72 



TENDENCY OF INTRODUCED FORMS TO PRODUCE EXTRA GENERATIONS 

 IN ADOPTED NORTHERN HABITATS. 



European introductions in the United States frequently produce one 

 or more generations in excess of the number that has been observed 

 and recorded in the northern countries of Europe where observations 

 have been made, and even attempt generations late, in the year, which 

 are often apt to perish by being overtaken by frosts before trans- 

 formation can be accomplished or suitable places sought out for 

 hibernation. 



Southern forms that migrate northward in time appear to become 

 perfectly at home in northern localities; in fact, thoroughly acclima- 

 ted, but this is apparent only, as there is every reason to believe that 

 many species attempt the production of one or more generations more 

 than similar northern species have; or, in other words, essay the 

 normal generations which they had in the south, which are apt to be 

 cut short by intervening cold weather before their completion. 



Examples of both forms are apparently more frequent in leaf- 

 feeding mandibulates, particularly the larvae of Heterocera or moths 

 and phytophagous Coleoptera, especially Chrysomelidse or leaf-bettles. 

 Several injurious forms of plant-lice are in the same category, although 

 these have not been given special study. Many genera are known to 

 feed in cold weather long after frosts, and inay even be taken on their 

 host plants under the snow. 



An excellent illustration of polygneutisni, or the production of sev- 

 eral generations annually in a species recorded as normally monogneutic 

 in its native home, is to be found in the imported elm leaf -beetle, 

 Galerucella luteola. There can be little doubt that this species is 

 monogneutic in Europe, but observations conducted at Xew Bruns- 

 wick, N. J., and Connecticut cities in the Upper Austral life area 

 have shown that there is an incomplete second generation. In the 

 more southern portions of the same life area there are invariably two 

 generations annually, and in exceptional seasons a third generation 

 is attempted; at least, beetles of the second generation have been 

 observed to lay eggs. 1 



An example of an extra generation being produced by a southern 

 species is found in the squash-vine borer, Melittia satyriniformis, 

 which is single-brooded on Long Island and northward, apparently 

 single and partial^ double-brooded in New Jersey, while in the lati- 

 tude of the District of Columbia the species is both single- and double- 

 brooded, as shown by the writer in recent years (Bui. No. 19, n. s. Div. 

 Ent., p. 39). This peculiarity in reproduction is evidently a survival 

 of the time when this species lived in a tropical climate, where it was 



1 Even some of our native species closely allied to the elm leaf-beetle, e. g. , Gale- 

 ruceUa americana Fab., have been observed by the writer to lay eggs for a second 

 generation late in July (Proc. Ent. Soc. Wash., Vol. Ill, p. 275), but this is, with 

 little doubt, exceptional. 



