418 



THE INSECTA. 



§336. 



on the brain that their optic nerves consist only of small papillae on this 

 lust ;''^ but, when further removed from the -brain and grouped together, 

 the optic nerves arise by a common trunk fl-hich divides into as many 

 branches as there are eyes.*''' 



The number and disposition of the stemmata vary very much in the 

 dift'erent orders. When they alone constitute the visual organs, they are 

 always situated on the lateral parts of the head, — where they may be 

 disposed either, as one on each side, or as several irregularly grouped 

 together {^Ocelli gregati), or regularly arranged in rows {Ocelli seriati). 

 There is only one simple eye on each side with the Pediculidac, Nirmidae, 

 Coccidae, the larvae of the Phryganidae and Teuthrediiiidae, and the aquatic 

 ones of very many Diptera. These organs are in groups of four to eight 

 with the Poduridae,*"' with the larvae of Lepidoptera, the hexopod larvae 

 of the Strepsiptera, the larvae of the Hemerobidae, Mymeleonidae, llaphi- 

 didae, and with the hexapod ones of the Coleoptera.*'^* The winged males of 

 the Strepsiptera have the largest number of stemmata aggregated in groups ; 

 they here form two lateral, globe-like projections, and constitute the transi- 

 tionary form to the faceted eyes, for there are fifty to seventy on each side, 

 separated from each other only by hairs.'"' Very many Insecta with two, 

 faceted eyes, have, also, on their front, three stemmata disposed in a 

 triangle.'^"' 



2. The Compound eyes, or those whereof the cornea is faceted, are com- 

 posed of simple eyes so thickly set together that their more or less thick, 

 slightly convex, quadrangular, or hexagonal corneae are contiguous.'"' 



The size of these facets is not uniform even in the same eye, for some- 

 times those above, or those in the centre, are the larger.*^-' Behind each 

 cornea is situated, in place of a lens, a transparent pyramid the apex of 

 which is directed inwards and received into a kind of transparent calyx 

 corresponding to a Corpus vitreum. This last is surrounded by another 

 calyx formed by the expansion of a nervous filament 



f 



arising from the 



lula, see Treviranus, Beitr. &c. p. 84, Taf. II. 

 fig. 25-35. 



S Bombus, Apis, F'espa ; see Treviranus, Bi 

 ologie, V. Taf. II., and his Beitr. &c. Taf. II. fig. 29 ; 

 and Brandt and Ratzeburg Medizin. Zool. II. Taf. 

 XXV. fig. 31, 32. 



i> With many of the larvae of the Lepidoptera 

 and tlie Coleoptera, the optic nerves arise by two 

 more or less long roots ; see Lyonet, Traite. &c. 

 p. 581, PL XVIII. fig. 1, No. 1, and fig. 6 (larva of 

 the goat-inoth) ; Suckow, Auat. physiol. Unter- 

 such. p. 41, Taf. III. fig. 34 (pine caterpillar), and 

 Barmeister, Trans. Entom. Soc. I. p. 239, PI. 

 XXIII. fig. 7 (larva of a Calosoma). The three 

 stemmata of Cicada receive their nerves from a 

 common trunk arising from tlie middle of the brain; 

 see Treviranus, Beitr. Taf. II. fig. 24, and L. 

 Dufour, Kecherch. sur lea Hemipt^res, &c., PI. 

 XIX. fig. 203. 



" See Nicolet, Recherch. sur les Podurelles, loc. 

 cit. p. 28, PI. II. III. 



tf Such are the carnivorous larvae of the Cara- 

 bidae, Stajihylinidae, Dytiscidae, Dermestidae, Sil- 

 phidae, &c., and the herbiferous larvae of the 

 Chrysomelidae. Those of Cicindela have only 

 two large stemmata on each side of th? heail, and 

 those of Lycus, Meloc, Lampyis and Cantfiaris, 

 have only one. 



9 See Templeton, Trans. Entom. Soc. m. p. 54, 

 PI. IV. 



10 There are three frontal stemmata with many 

 of tte Orthoptera (Mantidae, Acrididae, Libelluli- 



dae, Perlidae, Psocidae, Ephemeridae and some 

 Phasmidae) ; with some Neuroptera(//e7neroftiu.s, 

 Panorpa, Phryganea), and Heniiptera {Penta- 

 toma, Cureus, Berytus, Cicada). This is the case 

 also with many Dijitera, such as the Muscidae, 

 Syrphidae, Stomo.\idae, Bombylidae, Anthracidae, 

 Oestridae, .\silidae, Enipidae, &c.; they are want- 

 ing with TaOanus, Haemalopota, Conops, Hip- 

 pobosca, Melophagus, and many of the Tipulidae. 

 With the Ilymenoiitera, they are constantly pres- 

 ent except with the neuter ants and with the fe- 

 males of Mutilla and Myrmosa ; there are only 

 two of these eyes with most of Gryllus ; Sciopki/a, 

 Myce.tobia and Leja, of the Diptera ; Sesia, Eu- 

 prepia, Pyralis .and a great number of the Noctu- 

 idae, of the Lepidoptera ; Gry/lolalpa, Blatta and 

 Termes, of the Orthojjtera ; and Omalium and 

 Anthophagus, of the Coleoptera. 



11 For the intimate structure of the compound 

 eyes, see Straus (Consid. fee. p. 411, PI. IX.), 

 Vugis (Ann d. Sc. Nat. X.X. 1830, p. 341, PI. 

 XII., or in Froriep's Not. .X.\IX. p. 257), R. 

 tVagner (jyiegmami's Arcliiv, 1835, I. p. 372, 

 Taf. v.), and especially fVill (Beitriig. zur Anat. d. 

 zusammengesezten Augen mit facettirt. Hornhaut. 

 1840). 



1-' These differences in the size of the facets had 

 been observed by Marcel de Serres (loc. cit. p. 45) 

 with Libellula. They exist also in the eyes of 

 Lagriajlava, gibbosa, atra, Tabanus rusticus, 

 and some other Diptera ; see Ashton, Traus. En- 

 tom. Soc. II. p. 253, PI. XXI. 



