No.isio. NORTH AMERICAN TIIYSAXOPTERA—nTNDS. 121 



fore a sign that the field is becoming exhausted. Such fields should 

 he plowed, and it is advisable to plant ^ith some cultivated crop for 

 at least one season before re-seeding. 



CHAKACTEES OF THYSANOPTERA. 



Small insects; length ranging from one-fiftieth to one-third of an 

 inch. Wings usually present; four in number, long, narrow, mem- 

 branous, never folded, with at most two longitudinal veins and few or 

 no cross veins; hind margin alwaj's, front margin usually, fringed 

 with long, slender hairs much exceeding in length the breadth of the 

 moml)ranoas part of the wing; wings laid horizontally along the 

 abdomen when at rest; wings sometimes reduced to short pads not 

 reaching bej'^ond the hind edge of the thorax and entirely absent in 

 a few species. 



Mouth parts intermediate in form Ijetwecn those of sucking and 

 chewing insects, but probably used almost entirely for sucking; 

 ari'anged in the form of a cone situated on under side of head and 

 placed so far back that it lies almost entireh' under the prothorax (see 

 Plate X, fig. Ill), and is more or less concealed from the side by the 

 fore coxa^ and femora. Mouth cone formed hy the labrum, the broad, 

 flat, triangular, external portion of the maxilke bearing each a tvro or 

 three segmented palpus, and the labium ])earing two or four seg- 

 mented palpi; these external parts grown together and not freely 

 moA'able. Mouth always asymmetrical, only the left mandible being 

 developed. Mandible and lol)es of the maxilla? modified as internal, 

 protrusile, bristle-like piercing organs. 



AntenniB quite slender, six to nine segmented, situated closely 

 together upon vertex of head. Ocelli always present when long 

 wings are present, alwaj^s absent in entirely wingless forms; usually 

 present, sometimes absent, when wings are reduced to pads. Protho- 

 rax distinctly separated from mesothorax* and freely movable. Meso 

 and metathorax firmly and closely united; metanotum longc^r than 

 mesonotum. Tarsi usually two but sometimes one segmented; the 

 terminal segment fitted at the tip with a protrusile, bladder-like organ 

 which can be withdrawn entirely within the segment so as to be invis- 

 ible. Abdomen ten segmented. Terminal segment either conical or 

 tuljular. Three pairs of stigmata are always present and a fourth 

 pair is found in all Tubulifera and man}^ Terebrantia. In the adult 

 these are situated one pair each upon mesothorax and first and eighth 

 al)dominal segments. The metathoracic pair in Terebrantia is small, 

 invisible except in carefully prepared specimens, and in some cases I 

 have been unable to find au}^ trace of it. In the larva the stigmata 

 are distributed in the same wa}" except that they are present on the 

 second abdominal segment and not on the first. 



Young resemble adults in general form, structure of mouth parts, 

 and in food habits. There is, however, a distinct pupal stage during 



