INSECTA. 213 



the approach of foreign bodies. Each division of the compound ej-e 

 has its lens, which combines the characters of both crystalline and 

 vitreous humours : it is always of a more or less elongated conical 

 form, having its base applied to thecorneule^ and its apex to the optjc 

 nerve. The base is not immediately in contact with the cornea, but 

 is separated by a minute aqueous chamber into which a process of the 

 pigmental membrane penetrates, leaving a small pupil opposite the 

 middle of the base of the lens. The pigment is continued along the 

 crystalline vitreous cone to its apex, forming a sheath around it, and 

 enveloping also the adhering filament of the optic nerve ; at once 

 separating and connecting together the component ocelli of the com- 

 pound eye. Fine tracheal ramifications have been traced upon the 

 pigment, which displays very various, and often brilliant or metallic, 

 hues in its outer layer. 



The larvae of the Coleoptera, Lepidoptera, Neuroptera, some Hy- 

 menoptera, and Diptera, have merely simple eyes. Two or three of 

 such ocelli are retained, with the superadded compound eyes, in all 

 the preceding orders save Coleoptera, in which only compound eyes 

 are present in the perfect state. 



The high degree in which the power of discerning distant objects 

 is enjoyed by the flying insects corresponds with their great power of 

 traversing space. 



The few exceptional cases of blind insects are all apterous, as the 

 Claviger, and often peculiar to the female sex, as in the glow-worm 

 and cochineal insect. 



LECTURE XVII. 



INSECTA. 



The extraordinary powers of locomotion possessed by insects, the 

 variety of elements which tliey can traverse, their aptitude to gain 

 access to every situation where organised matter may be obtained, 

 prepare us to expect that they should manifest all the modifications 

 of the digestive system which may be required for the assimilation 

 of the different kinds and conditions of the solids and fiuids of plants 

 and animals. 



One insect preys upon another; pursues and attacks, like the 



p 3 



