46 The Mechanism of Evolution in Leptinotarsa 



First larval stage : Head, pronotum, and legs, black ; body yellow, with full 

 system of body color-markings, each with one or more spines. Length at end 

 of stage, 3.5 to 4.5 mm. 



Second larval stage: Head and legs black, pronotum with posterior edge 

 black, anterior border and sides to middle, yellow or brown ; body bright yellow, 

 ornamented with spiracula, wing and basi-pleural spots, and a variable number 

 of the anterior members of the inner tergal spots. Length at end of the stage, 

 6.5 to 8.25 mm. 



Third larval stage : In color and pattern identical with the second. Length 

 at end of the stage, 10 to 18 mm. Length of larval period varies from 13 to 25 

 days, average 17 days; but much depends upon surrounding conditions of 

 growth. 



Pupa: Pupates in ground, from 2 to 6 inches in depth ; pupa yellow. Period 

 of pupation, 6 to 15 days, but average about 9 days. 



Length of ontogeny: From 19 to 40 days; usual average is between 25 and 

 30 days. The larval stages are very constant in this geographical variety and 

 all of its minor divisions. 



Habitat and Ecology. 



This form passes the winter — i. c., the dry season, from the middle of 

 September to May — in hibernation in the ground as an imago. It emerges 

 from hibernation in May, usually about the 15tli, in the region of Mexico 

 City, and begins breeding about June 1 to 15, depending upon the advent of 

 the rainy season. It produces a brood that becomes adult about July 20, 

 which in turn breed at once and give a second brood, which emerges about 

 the end of August. These latter feed for a few days, and with the cessation 

 of the rains and cooler nights of September, enter into hibernation, and 

 by September 15 or October 1 they have all gone into hibernation, with the 

 exception of a few that never do enter the ground. These latter are curious 

 hybrid forms produced in the following manner : Some members of the over- 

 wintering hibernating population may remain alive and breed with their 

 descendants, giving a cross of summer fornix winter form, thus producing 

 hybrids which are variable in their breeding activities and some of which are 

 extracted summer forms in Fo, and do not hibernate, and are, therefore, elimi- 

 nated by starvation and desiccation during the dry season. 



Geographical Distribution. 



The range of this habitudinal variety and its minor divisions is, as indicated 

 on plate 5, limited to the southern end of the Mexican Plateau, between the 

 altitudes of 5000 and 8000 feet. It is especially characteristic of and common 

 in the valley of Mexico (7400 to 8000 feet), the plains of Apam, and eastward 

 to the edge of the plateau (6000 to 8000 feet), the plateau portion of the State 

 of Puebla, especially the headwaters of Eio Atoyac, and less common on the 

 high plateau of the portion of the valley of the Eio Lerma and westward to edge 

 of plateau; northward it passes over into northern habitudinal variety inter- 

 media, in the region of the headwaters of the Rio Panuco. 



Stal gives Mexico as its habitat; Jacoby, 1883, 1891, gives Toluca, La Parada 

 (Salle), Cerro de Plumas (Hoge) ; Tower, 1916, Z. c, records it from Puebla, 

 Atlixo, Matamoros de Izecas, San Marcos, in the State of Puebla ; Apizaco and 



