PROC. ENT. SOC. WASH., VOL. 23, NO. 1, [AN., 1921 9 



Cogan (1916). Taking the attachment of this bar to head plate 

 C as a definite anatomical landmark, these authors are forced to 

 regard the large facial plate (figs. 4, 5, 6, Ft) as the "clypeus," 

 of which the lateral plates A and A become posterior lobes. 

 The free sclerite (Clp) in front of the mouth then is the "labrum " 

 and the terminal grooved piece, holding the bases of the setae, 

 the "epipharynx. " 



With these interpretations, however, we are involved in 

 several difficulties which the writers proposing them do not con- 

 sider. First we have the anomaly of the hypopharynx forming a 

 bridge between the posterior lobes of the assumed clypeus 

 (fig. 7, A, a, Hphy, a, A). Internally there are more perplexing 

 complications. The dilator muscles of the pharynx (fig. 15, 

 PhyMcl) are attached to the plate (Ft) which in other insects, 

 including the Thysanoptera, is the front, but which is now 

 presumed to be the clypeus! Finally the protractor (extensor) 

 muscles of the mandible become attached to the clypeus (not 

 to the gena or to a ridge between the gena and the front as is 

 normal in other insects). These are startling innovations and 

 the same writers, Muir and Kershaw (191 1 a) do not apply them 

 to the head of a thrips in making comparisons between the 

 Hemiptera and the Thysanoptera. In the thrips they name the 

 facial plate, to which the dilator pharyngeal muscles are attached 

 the front; the plate below this and in front of the maxillary 

 plates they identify as the clypeus; while the terminal sclerite 

 forming a sheath for the seta;, as Lm of figures 6 and 12 does in 

 the cicada, they call the labrum. Peterson (1915) identifies 

 these parts of the Thysanoptera the same as do Muir and Ker- 

 shaw. Yet the parallelism between the head sclerites of Hemip- 

 tera, Thysanoptera and Orthoptera is so apparent that the 

 present writer has adopted the identifications given on figure 4 

 for the Hemiptera. Since the protractor muscles of the first 

 seta (fig. 13, JPMcl) are attached to sclerite A and to a high 

 ridge between A and the apparent front (Ft) it seems reasonable 

 to regard A as at least a part of the gena, and yet, how can the 

 mandibular articulation be behind any part of the gena? In 

 any case the mandibular bases must be admitted to have shifted 

 to a position behind the base of the hypopharynx, so we mi^ht 

 regard the sulcus between plates A and R as a split in the gena 

 produced by the downward growth of the lateral parts of the 

 head. 



However, again we are invoking far-fetched explanations, and 

 accepting the point g as the articulation of the mandible only 

 leads us into still further difficulties when we study the internal 

 connections of the first setae. Ifgis the mandibular articulation, 

 then the strip of chitin running from it to the funnel like base of 

 the first seta must be the base of the mandible (figs. l.\ 14, /). 

 In which case the protractor (extensor) muscles are attached 



