1915] The Head and Mouth-Parts of Thysanoptera 41 
_ Cephalothrips yucce.—The maxillae of Cephalothrips (fig. 
8, 11, 17, 18, 20, 22, and 33) are typical of the suborder Tubu- 
lifera. The maxillary sclerites (mx. s.) of the nymph (fig. 17) 
and the adult (fig. 8) are dissimilar in comparative size and 
symmetry, but both possess a two-segmented palpus (mx. pl.). 
The long maxillary sclerites of the nymph are nearly symmetrical 
while those in the reduced mouth-cone of the adult are asym- 
metrical. This asymmetry in the adult is characterized by a 
distinct knob (n.) at the cephalic end of the mesal margin 
of the maxillary sclerite. This knob fits into a notch on the 
left margin of the clypeus and appears to be a portion of the 
maxillary sclerite, but a close examination shows that it is 
only connected with the maxillary sclerite by a narrow strip 
at its latero-cephalic corner; and a distinct fissure, which extends 
cephalad and laterad from the point where the clypeo-maxil- 
lary suture turns mesad, separates it from the maxillary 
sclerite. The asymmetrical piercing organ of the adult is 
associated with the above asymmetry. On comparison with 
the asymmetry found in the Terebrantia the asymmetry of the 
Tubulifera is of a decidedly different nature. 
The mesal extensions (et.) of the lateral edges of the maxil- 
lary sclerites resemble the general description of these parts 
(fig. 22, 33 and 35). However, a unique modification of these 
extensions is found in the development of two long, trough- 
like, cephalic extending arms (mx. g.). These parts serve to 
guide the long maxillary sete and have been designated as 
the maxillary guides. These guides are only present in the 
Tubulifera, and undoubtedly they have been developed in this 
suborder on account of the necessity of some kind of a guiding 
structure for the exceedingly long maxillary seta. The max- 
illary guides arise from the mesal part of the sheath (et.) that 
extends between the lateral edges of the maxillary sclerites at 
the distal end of the mouth-cone, and project forward into the 
dorsal portion of the head-cavity to the region of the compound 
eyes. Cross-sections of the pharynx (fig. 53 and 54) show 
that in this forward projection they unite with the pharynx. 
In the nymph they are narrow at their proximal ends but 
broadly rounded and separate at their distal ends, while in the 
adult they are trough-like and their distal ends are united in a 
transverse plate. The long maxillary sete pass around the 
