27] THE HEAD-CAPS CLE OF COLEOPTERA—STICKyEV 27 



nephron (Fig. 20), Photinus (Fig. 37), Othnius (Fig. 52), Clielonarium 

 (Fig. 73), Eurymycter (Fig. 138), and others, is large and membranous. 

 The labrum of Aphodius (Fig. 260) is large but very thin and delicate. 

 The labrum of Scolytus (Fig. 295) and Dendroctonus (Fig. 296) is prob- 

 ably membranous. In the latter, a significant looking slightly chitinized 

 structure is located in the membrane within the mouth that may represent 

 the labrum. In Thecesternus (Fig. 292) there is a membranous area closely 

 joined to the postclypeus that probably represents the labrum. In 

 Eupsalis (Fig. 284), Lixus (Fig. 291), Sphenophorus (Fig. 293), and 

 Rhynchites (Fig. 288), there is an area within the mouth, bounded by 

 furrows, that may be the labrum. Such are particularly suggestive in 

 view of the fact that in the same location and lying flat against the post- 

 clypeus a very thin but a relatively large and well chitinized labrum was 

 found in Epicaerus (Fig. 290) and Attelabus (Fig. 289). The labrum in 

 Isorhipis (Fig. 210), Nosodendron (Fig. 224), Phyconomus (Fig. 230), 

 and Derobrachus (Fig. 269) is considerably reduced in size. It is present 

 in every species, except possibly the rhynchophorous genera named 

 above, where it is always said to be wanting. 



The prominent opening in the caudal part of the head is known as the 

 occipital foramen. It is generally very large, but in some species, such as 

 Connophron (Fig. 168), Cephaloon (Fig. 194), and Macratria (Fig. 203) 

 is reduced in dimensions, due to the constriction of the caudal end of the 

 head. In Calopteron (Fig. 182), Photinus (Fig. 183), Alaus (Fig. 209), 

 and Tetraopes (Fig. 272), the occipital foramen is extraordinarily large. 



In order to understand clearly the developmental processes that take 

 place on the ventral surface, it is necessary to consider a sclerite, belongmg 

 to the mouth-parts, the submentum. In generalized insects the sub- 

 mentum is not only adjacent to but is one of the covering parts of the 

 occipital foramen. Such a position is not found in the Coleoptera. Here, 

 it is always located cephalad of the occipital foramen, with a distinct area 

 between the two. In the vast majority of heads this area is very wide. 

 It is considered as having been present in primitive forms, and is shown on 

 the hypothetical type. The submentum in generalized insects is a large 

 quadrangular movable plate. Many genera of Coleoptera show a similar 

 size, form, and mobility, as in Leptinus (Fig. 165), Necrophorus (Fig. 166), 

 Stenus (Fig. 170), Nacerda (Fig. 195), Neopyrochroa (Fig. 202), Alaus 

 (Fig. 209), Heterocerus (Fig. 216), Cyphon (Fig. 220) and Byrrhus (Fig. 

 223). In these three characters, and the additional one of position in re- 

 spect to the paracoila, which in generalized insects is normally found 

 beneath the submentum, that of the Adephaga seems to be the most primi- 

 tive. The innumerable sizes and forms assumed by this structure through- 

 out the entire series of families can best be judged by glancing at the figures. 

 It is extraordinarily large in Rhysodes (Fig. 225). 



