1972] 
i Hlavac — Prothorax of Coleoptera 
133 
23 24 
Fig. 23. Generalized 1 Prothorax of the Lower Holometabola ; Fig. 24, 
Hypothetical stem prothorax of Coleoptera. Membranous regions stippled. 
two parts by an apophysis, or pleural suture is a common feature of 
pterothoracic segments. Among Holometabola this arrangement is 
preserved in the prothorax only in Ithone and a few related forms 
(fig. 20, Eps, Epm, Invg). In the Trichoptera, a detached epimeron 
is present (fig. 22). In all other Holometabola the posterior rim of 
the apophysis is membranous. Ventral enlargement, and fusion of 
sternum, pleuron and even the cervical sclerites of the Corydalidae 
and Raphidioidea are doubtless specialized features readily derived 
from a generalized configuration (Kelsey 1954, figs. 1, 7; Ferris 
and Pennebaker 1939, fig. 61). 
Coleoptera and the Ancestral Group 
Differences in Prothoracic Structure 
Comparison of the generalized holometabolous prothorax (GH) 
with that of the hypothetical stem coleopteran (SC) yields differ- 
ences in five major categories (figs. 23, 24). The abbreviations (GH 
and SC) are employed below for simplicity and to emphasize the 
fact that two abstract assemblages are being considered rather than 
elements of actual organisms. 
A). Head-Prothoracic Joint. — In SC an anterior articulation- 
collar composed of notal, pleural and sternal flanges encloses part of 
the head and all cervical membrane as well as the cervical sclerites. 
The sternum is developed anteriorly and joined to the pleuron. An 
articulation-collar is absent in GH, the head may be slightly en- 
