196 THYSANOPTERA AND ORTHOPTERA. 



as badly infested as before. In 48 hours the older flower was quite 

 brown and was becoming dry; it bore 6 thrips. The younger flower 

 seemed past its prime and contained about 15 thrips. Here was an 

 opportunity to migrate to better material and to less crowded quarters 

 which was not seized until drying plant tissues compelled it. The 

 significance of the small number (15) found on the younger flower, as 

 compared with the probable 75 that were on the older one, is pointed 

 .out elsewhere. 



Possible factors operating against migration. In an attempt to explain 

 this variation in number in different localities, it is to be determined 

 why the thrips remain near the road. There are several possible solu- 

 tions. (1) The conditions maybe most favorable near the roads. 

 (2) They may be incapable of sustained flight, w^hile leaping or 

 crawling is wholly inadequate to cover considerable distance. (3) 

 They may fly long distances, but without aim, so that most of the 

 migrating ones are lost. Or (4) they may be simply inert, that is, they 

 may "prefer" remaining in tolerably favorable circumstances to flying 

 away to a fate unknown. While I may not be able to answer the 

 question definitely, I hope to eliminate some of the suggested answers, 

 and to establish a strong probability. 



(1) Preference of location. The only difference I could detect be- 

 tween the situations near the road and those back in the fields was a 

 larger or smaller amount of dust in the former. Inasmuch as the food 

 of thrips is apparently the juice or tissues of the plants, dust can hardly 

 be other than a deterrent factor, if it is a factor at all. -The care taken 

 to have all the flowers practically alike, equally abundant, and 

 evenly distributed, precludes, I believe, any likelihood that anything 

 could have made one situation better than the other. 



(2) Incapability of sustained flight. The ability to fly may be 

 more directly tested. As stated above, the rate of flight in the species 

 observed was 30 to 90 cm. per second. It is evident that flight must 

 be wholly unsuccessful in the face of even an ordinary wind. But with" 

 the wind, or in calm weather, the case may be different. Owing to the 

 small size of the insect, it is generally impracticable to observe their 

 flight for any great distance. By darkening a room everywhere ex- 

 cept a portion of one window which admitted direct sunlight, and re- 

 leasing a thrips in the beam of light, where it could be observed against 

 a dark background as it flew towards the window, I have been able to 

 trace continuous flight for about three meters. But there is indirect 

 evidence of much greater distances. Parth" to test this, and partly to 

 observe the reaction of thrips to light, two adjoining rooms were dark- 

 ened, except a south window in one of them. There was an open door 

 between the two rooms so that the farthest side of the second room was 



