894 



INSECTA. 



the cornea;, but the ridge is absent, while the 

 transverse ridges between the two portions of 

 the clypeus are distinct, and also their corres- 

 ponding sutures on the exterior. On the other 

 hand, in the large Buprestis chrysis, the longi- 

 tudinal portion of the epicranial suture is very 

 distinctly marked on the upper surface, and ex- 

 tends as fax forward as the middle between the 

 corneae, while internally the ridge is so largely 

 developed that it extends downwards into the 

 cavity of the head, like the ossified falx in the 

 head of some Carnivorous Mammalia, partially 

 dividing the posterior region of the head into 

 two halves. But the clypeus anterior and pos- 

 terior are so solidified together, and united with 

 the epicranium, that they are not easily distin- 

 guished. This is also the case in the rapacious 

 ground-beetles, Cicindelida, in which all the 

 parts of the cranium are completely united, and 

 the true clypeus is reduced to a narrow trans- 

 verse plate, with which the labrum is freely ar- 

 ticulated. But in the rapacious water-beetle, 

 Dyticus marginalia, although the ridge of the 

 epicranial suture is wanting, as in Cicindetida, 

 the suture itself is remarkably distinct, and the 

 anterior and posterior clypeus are well marked, 

 and are very clearly seen owing to their thin- 

 ness and translucency, when examined by 

 transmitted light. 



The inferior surface of the head affords us 

 equal reason with the superior, for believing 

 that this part of the insect is formed of an ag- 

 gregation of several segments. We shall ex- 

 amine them more particularly when speaking 

 of its development. On its interior surface 

 are parts which tend much to confirm the opi- 

 nion. In Hydrous piceus on each side of the 

 occipital foramen there arises a strong bony 

 plate, lamina posterior (), which, bending a 

 little towards the median line, extends across 

 the basilar portion of the skull, as far as the os 

 transversum (.r), with which it is united. At a 

 short distance from the occipital foramen the 

 lamina of one side is connected with its fellow 

 of the opposite by a narrow bony arch (u), 

 which has been called by Straus Uarcade, and 

 by Burmeister, who has described it in Dyticus, 

 the tentorium. The two laminae beyond this 

 are expanded upwards and laterally, and uniting 

 anteriorly by a thin process form a cradle, or 

 bed, which, as Straus and Burmeister have re- 

 marked, supports the first sutwesophageal gan- 

 glion, while the two lamina postertores inclose 

 between them, as in a canal, the anterior por- 

 tion of the spinal cord, which passes under 

 the tentorium in its exit from the cranium 

 through the occipital foramen. Each of these 

 expanded portions of the laminae are united by 

 their superior angles with a narrow process (s), 

 which articulates, as before noticed, with one 

 of the apophyses of the upper surface or vault 

 of the cranium. The orbital plates (w) above 

 described are continued around the margin of 

 the cornea?, and form the inferior lateral boun- 

 <lary of the basilar portion of the cranium. 

 Between the anterior margins of the corneas, 

 extending across and dividing the basilar 

 part of the skull from the sub-mentum, is a 

 thick elevated ridge, the os transversum (x). 



On its anterior border the os transversum is 

 connected with a minute bony ridge, which ex- 

 tends forwards on each side of the sub-mentum, 

 and it has also two articulating surfaces. The 

 first and most internal of these (.r) is situated 

 close to the base of the sub-mentum, and is that 

 with which the hinge of the maxilla is articu- 

 lated. The second is situated more externally, 

 between this and the margin of the cornea. It 

 is a deep smooth cotyloid cavity (?/), which re- 

 ceives the external inferior angle of the man- 

 dible, and is separated from the articulation 

 for the hinge of the maxilla by an elevated 

 tubercle. Externally the base of the skull is 

 connected only by an indistinct suture with a 

 quadrate plate, the sub-mentum, which was sup- 

 posed by Straus-Durckheim to form a process 

 only of the basilar piece in melolontha, and 

 was called by him the pre-basilaire. We have 

 already seen that it is part of a distinct segment, 

 and seems to correspond to the clypeus poste- 

 rior of the upper surface. At the anterior mar- 

 gin of the sub-mentum, or rather extending 

 backwards upon that segment from the men- 

 turn, are two broad diverging laminae (z), 

 which support the fleshy pharynx and tongue, 

 in which respect they are similar in office to 

 the proper hyoid bones of vertebrata. They 

 serve as means of attachment for some of the 

 muscles of the pharynx, and are connected 

 with similar laminae that cover the upper sur- 

 face of the pharynx, and seem to be connected 

 with the clypeus, as in the Lucanus cervus. 

 The mentum, like the sub-mentum, to which it 

 is attached, is broad, quadrate, and supports 

 the diverging laminae which form the floor of 

 the mouth, and it also affords an attachment for 

 some of the muscles of the tongue and labial 

 palpi. The ligula, or most anterior portion of 

 the labium, is densely covered on its upper 

 surface with hairs. It is divided in the median 

 line into two halves, which, when developed to 

 a much greater extent, as in some other insects, 

 take the name of paraglosste. 



The general structure of these parts is similar 

 in most Coleoptera, but in some species there 

 is considerable variation of form and relative 

 size, owing to the greater development of one 

 part than of another. Thus in Lucanus cervus, 

 (l/ig.388,) in which the whole head is developed 

 to its greatest extent, and the epicranial and 

 basilar regions, with the mandibles (./*), have 

 very far exceeded their usual proportions, the 

 labrum is very minute, and soldered to the 

 clypeus (d), and the maxillae (g) are reduced to 

 small palpiform organs. Internally, the pos- 

 terior laminae (t) do not extend forward to an os 

 transversum, but are short, strong, triangular 

 plates, which, instead of being connected, as in 

 Hydrous and Melolontha, by an arcade, or ten- 

 toriura (u), support a double ring, or annul us, 

 like the ring of a vertebra, through which the 

 nervous cord passes before it arrives at the 

 occipital foramen. In Geotrupes stercorarius 

 there is a like annular form of the same parts, 

 but the laming squamoste, which are absent in 

 Lucanus, are thick and strong, and form a 

 complete cradle for the supra-oesophageal 

 ganglion, la like manner a similar change ia 



