146 
Psyche 
[September 
membrane for probing beaks. In compacted forms, elements of the 
proleg and sometimes the mesoleg as well are drawn tightly together 
and lie in thoracic cavities, so that exposed leg surfaces are flush 
with one another and with surrounding sclerites (figs. 53-55).. Fre- 
quently the antennae are concealed in pronotal grooves and the 
mouthparts by the prosternum. 
In a few groups of Polyphaga, the antennae, mouthparts, front, 
and sometimes middle legs are enclosed, in defense position, within 
a complex cavity formed from elements of the head, pro- and ptero- 
thorax (figs. 50-52). A “roll up in a ball” strategy requires exten- 
sive structural and mechanical modifications which result in great 
reduction of prothoracic volume. 
The pro-mesothoracic joint, then, varies greatly in structural in- 
tegrity and may contain locomotory and defense adaptations. 
Adephaga. This group consists of a broad adaptive array of sub- 
strate and surface inhabitants as well as several exclusively aquatic 
families. A uniform pleuro-coxal mechanism and high degree of 
structural integrity are maintained throughout (see subordinal di- 
agnoses above). Structural variation occurs primarily in the com- 
pleteness and quality of the posterior articulation collar, which is 
incomplete only in a few apparently primitive taxa. In all major 
groups of ground and arboreal carabids the collar is complete; re- 
versal following shift in zone seems unlikely (Hlavac 1971). Pro- 
thoracic size is quite variable and is, of course, strictly correlated 
with ecology, as in Polyphaga. In the Adephaga, prothoracic elonga- 
tion is particularly important and is graphically seen in the increased 
ventral inclination of the pleuro-sternal joint (figs. 61-65). The 
pattern of prothoracic evolution in the Adephaga is similar to that 
found in several biologically diverse higher taxa of Polyphaga with 
specialized pleuro-coxal mechanisms and relatively little structural 
variation, e.g., Scarabaeidae (figs. 45-49). 
Discussion 
As seen above, similar prothoracic configurations and characters 
occur in members of unrelated taxa which share a common band on 
a broad ecological spectrum. Reasons why convergence and paral- 
lelism are an important aspect of prothoracic differentiation can be 
seen from a consideration of the relative breadth of adaptive pathway 
within each functional variable — structural integrity and power. 
The plasticity of structures varying morphological strength and en- 
closure of membrane is sharply limited by geometry, i.e., the adaptive 
pathway is narrow. For example, there are just two modes of pos- 
