1972] 
Hlavac — Prothorax of Coleoptera 
139 
Since a number of forms are to be compared in some detail, it is 
convenient to treat variation separately in three functional units : 
1) Size (volume, muscle housing) 
2) Pleuro-coxal mechanism (housing, motility of coxa, trochantin, 
pleuron ) 
3) Intersegmental attachment (pro-mesothoracic joint) 
Size. Variation in muscle volume and consequently in gross pro- 
thoracic size represents adaptive equilibria to the sharply different 
power requirements of substrate and surface locomotion. Obviously, 
compression of substrate requires more power than simply counter- 
ing the force of gravity on a surface. Changes in prothoracic size 
are most easily demonstrated through consideration of one dimension 
height, i.e., prothoracic height and that of component structures/ 
height of metathorax plus elytra (= Thi, etc./Th3). 
In the high volume prothoraces of substrate dwellers, the notum is 
tall(Ni/Th3 ca. 60%), the sternum extends to near the coxal apex 
and the entire assemblage is frequently ca. 85% TI13 (figs. 27, 29, 
32, 34, 37, 39, 42, 44, 47, 49). At the other extreme, in the low 
volume prothoraces of a surface or interstitial space inhabitants the 
Figs. 25-49. Prothoracic variation within five higher taxa of Polyphaga. 
Each set, bounded by brackets, compares two forms differing widely in 
prothoracic volume and consists of five figures: a pair of lateral views, 
drawn parallel to the coxal long axis, a smaller pair of posterior views 
and a set of graphs comparing the height, length and width of the pro- 
thorax with the dimensions of the metathorax. The set on the left, e.g. 
fig. 29A, contrasts the cumulative heights of prothoracic structures expressed 
as a percentage of height of metathorax plus elytra, measured at the level 
of the posterior edge of the noto-sternal joint, and depicted in the same 
order as the drawings. While the paired, crossed lines on the right, e.g. 
fig. 29B, contrast the length (vertical line) and width of the pronotum 
expressed as a percentage of maximum width of metathorax plus elytra ; 
the percentage scale on the right applies here as well. 
Figs. 25-29; Elateriformia ; figs. 27, 28 Lutrochus geniculatus (Limni- 
chidae) ; figs. 28, 29, Perothops muscida (Perothopidae) . 
Figs. 30-34, Cleroidea ; figs. 30, 31 Malachius aenaeus (Melyridae) ; figs. 
32, 33, T emnochila chorodia (Trogositidae) . 
Figs. 35-39; Staphylinidae ; figs. 35, 36, Philonthus cyanttipennis ; figs. 37, 
38, Trigonurus crotchii. 
Figs. 40-44; Cucujoidea; figs. 40, 41, Epicauta pennsylvanica (Meloidae) ; 
figs. 42, 43 Alobates pennsylvanica (Tenebrionidae) . 
Figs. 45-49; Scararbaeidae ; figs. 45, 46, Euphoria limbalis; figs. 47, 48, 
Copris fricator. 
Figs. 50-55 ; compaction and enclosure in Polyphaga. Figs. 50-52, lat- 
eral, posterior view of prothorax and lateral view of body of A gathidium 
sp. ( Anisotomidae) . Figs. 53-55, same of Chelonarium lecontei (Chelonari- 
idae). See text for explanation. 
