22 DISTRIBUTION AND DISPERSION OF I.EPTINOTARSA. 



rainfall are the controlling factors. Whenever and wherever there is suffi- 

 cient moisture this species and its food plant thrive ; wherever moisture is 

 lacking for a year both fail to develop and lie dormant in the earth in 

 aestivation until such time as the rains come to awaken them. This 

 direct control by the precipitation is productive of an irregular distribution 

 and complete absence from many otherwise favorable localities, and is due 

 to its living in the transitional zone between grasslands and desert. The 

 photograph of its general habitat (plate 7) shows the nature of this zone. 



A rather striking feature of the distribution of this species is the fact that 

 it is most abundant along highways of human trafl&c, and often present there 

 only. Since the beginning of the occupation of northern Mexico by the 

 Spanish two chief routes of travel to the north, those along the eastern and 

 western edges of the plateau, have been in almost constant use. Along 

 these — the eastern leading into Texas, the western into New Mexico and 

 Arizona — Solanum rostratum is distributed as a weed, growing largely, not as 

 a native plant, but as one introduced through transportation by beasts of 

 burden. It everywhere grows best about corrals, stables, and watering 

 places where cattle congregate. Mr. Pringle, who for twenty years has 

 studied and collected extensively the flora of Mexico, tells me he is fully 

 convinced that these plants, as well as others, have been carried from places 

 in southern Mexico northward into the regions where we now find them by 

 Spanish pack-trains and caravans. From the study of the distribution of 

 intermedia, I had also come to the conclusion that these insects had been 

 able to spread northward only by means of some outside help, and Mr. 

 Pringle' s observations concerning the food plant is strong evidence in sup- 

 port of this conclusion. It is highly probable that previous to Spanish occu- 

 pation and exploration these beetles and their food plant were limited to the 

 area at present occupied by multitcEuiata . 



From the conquest of Mexico, in 152 1, until the close of the sixteenth 

 century numerous exploring expeditions traversed the country north, west, 

 and south. Early in the seventeenth century colonies were established in 

 northern Mexico, Texas, New Mexico, and Arizona, and the final conquest 

 of these regions was completed in 169 1. It was during this period that the 

 main routes of travel to the north became established, and it is probable, if 

 the above hypothesis be true, that it was during this period that Solanum 

 rostratum and L. multitceniata were able to extend their habitats into the 

 northern part of the Mexican area through aid in transportation. L. inter- 

 media, therefore, has probably occupied its present area of distribution since 

 early in the eighteenth century. 



Whether the above hypothesis be true or not one fact is certain, that inter- 

 media exists to-day most abundantly along the three chief routes of Spanish 

 travel and commerce, and often there only. No feature of topography or 

 climate could be productive of such a distribution. Moreover, I have found 



