JOUENAL OF ENTOMOLOGY AND ZOOLOGY 127 



The record for southeastern Idaho refers to a species which I 

 found in some abundance around Poeatello in the summer of 

 1911. If it was not our common species, it very probably was 

 T. flavus Ewing, wliich is so common in some sections of eastern 

 Oregon. 



Eegions Found to Be Free fkom General Infestation 



Although we have five authentic records of the common spider 

 mite occurring along the Columbia river east of the Cascade 

 mountains, yet it is not generally distributed in this section. I 

 have made many trips to this jjart of the state and find that the 

 sj^ecies is not generally distributed here. 



The common red spider apjjarently is not found in the Coast 

 Range mountains or in the forest reserves along the Cascades 

 in the state of Oregon, and the same is probably true of most of 

 the other national forests. 



The great plains regions between the Rocky mountains and the 

 eastern parts of the Dakotas, Nebraska, Kansas, Oklahoma, and 

 Texas, are probably free from general infestation. These regions 

 are largely occupied either by native grasses or by field crops 

 that are not commonly host plants of the red spider. I find that 

 our native plants are remarkably free from attacks of the intro- 

 duced red spider, hence it is probable that the native plants in 

 this vast stretch of land are free from attack at the present time. 

 Professor Waldron writes that the only places where the red 

 spider has been found in North Dakota are upon j^lants that have 

 been introduced. Hence in that state it does not appear to be 

 generally distributed. 



Factors Affecting Geographical Distribution 



The chief factors affecting the distribution of T. telarius Linn, 

 are those of climate and of host plants. Wherever the mean 

 daily temperature falls below about 50 degrees Fahrenheit this 

 mite will not reproduce, but will hibernate, under advantageous 

 conditions. Unless there is a summer mean considerably above 

 50 degrees Fahrenheit we would not expect this species to be 

 present. Red spiders have been reported at elevations of from 



