§ 337. THE INSECTA. 419 



ganglion on the extremity of the optic nerve, a short distance from the 

 brain/^^' Each lens-like pyramid with its vitreous body and nervous fila- 

 ment is enveloped by a Chorioidea usually of a brown color, which forms, 

 behind the cornea, a kind of pupil,*''" but to which are due, by no means, 

 the beautiful colors so often observed in the eyes of these animals. <'■'> 



The size and form of the compound eyes, as also the number of their 

 facets, are very varied.*'"* The larvae and pupae of the hemimetabolic 

 Insecta have, usually, a less number of facets and consequently smaller 

 eyes, than the perfect forms. With the Libellulidae, and Diptera, the eyes 

 are very large ;*''^' while with the Formicidae, they are perhaps the smallest 

 of all. With many Diptera, and some Hymenoptera, those of the males 

 are much larger than those of the females, and are often contiguous in 

 front or above. <"^> With some Hymenoptera, and Diptera, they are 

 pilose, — the hairs being inserted in the angles of the facets.*''" 



The compound eyes are usually spherical or oblong ; and, with many 

 Cerambycidae, and with the Vespidae, they are deeply eraarginate in front, 

 or on their internal border. With Diopsis,^^^ they have a very singular 

 appearance, being supported on two very long, rigid, frontal processes, and 

 their direction cannot, as with other Insecta, be changed without a turning 

 of the head.<2'> 



CHAPTER V. 



DIGESTIVE APPARATUS. 



§ 337. 



The Insecta very often use their labial and maxillary palpi to seize and 

 to convey food to the mouth, and even to introduce it wholly within this 

 last. With many species, the fore-legs are used to seize and retain the 

 food, and the first pair is sometimes changed for this purpose even into 

 rapacious organs."' With the larvae and pupae of the Libellulidae, there 



13 According to Mailer (Arch. 1835, p. 613), l!" The largest eyes are observed with the Heno- 

 these retinae are formed only by a prolongation of pidae, where they cover nearly the whole head ; 

 the neurilemma, while the proper nervous sub- see firicAson, Entomographien. Hft.I. p. 132, Taf. I. 

 stance does not extend beyond the extremity of the IS Among the Hymenoptera, the genera Aetata, 

 vitreous body ; but fVill denies this {Mailer's Larra, Tachytes, Apis ; and among the Diptera, 

 Arch. 1S13, p. 349). the Muscidae, Syrphidae, Leptidae, Tabanidae, 



14 Each of these pupils, according to IVill {Mai- Stratiomydae, and- many other families. 



/er-V Arch. 1813, p. 350), is moved by thirty to WWith Apis,Tabanus, Anthomyia,Eristalts, 



thirty-five delicate fibres which arise on the four Volucella, and other Diptera. 



transparent cylinders surrounding the pyramidal 20 gee Linne, Amoenitates academicae. VIII. 



lenses ; but Brants (Tijdsk. voor natuurlijke ge- Tab. VI. and Dalman, in Fuessly\i Archiv d. 



schied. en physiologic. 1844, II.) regards them as Insekt. lift. 1, Taf. VI. or Isis, 1820, p. 501, Taf. V. 



tradiean branches and not contractile fibres. -1 The. Insecta scarcely move their head when 



l") The beautiful emerald color of the eyes of they look in different directions. This renders very 



many Libellulidae, Tabanidae, Hemerobidae, &c., is singular the extended mobility of the head with 



due to the corneae ; for the chorioideae are of the Mantis religiosa, which, in watching for its prey, 



name dead color as those of other Insecta. looks on all sides. 



Ifi There are sometimes several thousands of 1 For example, with Syrtis, Naucoris, Nepa, 



these facets in tlie eyes of large size ; see Mulle.r, Ranatra, Hemerodromia, Mantis, Mantispa, 



Zur vergleich. Physiol, d. Gesichtssinn, &c., p. 340 ; &c. 

 and mil, Beitrag. &c. p. 10. 



