INSECTA. 



893 



occipital foramen to about midway between the 

 cornea, where it becomes much thickened and 

 expanded, and then divides into two portions, 

 which pass forwards and outwards m a dia- 

 gonal direction, to the anterior margin of each 

 cornea. These ridges on the internal surface 

 exactly correspond to the faint indication of the 

 epicranial suture on the external. They serve 

 for the attachment of muscles, and divide the 

 epicranium from the clypeus posterior. At 

 the external angles of these ridges, immediately 

 anterior to the corneae, are two articulating apo- 

 physes, the most external of which, the torulus 

 (r), is smooth and rounded on its anterior sur- 

 face, and articulates with the broad concave 

 extremity of the scapus} or basial joint of the 

 antennae, and the external one (<?), (cuviti coty- 

 loid, STRAUS,) is smooth, rounded, and con- 

 stricted in its middle, and articulates anteriorly 

 with the superior external condyle of the man- 

 dible, and posteriorly with a process of the 

 lamin< i>quamos( (s), which support and pro- 

 tect the brain, and are united with other la- 

 minae (v) (hunes laterales, STRAUS,) which 

 arise from the inferior surface of the cranium. 

 The torulus (r) is attached externally to the 

 most anterior portion of a thin broad lamina, 

 the orbital plate (w}, which extends backwards 

 to the posterior angle of the cornea, in an 

 arched direction, separating the cavity of the 

 orbit from the interior of the cranium, with 

 which it communicates only by means of a 

 round foramen for the passage of the large 

 optic nerve and its tracheae. The superior half 

 of this plate consequently belongs to the epi- 

 cranial, and the inferior to the basilar portion 

 of the skull. Immediately anterior to the epi- 

 cranial suture is situated the clypeus (rf), the 

 middle portion of which is smooth and slightly 

 concave, and forms the covering of the ante- 

 rior part of the head. On either side it has a 

 smooth broad inflected margin, which is not 

 included within the interior region of the head. 

 At the anterior margin of the clypeus is arti- 

 culated the freely moveable labrum (e\ the 

 under surface of which is smooth and shining, 

 and gives no attachment to muscles, excepting 

 along its posterior margin. The ridge of the 

 epicranial suture is developed to a greater ex- 

 tent in the head of Hydrous than in any other 

 species we have yet examined. Its perfect 

 correspondence with the faint indication of the 

 suture on the exterior of the head clearly in- 

 dicates the boundary of the epicranium, and is 

 of very great importance, as we shall hereafter 

 see, in enabling us to determine the number 

 of segments of which the head is composed. 

 This suture exists in every species we have 

 examined, more or less developed in different 

 individuals. Its existence appears to have 

 been entirely overlooked by Straus-Durckheim 

 in the head of Melolontha, in which, indeed, 

 it is almost obliterated externally, but when 

 the cranium is well cleansed, and then ex- 

 amined by means of transmitted light, a trace 

 of it may still be observed, and its situation 

 internally is indicated by a shallow triangular 

 furrow, which extends backwards from the 

 anterior portion of each orbital plate to within 



a short distance of the occipital foramen in the 

 middle line, the longitudinal portion being 

 exceedingly short. But in the larva of the 

 same insect the suture is very distinct on 

 the exterior of the epicranium, and the ridges 

 corresponding to the suture are developed on 

 the interior. Anterior to this suture in the 

 same larva is a triangular piece, which is 

 bounded in front by a freely articulating plate, 

 the anterior clypeus. It is the part correspond- 

 ing to this, and which is consolidated with the 

 true clypeus in the head of Hydrous, as in- 

 dicated by the diagonal depressions before 

 noticed on the external surface of the head, 

 which we shall distinguish in all insects as the 

 clypeus anterior. 



It will thus be found that in some insects 

 the clypeus anterior and posterior have hitherto 

 been confounded under one name, and in 

 others the clypeus posterior and epicranium. 

 We believe, however, that these are distinct 

 parts in all insects, but are less readily distin- 

 guished in some than in others. The upper 

 surface of the head is thus shewn to be formed 

 of at least four clearly indicated portions, both 

 in the larva and perfect insect. In the larva of 

 melolontha there is also a slight indication of a 

 .fifth, segment, of which the antennas, or ante- 

 rior prolongations of the spinal columns, are in 

 reality the proper appendages. The indication 

 of this segment exists in a triangular line, 

 parallel with, but a little anterior to, the suture 

 behind the clypeus posterior, and in the space 

 included between it and the epicranial suture 

 the antennae seem to be inserted. But al- 

 though we believe in the existence of the fifth 

 segment in all insects, it must be acknowledged 

 that it is not easily demonstrated. Four seg- 

 ments are, however, readily detected, yet in 

 some species one of these has almost disap- 

 peared. Thus in Geotrupes stcrcorarius, the 

 epicranial suture has become very indistinct on 

 the upper surface of the head, and the ridges 

 are entirely absent on the interior, as in melo- 

 lontha, while the clypeus posterior exists only 

 as a narrow triangular space, bounded by the 

 suture posteriorly, and anteriorly by a ridge cor- 

 responding to the boundary of the proper an- 

 terior clypeus on the exterior of the head ; the 

 labrum also, as in all insects, being quite dis- 

 tinct. In Lucanus cervus, in which the head 

 has reached its maximum of development, and 

 is much broader than the pro-thorax, there is no 

 indication whatever of the triangular suture in 

 the male, all the parts of the head being firmly 

 consolidated together. But in the female there 

 is a faint depression internally, as in melolon- 

 tha, and the trace of a corresponding line is 

 apparent in some specimens externally. In 

 some specimens of Melue cicatricosus there is a 

 distinct indication of the suture externally, ex- 

 tending from the occipital foramen to near the 

 middle line between the eyes, while internally 

 the ridge is distinctly elevated ; but we have 

 not been able to trace the clypeus posterior, 

 which may be supposed to have merged in the 

 largely developed epicranium. In Blaps mor- 

 tisaga the epicranial suture is usually distinct 

 on the upper surface of the head, posterior to 



