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Psyche 
[September 
wholly enclose these structures as well as surrounding membrane 
(figs. 56-60). Enclosure is accomplished through ventral develop- 
ment (below the noto-sternal joint), overlap, or fusion of notal and 
sternal cowlings (figs. 27, 57, 58, 60) ; through dorsal development 
of the notal cowlings (figs. 5, 45, 47, 59) ; or through a combination 
of these two methods (figs. 40, 44, 59). 
Ventrally, the sternal projection may be developed below the 
coxa, thereby protecting the most ventral section of pericoxal mem- 
brane. A sterno-coxal articulation is frequently present in high vol- 
ume forms and prevents the coxa and trochantin from being deflected 
(figs. 27, 32, 42). 
Structures which increase structural integrity by enclosing the 
coxa, trochantin and surrounding membrane are common in the 
Polyphaga and are most prevalent in but not restricted to substrate 
dwelling forms. 
Function. Coxal movement, the end product of the pleuro-coxal 
mechanism, consists, in the Polyphaga, of rotation and sometimes 
flexation as well. Rotation is simply circular movement about the 
coxal long axis. Flexation results in antero-posterior motion of the 
coxal apex and is generated only through movement of the pleuron 
against its notal attachment (fig. 35). The coxa rotates but does 
not flex against the pleuro-trochantinal joint. Each type of coxal 
movement is most suited for locomotion in one adaptive zone. 
Flexation is greatly emphasized in many low notal volume, ex- 
treme surface grade forms and in a few cursorial interstitial space 
inhabitants (figs. 30, 35, 40). In both sites, locomotory require- 
ments for power are minimal, so that the quantity of forward mo- 
tion generated per stroke is a valid measure of coxal performance,, 
which is maximized by employing a combination of rotation and 
flexation. The effectiveness of flexation is a function of the radius 
and swing angle. The radius is increased by lengthening the coxa. 
Anterior and posterior clearances are necessary for a long coxa to 
traverse a broad arc. Anterior clearance is achieved by sternal re- 
duction so that in extreme cases, its ventral plane lies just below the 
trochantinal apex permitting the coxa to slide under the sternal rim 
(fig- 35)- Posterior clearance is obtained by reducing and/or flat- 
tening the notal projection thereby decreasing the amount of inter- 
segmental overlap and exposing membrane. Modifications permitting 
extensive flexation also allow a large coxa to rotate extensively. A 
flexing coxa then imposes strict design limitations on surrounding 
structures. 
Rotation is the sole coxal movement in almost all substrate forms 
