20 DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



tion with the horizontal extent of its habitat that this species shows the 

 greatest range of individual variation of any in the group, a condition 

 dependent upon fluctuating differences in its habitat which are sufficient to 

 produce marked individual variation, but are not of enough magnitude to 

 act as barriers to its distribution. From the optimum conditions there is a 

 continuous series of gradations in the environment to the minimum, whereas, 

 in the case of the species previous^ described, the changes in the habitat are 

 rapid, often abrupt, and serve as effectual barriers. 



Closely associated with this species are the forms or species mclanothorax , 

 oblongata, intermedia, rubicunda, and decemlineata, which are the direct 

 descendants of this ancestral stock. In some cases the difference between 

 the species is so slight that separation is difficult, and is especiall)' so in 

 museum material. In life, however, the differences are abundant and defi- 

 nite, so that the various species can be recognized easily. 



LEPTINOTARSA OBLONGATA. 



This species, clearly distinct from the preceding, has been by previous 

 writers confounded with multitceniata. The food plant and the conditions 

 which govern its position in any particular place are the same as for multi- 

 tczniata, and need not be restated. In the upper part of the Rio Atoyac 

 Valley the two species live together upon the same plant, but are, as I have 

 determined by breeding experiments, physiologically isolated. The same is 

 true in the Valley of Mexico, where this species also occurs, although rather 

 sparingly. From the Plains of Apam this species extends southward into 

 the Rio Balsas Valley and the Oaxaca-Guerrero Highlands, but does not 

 reach as far south as the Isthmus of Tehuantepec. Everywhere its habitat 

 is narrower than that of the preceding species, and its range of variation is 

 less. This form is a direct descendant of multitaniata, which has become 

 established in the region where it now lives. 



LEPTINOTARSA MELANOTHORAX. 



Associated with multitceniata is a form described under the above name. 

 It is never common only a few specimens appearing at any one time. Its 

 food is Solatium rost ration , and the controlling factors of its habitat are the 

 same as those for multitcrniata. Its distribution is irregular, as is also its 

 occurrence in any locality. I have reared this form from eggs laid by 

 multitaniata. It is an elementary species which appears at various localities 

 at frequent intervals, but which has not yet been able to become established 

 as a member of the fauna of the habitat into which it is born. The point 

 to be noted in this connection is the fact that the appearance of this ele- 

 mentary species occurs at several points rather constantly, as is shown in 

 the data of its distribution. We shall have occasion to consider this case 

 again in a later portion of this paper. 



