154 APPLIED ENTOMOLOGY 



Thrips feed on plant juices, puncturing the tissues and extracting 

 the sap, leaving white marks or streaks where the cells without their 

 juices have dried. They attack stems, leaves and blossoms, in the last 

 case often blighting them and preventing the setting of fruit. On leaves 

 of plants the under surface appears in most cases to be the preferred 

 place of attack and the insects do not move about much. With grasses 

 and cereals the stems as well as the leaves suffer, thus checking the growth 

 of the top, and in some cases the kernels of growing grain are also fed 

 upon. Some species live under loose bark and a few have been reported 

 as feeding upon other insects. In many cases the injury caused by these 

 insects is very serious. 



In one section (Suborder Terebrantia) the female has an ovipositor 

 with which she saws slits in the epidermis of plants, placing an egg in 

 each slit. In the other section (Suborder Tubulifera) there is no ovi- 

 positor and the eggs are laid upon the surface of the food material. The 

 larvae considerably resemble the adult. After from two to four molts 

 they leave their food to find some more protected place and there molt 

 again, at which time wing stubs appear and other changes can be seen. 

 Another molt and now the insect becomes quiet unless disturbed, not 

 feeding, and marked changes become evident, bringing it more nearly 

 like the adult, and the completion of these changes is followed by a molt 

 which produces the adult itself. This is more than a typical incomplete 

 metamorphosis, yet not entirely comparable with a complete one. It 

 may be regarded therefore as intermediate between the two. 



In some cases parthenogenesis, i.e., the production of the next gener- 

 ation by unfertilized females, occurs. This is perhaps to some extent 

 determined by weather conditions, in this group. Parthenogenesis is 

 frequently present here and there among insects and will be considered 

 more fully elsewhere. Driving rains are very destructive to all kinds of 

 Thrips. Lady beetles and other insects of several species feed freely 

 upon them. 



The Wheat or Strawberry Thrips (Frankliniella tritici Fitch). This 

 is probably the most widely distributed species of the group in this 

 country. It feeds on wheat, strawberry, apple and many other plants and 

 where the blossom is attacked as in the case of the strawberry, it is blighted, 

 preventing the formation of the fruit and producing the stunted struc- 

 tures known as " buttons," instead. Leaves attacked often curl and be- 

 come malformed, the particular parts injured soon turning brown and 

 dying. In California it is a particular pest of alfalfa. 



The adult is about a twentieth of an inch long, yellowish in color. In 

 the warmer parts of the South it is more or less active at all seasons of 

 the year, but in the North it winters in protected places, many probably, 

 like other species, in grass fields close to the ground. 



The life history in the South requires about 12 days but is probably 



