

 and perhaps some day the western and eastern parts of this area may be completely 

 disjointed, and, following this separation, its posterity may be so modified by isolation 

 as to form two distinct species, as has occurred to circumpolar species. 



In tbe discussion which followed, M. de Selys Longchamps speaks of 

 the ditiiculty of separating Pachytylus migratorius (Linn.) and cinerafcens 

 (Fabr.), which he had at first regarded as varieties, but now considers as 

 a distinct species, the hitter being more sedentary and rejiroducing in 

 Belgium year after year : " M. F. H. Koi)pen not speaking of cineraseens, 

 it would be interesting to know whether he admits this species, and if 

 in the affirmative, whether all his remarks apply alone to the true 

 migratorius type, notably that which he says normally sojourns at 

 Bayoiine, where I have taken only cineraseens^ variety virescens, whose 

 characters are the same as in Belgium and Fiankfort-on-the-Main. It 

 is also cineraseens that M. von Heyden has taken." 



Some notes on the Algerian locusts {Acrydium peregrinum, migrato- 

 rium, «S:c.) by Conre, have been communicated to the Entomological 

 Society of France by Giraud. In them, mention is made of a special 

 work on the same subject, which the recorder has not yet seen. (Bull. 

 Soc. Ent. Fr., 18G7, pp. x, xiii.) The locusts visiting Algeria come from 

 the south, and arrive in May. Thej'- lay their eggs soon after their 

 arrival, and the young animals produced fioin these eggs usually become 

 adult in July. In August all usually disappear. Coure also notices the 

 arrival in Algeria in the early jtart of January, 1807, of a flight of 

 locusts. The color of these was stated to be reddish. It appears thai 

 on lirst attaining their adidt form, these insects are of a rosy tint, and 

 afterward change; and Coure thinks that it is not until after their 

 change of color that they are fitted for reproduction. Lallemant states 

 (/. c, p. xiii) that the locusts, which live for a long time iu the adult 



