INSECTA. 



887 



which live on the pollen of flowers, the eyes 

 are very protuberant. From these circumstan- 

 ces it may be inferred that all those insects in 

 which the eyes are either protuberant or very 

 large are directed by sight alone to some parti- 

 cular object of their search, whether this be the 

 female of the species, as with the glow-worm, 

 &c. or the active living prey, as with the rapa- 

 cious beetles; and consequently in these in- 

 stances a more extended field of vision is re- 

 quired than in those whose object of search is 

 more easily discovered, or whose means of sub- 

 sistence is less precarious. In many Coleop- 

 tera each eye is divided anteriorly by a process 

 of the epicranium, the canthus, as is particu- 

 larly the case in the Lamellicornes (Jig. 333). 

 The extent to which this is developed in dif- 

 ferent insects varies considerably, and seems to 

 be greatest in those species which are constantly 

 engaged in burrowing. Thus, while it is ex- 

 tended only a little way into the eye in Ceto- 

 niid<f, it is carried half way across it in Copris, 

 and in the female of Lucanus cervus, but less 

 than half in the male; in the genera Ateuchus 

 and Dorcas more than half way across ; while, 

 according to Kirby and Spence,* in another 

 genus, Ryssonatus, it completely divides the 

 eye into two. In other instances the canthus 

 is not produced, but the eye is encroached 

 upon anteriorly by a portion of the epicranium 

 or by the base of the antenna, which sometimes, 

 as in the Cerambycidtf, appears as if inserted 

 into the eye itself. In other families, as in the 

 Gyrinidte, the middle of the eye is excavated 

 across its whole surface by a deep furrow, 

 which gives the appearance of two distinct eyes 

 on each side of the head. In some insects the 

 eyes are entirely absent, an instance of which 

 occurs in one of the Xylophagi, Annomatus 

 terricola, recently discovered by M. Robert, 

 near Liege, and an account of which was read 

 before the Royal Academy of Sciences of Brus- 

 sels by M. Wesmael, in Oct. 1835. This in- 

 sect, whose habits are believed to be entirely 

 subterraneous, is without any external organs 

 of vision .f 



The ocelli, stemmata, or single eyes, are 

 simple, convex, hemispheric lenses, varying in 

 number from one to three. They are always 

 situated, in those insects in which they exist, 

 on the superior part of the epicranium, poste- 

 riorly to the triangular suture. They are en- 

 tirely absent in Hydrous and all Coleoptera 

 except the Dermestidte, in which there is a 

 single ocellus situated on the centre of the epi- 

 cranium, a little posteriorly to the true eyes; in 

 one of the Pamsidac, and in some of the smaller 

 Brachelytra ;J but they almost invariably exist 

 in some of the other orders, as in the Hymen- 

 optera, Neuroptera, &c. 



The under surface of the head is formed chiefly 

 by the posterior and lateral parts of the gula 

 (jig- 369, n), which unite with the lateral parts 

 of the epicranium and occiput. It is bounded 

 anteriorly by an indistinct suture, and laterally 

 by the inferior portions of the cornea. In Me- 



* Introduction to Entomology, vol. iii. p. 502. 

 f Fr. Knt. Soc. vol. ii. Proceeding, p. xii. 



* Entomologist's Text-book, p. 238. 



lohmthid* it is of great extent, and is the 

 piece basilaire of Straus Durckheim. In Hy- 

 drous it is excavated in the middle line, on 

 each side of which are two elevated ridges, the 

 remains of the basilar parts of the mandibles (<>), 

 the proper appendages of the fifth sub-segment, 

 or basilar portion of the head, with which they 

 have become consolidated. The sub-mentum 

 (;/?), piece pre-basilaire of Straus, is the most 

 posterior of the parts that form the under 

 lip. Straus Durckheim and others appear to 

 have considered this part as a process of the 

 immoveable structure of the head, with which 

 at first it appears to be firmly united. Mr. 

 Westwood remarks, that although it appears to 

 be articulated in some beetles, it is immove- 

 able, and forms part of the under surface of 

 the head.* We have but little doubt that 

 it is a distinct piece, and is part of the third 

 sub-segment of the head, however it may be- 

 come anchylosed to the gula by the obli- 

 teration of the fourth in some instances, or 

 be itself entirely obliterated in others. In 

 Melolontha it is exceedingly short, but of 

 great width. In Hydrous it is very distinct, 

 and the maxillae are articulated to the skull on 

 each side of its base, as is the case also in 

 Melolontha and most other instances. It is a 

 little narrower posteriorly than anteriorly, and 

 its length is not more than one-half its breadth. 

 It is articulated anteriorly with the mentum (I); 

 this is a short transverse plate, in Hydrous 

 somewhat lunated on its anterior margin, rather 

 broader than long, but not so short as the sub- 

 mentum. In Dytictts it is excavated at its 

 anterior margin, the sides being carried forward 

 like separate lobes. In this genus it forms 

 with the sub-mentum, from which it is sepa- 

 rated only by a slight transverse articulation, a 

 broad plate, rounded on its edges, and cover- 

 ing nearly the whole of the under surface of the 

 mouth ; in some of the Staphylinida it is ex- 

 ceedingly short and broad, in Melolontha it is 

 nearly of a square form, but its anterior mar- 

 gin is acute; in Cetonia aurata, on the contrary, 

 its anterior margin is much wider than its pos- 

 terior, or articulation with the sub-mentum. 

 In Arnphimalla it is quadrate as in Melolontha, 

 and forms with the palpiger a nearly square 

 plate. The palpiger, first described by Mr. New- 

 man^ is not developed in the lip of Hydrous. 

 In those genera in which it is found, as in Dy- 

 ticus, it is an articulation which, as its name 

 implies, bears the labial palpi, and is situated 

 between the mentum and ligula, of which it 

 seems to be only a portion. It is subject to 

 great diversity in size and shape, and in conse- 

 quence is often confounded with the ligula itself. 

 It is said to be very distinct in most of the Co- 

 rabidse, and in Cychrus roslratus, as remarked 

 by Mr. Newman, it seems at first to have entirely 

 taken the place of the ligula. In the Staphylinid# t 

 Goerius, it is much narrower and longer than 

 the mentum, with which it forms as it were a 

 con e . I n one of the Endomych id#, Lycoperdina 



Op. cit. 1838, p. 256. 



f Entomol. Magazine, vol.ii. p. 82 et seq. . 

 a Paper on the Nomenclature of the Parts of 

 Head in Insects, p. 19. 



Also 



the 



