134 
Psyche 
[September 
closed by the notum, cervical membrane, and sclerites are exposed, 
and the sternum is not externally joined to the pleuron. 
B) . Pro-Mesothoracic Joint. — In SC a partial posterior articu- 
lation-collar is formed from notal and pleural flanges, which rest on 
the mesothorax and enclose dorsal and lateral intersegmental mem- 
brane. Such flanges are absent and intersegmental is exposed in GH. 
C) . Not'O-Plenral and Pleuro-Sternal Joints. — In SC these con- 
nections are of the rim-fold type and result in a rigid frame pro- 
thorax. The horizontal invagination that forms the endopleuron is 
located close to the notal rim. In GH, the noto-pleural joint is 
loose and there is neither an external pleuro-sternal connection nor 
a broad endopleuron. 
D) . Trochantinal and Coxal Articulations. — In SC the coxa 
and trochantin are connected to one another and to the sternum and 
pleuron via loose rim-fold joints, which enclose peritrochantinal and 
pericoxal membrane (fig. 56). Loose rim-fold joints are absent in 
GH and these membranous regions are exposed. 
E.) Attachment of Sternal and Pleural Invaginations (or Apo- 
physes). — There is no vertical pleural apophyseal invagination in 
any extant coleopteron; the sternal apophyses are always present. In 
all other Holometabola, both sternal and pleural apophyses are pres- 
ent, and these invaginations are solidly fused together (fig. 21 ). 
These differences are of two major types, those that result in the 
enclosure of membrane between moving parts (A, B, D) and one 
that results in a rigid frame prothorax (C). 
Origin of the Coleopterous Prothorax 
Major features of the hypothetical stem prothorax of beetles re- 
sult in great improvements in structural integrity over the ancestral 
condition due to development of rigidly attached segmental walls, 
and widespread enclosure of membrane. Two differences seem to be 
side effects of an increase in structural stability. An anterior, exter- 
nal sternal attachment provides potentially rigid attachment for this 
sclerite to the pleuron or notum, and permits the sternum to form 
the ventral body wall, and part of the anterior collar. An internal 
attachment can give only rigidity. To have both is redundant. Loss 
of both the internal sterno-pleural attachment and the pleural apo- 
physis itself may be a structural simplification occurring after devel- 
opment of a multi-purpose anterior sternal attachment. Parenthet- 
ically, the line of fusion between the posterior rim fold and the body 
