1915] The Head and Mouth-Parts of Thysanoptera 37 
they are homologous with the so-called maxillary plates 
described by Muir and Kershaw. The segmented palpi 
however are wanting on the maxillary plates of the Hemiptera. 
Their presence in Thysanoptera indicates a more primitive 
condition and also is conclusive evidence that these sclerites 
are maxillary in origin. 
The lateral margins of the maxillary sclerites (mx. s.) are 
turned into the mouth-cone, thus giving rise to ento-mesal 
extensions (et.) or plates. These extensions, toward the distal 
end of the mouth-cone, unite with the lateral edges of the 
pharynx and form sheaths or troughs over which the needle- 
like portions of the paired piercing organs (mx.) pass. This 
is clearly shown in cross-sections of the pharynx (fig. 38-44 and 
51-57). 
Two sets of ps, eres are found in the mouth-cone of 
all Thysanoptera (fig. 22, 27, 28 and 33). The paired, sym- 
metrical set will be considered first because of their relation 
to the maxillary sclerites. Each one of the paired piercing 
organs is composed of three parts (mx.). Its distal portion 
is long, grooved, needle-like, and swollen at the proximal end. 
The middle portion is short and thick and separated from the 
distal portion by a distinct suture while its proximal end is 
connected to the proximal portion by a movable joint. The 
proximal portion is a heavy piece and in some cases three or 
four times as long as the middle portion. Also its proximal 
end articulates against. the cephalic edge of the maxillary 
sclerite (mx. s.) or in an acetabulum on an elevated pillar 
(mx. p.) arising from the head-capsule. This articulation in 
all cases is either directly or indirectly associated with the 
maxillary sclerite. The presence of two movable joints in 
this piercing organ makes its extension into a straight line pos- 
sible. This extension occurs when the organ functions. When 
the paired piercing organs are extended beyond the mouth-cone 
the grooved, needle-like piercing portions interlock and such a 
union naturally makes an effective tool. This interlocking is 
readily seen in sectioned material (fig. 25). 
The muscles which bring about the extension of the paired 
piercing organs (mx.) are attached to the proximal pieces. 
Each extensor muscle is connected with the ental surface of the 
maxillary sclerite and the mesal edge of the proximal piece 
when the piercing organs (mx.) are within the head-capsule. 
