No.i:uo. NORTH AMERICAN TlIY,SANOrTERA— HINDS. 117 



though it is also possible that more tender tissues may be an attraction. 

 Such species avoid the light and, if a leaf be turned over, the insects 

 will move around to the under side again. The constant su(^king of 

 myriads of larv» and adults soon causes the feeding ground to wither, 

 the leaf becomes encrusted with dead cells and dark colored spots of 

 excrement and it is not long })('fore its death results. Unless disturbed, 

 most species do not travel much, and thus in time there appears to be 

 something of a colony feeding around the place where the mother has 

 fed and deposited her eg*gs. Though many plants thus suffer from 

 the destruction of their leaves, the onion seems to be most severel}^ 

 afflicted. (See Thrips tahaci, p. 183.) 



Grasses and cereals may be included in a third class in which the 

 nature of the injury is somewhat different. Besides the abstraction of 

 sap from the leaves of these plants, Thrips cause a greater injury by 

 attacking the tender axial stems, thus cutting oft' directly the supply of 

 sap to the head, w^hich therefore fails to bear fruit and may be entirel}^ 

 killed. This is the waj^ in w^hich "Silver Top" is caused, and it is 

 impossible to estimate with any degree of accuracy the damage which 

 results to the hay crop. Besides working in this way, Thrips are 

 charged with attacking directly the growing kernels of cereals. In the 

 case of wheat, rye, oats, etc., they suck the nutritious milk directly 

 f j'om the growing kernels in the ear and produce an abortive condition 

 of much, if not all, of the head, which is then called "pungled.'" 



Greenhouse species appear to be l)ecoming more numerous and more 

 injurious each year. The principal injury here is done to the leaves, 

 and nearly all kinds of greenhouse plants are subject to attack. Thrij^s 

 tahaci^ which has recently come into prominence, especially in cucum- 

 ber and carnation houses, has an unusually wide range of food plants. 

 It has already proved to be a serious pest, capable of the complete 

 destruction of a crop, and is exceedingly difficult to control. 



BENEFICIAL FORMS. 



Predaceous Thrijys. — The late B. D. Walsh once expressed the opin- 

 ion that Thrips "are generally, if not universally, insectivorous, and 

 that those that occur on the ears of the wheat, both in the United 

 States and in Europe, are preying there upon the eggs or larvse of the 

 Wheat Midge {Diplosls tritici)^ and are consequently not the foes, as 

 has been generally imagined, but the friends, of the farmer" (127 and 

 132). Such an opinion from so eminent an entomologist is likeh^ to 

 have some basis in fact, though w^e question whether his conclusion is 

 even usually correct. Thrips have been frequently found in the galls 

 caused by other insects, either with the makers of the galls or alone, 

 and the conclusion has been drawn, though frequently, we suspect, 

 without a direct observation to that effect, that the Thrips were prey- 

 ing upon the makers of the galls. Walsh also wa-ites that he has 



