116 PROCEEDINGS OF THE NATIONAL MUSEUM. 



ECONOMIC CONSIDERATIONS. 



INJUKIOUS FORMS. 



Small and apparently insig-nificant though these insects are, they can. 

 not be disregarded from an economic standpoint. Only a few species, 

 to be sure, must be considered as decidedly injurious, but these arej 

 widely spread and hard to control. Doubtless much damage, really^ 

 caused l)y these tiny foes, has been attributed to more conspicuous but; 

 less injurious insects. The most important species in this country 

 belong to the family Thripidfe. The economic importance of each oft 

 these "species is considered in connection with its description, buti 

 there are, however, some general points worthy of note which may, 

 be considered together here. 



Feeding hah Its. ^Thvi^s are found upon most flowering and some 

 liowerless plants. The general mode of feeding is the same through- 

 out the order. The green parts of the plant, chiefly, are punctured 

 by the piercing mouth parts and the sap withdrawn therefrom by suc- 

 tion The emptied plant cells become white and shriveled as they dry 

 up and the insect, standing usually parallel to the veins of the leaf, 

 moves on to fresh cells. The traces of their feeding are thus left in 

 irregular streaks of dried, whitened cells. Behind them, as they feed, 

 they leave rows of dots of dark colored excrement, which, it seems,; 

 have sometimes been mistaken for eggs. ^ ' 



On flowers Thrips are most abundant in summer. Burmeister states 

 that the nectar of flowers furnishes them with nourishment, and Per- 

 gande has expressed a similar opinion (219); but this does not seem tc 

 me to be the case, as when present on flowers they are found sucking 

 sap not nectar. Thev feed to some extent upon the petals, but nol 

 so freely as upon the green parts. The inner surface of the sepals ui 

 a favorite place for feeding and oviposition. The essential parts oi 

 the flower come in for their part of the general attack and it is jusi 

 here that the greatest injury to the plant is caused. Injury of this 

 sort has been reported, especially upon strawberries by Quaintanc( 

 (151), and upon apple and other fruit blossoms by Osborn (218, 223 

 221),'in which cases they caused so much injury to the styles by then 

 punctures as to prevent fertilization and the setting of the fruit. (Set 

 . Buthrips trUici, p. 152.) Probalfly Euthnp. fntlcl and Thnps tahac 

 are guilty of most of such injuries. Mally has recorded a very simila; 

 injury to cotton bolls by an undetermined species of the family Thrii 

 pidte,' (311). Many other flowers, though perhaps of less economij 

 importance, are similarly attacked. Flower species feed also up| 



On the leaves of plants and trees may be found a large variety^' 

 species, most of which feed mainly upon the under surface of th, 

 leaves, probably chiefly for protection from sun, rain, and enemieg| 



