6 



BULLETIN 421, V. S. DEPARTMENT OF AGRICULTURE. 



0.315 mm.; one pair of setse on each lateral margin of the prothorax, and one on the 

 posterior margin; two pairs on the dorsum of the mesothorax and metathorax. 



Abdomen tapering gradually posteriorly until the ninth segment, then abruptly. 

 Segments 9 and 10 much narrower than the rest and bearing the rudiments of a pair 

 of obtuse spines on the posterior margins. The first abdominal segment bears one 

 pair of lateral setse; segment 9, one pair dorsally and one laterally; segment 10 bears 

 one pair; the remaining segments bear two pairs dorsally and one pair laterally. 



Third stage (fig. 5). — Head subquadrate, wider than long; length 0.116 mm.; width 

 0.166 mm. Eyes dark red, small; ocelli absent. Antennal cases, segments con- 

 fused; only five apparent; length 0.265 mm. 



Prothorax transverse, half as long as broad; length 0.133 mm.; width 0.266 mm.; 

 a row of setse on the lateral and posterior margins. Mesothoracic angles prominent; 



Fig. 3. — Newly-hatched nymph of 

 sugar-beet thrips. Enlarged about 

 160 diameters. (Original.) 



Fig. 4.— Second-stage nymph of 

 sugar-beet thrips. Enlarged 

 about 40 diameters. (Original.) 



width of mesothorax 0.299 mm. The wing pads reach to the third abdominal seg- 

 ment. Length of wing pads from the base to the tip 0.299 mm.; the hind pair the 

 longest. Abdomen fusiform, tapering slightly anteriorly; the last two segments are 

 much narrower than the rest, and bear on their posterior margins two large obtuse 

 spines, which extend upward. 



Fourth or last stage (fig. 6). — Color translucent white; length 1.469 mm. 



Head subquadrate, faintly reticulate; wider than long; length 0.12 mm.; width 

 0.199 mm. Eyes dark red; three ocelli present, lighter in color than eyes. Antennal 

 cases folded back over the head, the apices extending to the middle of the prothorax, 

 the cases coming in contact beyond the middle of the head. Segments confused; 

 1 and 2 protruding in front of the head. Segment 2 bears four setse, two long and 

 two short ones, extending forward. The head bears four setse, one behind each 

 eye and two small ones between, the antennal cases. The newly formed antennse 

 may be seen through the wall of the case 



