144 
Psyche 
[September 
brane enclosed (figs. 32, 33, 41, 43, 45-48). A complete collar 
(sometimes called closed coxal cavities in the taxonomic literature) 
then increases the structural integrity of a major body joint during 
movement and can be our adaptation for substrate locomotion. 
The posterior collar is only partially developed in many Poly- 
phaga. Commonly, the notal projection extends to the level of the 
trochantinal apex, and mechanically restricts the coxa to rotation 
and the sternal projection extends behind the coxa. When the pro- 
thorax is in its most ventral position, latero-ventral intersegmental 
membrane is enclosed but will be exposed if the prothorax moves 
dorsally (figs. 8, 26). The collar is least developed, providing pos- 
terior clearance in forms capable of extensive flexation (figs. 30, 35, 
40). In these extreme surface grade beetles, prothoracic movement 
does not contribute directly to locomotion but may aid in leg place- 
ment in a multi-planar environment. 
In a few substrate dwellers (e.g., Histeridae), the prothorax is 
rigidly held against the mesothorax during locomotion and the en- 
tire dorsal surface of the squat body is employed to compress mate- 
rial. In some streamlined aquatic groups, the pro- and mesothorax 
are tightly joined, sometimes by complex interlocking mechanisms, 
thereby avoiding potential turbulence during swimming. In these 
two cases, a non-motile joint plays a role in locomotion. Frequently, 
a potentially rigid joint is an important part of an anti-predator de- 
fense system. 
As pointed out above, structures increasing structural integrity 
can be adaptations for substrate locomotion, defense against preda- 
tors, or both. However, several specializations of the pro-mesotho- 
racic joint and peripheral structures function exclusively as part of 
an anti-predator system. 
Given a thick uncrackable cuticle and widespread enclosed mem- 
brane, the remaining vulnerable sites are the appendages and the 
major body joints, i.e., connections between head-prothorax, pro- 
thorax-mesothorax, metathorax-abdomen, elytra-body. 
The strength of the pro-mesothoracic joint is increased in some 
beetles (esp. Elateriformia) by a complex series of interlocking 
mechanisms involving ball-socket and groove- ridge devices (figs. 25- 
28, 53-55). Interlocking occurs apparently only after attack or dis- 
turbance; during walking the segments are widely separated expos- 
ing much intersegmental membrane. 
Appendages can be protected via two strategies: compaction and 
enclosure. In each case, a smooth continuous surface is formed that 
offers neither purchase for crushing mandibles nor a pathway to 
