200 THYSANOPTERA AND ORTHOPTERA. 



amined the flowers were nearly uniformly distributed, and were abun- 

 . dant, so that an insect might have flown almost anywhere and still 

 have been within eas}^ reach of food and protection. 



(4) Inertia. Three tangible factors which might have explained 

 the crowding of thrijis near roadways have been examined and found 

 insufficient. There remains one possible factor, namely, inertia. 

 The meaning of this term is explained at the beginning of this section. 

 There can be no direct evidence that inertia accounts for the non- 

 migration of thrips from roadways. It is merely the one factor that 

 is left after the other conceivable factors have been eliminated. By 

 proposing the name ''inertia" for what I suppose to be the responsible 

 factor in preventing migration, I do not mean to imply that any thing 

 very definite is known about it, but, on the other hand, it is not merely 

 a name. It is employed for convenience to indicate that we are here 

 dealing with an. internal factor, in contradistinction to the several ex- 

 ternal factors which have been excluded. 



It is to be noted that so far no explanation has been offered for the 

 congregation of thrips along the roadways in the first place. This I 

 suppose to be due to traffic. Uzel (1905, p. 368) mentions the possi- 

 bility of artificial spreading, as advocated by Blanchard (1851), but is 

 inclined to discount its eflficiency. Loads of hay in summer probably 

 contain thousands of thrips, since the insects remain in flowers until 

 the latter are rather dry. Many of these flowers (e. g., clover) are 

 dropped by the roadside. Loads of hay or straw in winter may bear 

 hibernating individuals or eggs. Bouquets may transport a few, es- 

 pecially in or near cities. Passing vehicles may intercept the flight of 

 thrips and carry them for longer or shorter distances. Stock pasturing 

 along the road, or the clothing of pedestrians probably carry some; for 

 after walking through a patch of white clover bearing thrips I have 

 several times found some of the insects on my leggings. Phloeophilous 

 species are doubtless carried in logs where lumbering is practised. In- 

 asmuch as the phenomenon of abundance near roads may be the cumu- 

 lative effects of years, the young remaining near their place of birth, I 

 believe the above considerations are sufficient to account for the ob- 

 served greater abundance of thrips along highways. 



It would be interesting to test this view by studying a region about 

 to be thrown open to settlement, both before its opening and at in- 

 tervals thereafter. If my interpretation is correct, it should be possil^le 

 to observe the gradual increase in the relative abundance of thrips near 

 lines of traffic. 



ENEMIES AND CHECKS. 



Predaceous Animals.. Among these, spiders must probably be num- 

 bered, as I have seen the empty skins of Anaphothrips striatiis entangled 



