1915] The Head and Mouth-Parts of Thysanoptera 30 
head-capsule. The invaginations of the anterior arms are very 
indistinct in this species due to the thick, highly chitinized 
and reticulated nature of the head-capsule. This characteristic 
also makes it difficult to differentiate the tentorial thickenings. 
However in Thrips physapus (fig. 3 and 6) the invaginations 
of the anterior (i. a.) as well as the tentorial thickenings are 
readily made out in dissected heads that have been stained. 
The invaginations of the dorsal arms (i. d.) with their ental 
projections show very clearly in the adult of Heliothrips and 
likewise in all thrips. 
In the nymph of Cephalothrips (fig. 22) a still greater 
reduction of the tentorium apparently exists. This apparent 
loss, however, may be due to the fact that it is impossible to 
distinguish all the parts on account of the extremely thin and 
membranous nature of the head-capsule. Only the invagin- 
ations of the dorsal arms (i. d.) with their ental projections 
show very clearly. By careful staining one can distinguish 
a thickening (a. a.) occupying the position of the left anterior 
arm of the tentorium. This thickening extends from the 
invagination of the left dorsal arm (i. d.) to the point where the 
asymmetrical piercing organ (1. md.) unites with the head- 
capsule. In this feature there is a remarkable similarity 
between the nymphal head of the Tubulifera and the adult 
head of the generalized Terebrantia. 
The tentorium of the adult of Cephalothrips resembles 
somewhat that of Heliothrips. In Cephalothrips the anterior 
arms of the tentorium are atrophied but the invaginations 
of the dorsal arms with their projections are very distinct 
and the invaginations of the anterior arms can be identified 
in dissected material that has been stained, if a careful examina- 
tion is made of the caudal boundary of the head-capsule (z.) 
at the point where the lateral margins of the clypeus are in 
contact with the front. Besides the thickenings that are similar 
to those of Heliothrips two thickenings extend cephalad (mx. p.) 
on the lateral areas of the head-capsule. These cephalic- 
extending thickenings clearly arise from the thickenings (z.) 
about the caudo-ventral margin of the head-capsule and 
terminate in enlarged, elevated ends which possess acetabula 
in which the proximal pieces of the paired, piercing organs 
(mx.) articulate. One can readily see that these mandibular 
pillars, as Muir and Kershaw call them, are of a tentorial origin. 
