NO. 1310. NORTH AMERICAN THY SANOPTETtA— HINDS. 87 



crystals, many of which were also floating throug-h the gh'cerin. 

 As a result these slides, containing most of the results of a sunmier's 

 collecting, had to be thrown awa3\ The exact composition of the 

 crystals was not determined. l)ut it is suppo.sed that they were mostly 

 phosphat(\s which had been dissolved in the juices of the insect's body. 

 As the juices were gradually drawn out, the phosphatic salts, not 

 being soluble in the glycerin, were deposited as the white crystals. 



There are still other objections to glycerin as a mounting medium 

 for Thysanoptera, though it may be all right for other insects. The 

 dark pigment of the eyes is frequently dissolved out by glycerin, and 

 spreads all through the head, suffusing it with a dark color, Avhich 

 obscures all details in that region. Furthermore, glycerin does not 

 preserve the tissues of the body for a very long time. They gradually 

 go to pieces, the segments spread apart, and the mount becomes 

 worthless in the course of a few years. Of course this objection to 

 glycerin does apply to the mounting of chitin which has been cleared 

 from all soft tissues by treatment with caustic potash solution, as 

 chitin is unatiected by glycerin. 



EXTERNAL ANATOMY. 



INTKGUMENT. 



Adult.— T\iQ chitinous skeleton of these insects is quite lirm. The 

 body wall is made up of strongly chitinized, rigid plates joined together 

 by thin and very flexible mem})ranes. The texture of the plates 

 appears usually to be quite uniform in diflerent parts of the same 

 specimen. In the head, especially, several of them are so smoothly 

 joined that no sutures arc visible. The thin connecting membrane 

 may ])e smooth and of a uniform thickness, or, as in many parts of 

 the Tubulifera, it may show a peculiar structure in the nature of regu- 

 lar, distinct, very minute, plate-like thickenings, varying in form but 

 often circular or hexagonal, giving a decidedly granular appearance to 

 the area. 



The chitin is frequently thrown into more or less distinct folds or 

 ridges, most frequently transverse in direction, but often branching 

 and running together to form a reticulated structure. The back of 

 the head and the pronotum are most frequently marked in this way. 

 Sometimes the ridges become very thick and pronounced, and form a 

 regular network over the surface so conspicuous as to be of use in 

 classification {Ildlothnp^, PdrthrnothupH, see Plate VI, fig. 64). This 

 reticulation may extend over the whole outer surface of the body, 

 legs, and even the fore wings, but always seems to be heaviest upon 

 the head and pronotum. It is not known to occur in the .Eolothripidie, 

 but is found in several species of Thripida^, and I have discovered it 

 in an undescribed species of Phloeothripidje. 



