INSECTA. 371 



serve the aquatic whirligigs to discern at the same time objects be- 

 neath them in the water, and above them in the air. 



The integument of the head, which passes uninterruptedly over 

 the compound eye, there becomes transparent^ and is subdivided into 

 a number of hexagonal corneules, varying in number from 50 in the 

 ant, to 4,000 in the house-fly, to above 17,000 in the butterfly, and 

 to more than 25,000 in the Mordella beetle. The size of these facets ' 

 is not uniform even in the same eye, for sometimes those above or 

 those in the centre are the larger ; e. g. Libellula, Lagria, Tahanus, 

 In general each corneule is thicker than it is broad, and thicker at 

 its middle than at its circumference ; a layer of pigment here in- 

 sinuates itself into the interspaces between the corneules. In bees 

 and flies fine hairs project from these interspaces, which must defend 

 the eye or warn the insect against the approach of foreign bodies. 

 Each division of the compound eye 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 the 

 corneule, and its apex to the optic nerve. The base is not imme- 

 diately 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 con- 

 necting together the component ocelli of the compound 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, Lepiclopfera, Neuroptera^ some Hy- 

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

 the larval ocelli are retained, with the superadded compound eyes, in 

 most of the winged 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, and often peculiar to the female sex, as in the glow-worm, 

 cochineal insect, and parasitic Stylops. 



B 2 



