40 
Psyche 
[September 
notum is low (Ni/ TI13 ca. 35%), the most ventral point of the 
sternum is far above the coxal apex, and the coxa is quite elongate so 
that Thi/Th3 is ca. 70% (figs. 30, 34, 35, 39, 40, 45, 49). Large 
changes in gross prothoracic size are associated with great modifica- 
tion in relative sizes of structural units due to the geometry of pro- 
thoracic musculature. 
Major locomotory muscles are housed within the notum (Larsen 
1966, fig. 64). Sharp increase in muscle mass, then, implies increased 
notal volume. And notal enlargement effects other prothoracic struc- 
tures. There appears to be a distinct upper limit to prothoracic 
height in most taxa; Thi/Th3 rarely exceeds 90%. Given this 
boundary to vertical growth, large notal increase is accompanied by 
sternal reduction and coxal modification. The complimentarity of 
notal and sternal heights effects coxal structure. 
In generalized forms, the most dorsal point of the coxa is level 
or nearly so with the noto-sternal joint (fig. 6). As notal height 
increases, the coxa is either reduced with the sternum, maintaining 
initial geometry (fig. 27) or the coxa is elongate and enclosed deeply 
within the notum (figs. 42, 45, 47). Each strategy has been adopted 
numerous times in the Polyphaga, while only the former occurs in 
the Adephaga (figs. 8, 61, 65). 
Variation in prothoracic length and width are correlated with 
height. The width of a prothorax is rarely greater than that of the 
metathorax (figs. 29, 34, 39, 44, 49). Prothoracic height and width 
are then usually limited by the corresponding dimensions of the 
largest segment of the body. 
Structure of substrate grade configurations can be explained as an 
optimization of muscle volume given geometric limiting factors. 
However, prothoracic structure of extreme surface grade inhabitants 
represents not only reduced muscle volume but also a mechanism for 
increasing coxal flexation, see below. 
Pleuvo-coxal Mechanism. This complex and highly variable sys- 
tem divides naturally into two subunits involving: housing and 
function of pleuron, trochantin and coxa. 
Plousing. Concealment of moving parts and surrounding mem- 
brane produces improved structural integrity. Enclosure can occur 
around the entire coxal perimeter, dorsally by notal and sternal 
cowlings, anteriorly and ventrally by the sternum and sternal projec- 
tion, and posteriorly by the notal projection. Dorsal and ventral 
enclosure are particularly variable. 
Enlargement of rim folds, which originally protect only membrane 
around the coxa and trochantin, forms cowlings that partially to 
