INSECTS AND ARACHNIDA. 231 



single eyes. Although the foregoing may be considered as the typical 

 structure of the Eyes of Insects, yet there are various departures from it 

 (most of them slight) in the different members of the Class. Thus in 

 some cases the posterior surface of each ' corneule ' is concave; and a space 

 is left between it and the iris-like diaphragm, which seems to be occupied 



ber of ocellites being reduced, and each one being larger, so that the 

 cluster presents more resemblance to that of Spiders, etc. Besides their 

 ' compound ' eyes, Insects usually possess a small number of f simple ' 

 eyes (termed ocelli or stemmata) seated upon the top of the head (Fig. 

 423, , a, a). Each of these consists of a single very convex corneule; to 

 the back of which proceeds a bundle of rods that are in connection with 

 fibrils of the optic nerve. Such ocelli are the only visual organs of the 

 Larvas of insects that undergo complete metamorphosis; the ( compound ' 

 eyes being only developed towards the end of the Pupa-stage. l 



627. Various modes of preparing and mounting the Eyes of Insects 

 may be adopted, according to the manner wherein they are to be viewed. 

 For the observation of their external facetted surface by reflected light, 

 it is better to lay down the entire head, so as to present a front-face or 

 a side-face, according to the position of the eyes; the former giving a 

 view of both eyes, when they approach each other so as nearly or quite 

 to meet (as in Fig. 423); whilst the latter will best display one, when the 

 eyes are more situated at the sides of the head. For the minuter examina- 

 tion of the ' corneules,' however, these must be separated from the hemi- 

 spheroidal mass whose exterior they form, by prolonged maceration; and 

 the pigment must be carefully washed away, by means of a fine camel- 

 hair brush, from the inner or posterior surface. In flattening them out 

 upon the glass-slide, one of two things must necessarily happen; either 

 the margin must tear when the central portion is pressed-down to a level; 

 or, the margin remaining entire, the central portion must be thrown into 

 plaits, so thab its corneules overlap one a*nother. As the latter condition 

 interferes with the examination of the structure much more than the 

 former does, it should be avoided by making a number of slits in the 

 margin of the convex membrane before it is flattened-out. Vertical sec- 

 tions, adapted to demonstrate the structure of the ocelli and their rela- 

 tions to the optic nerve, can be only made when the insect is fresh, or 

 or has been preserved in strong spirit. Mr. Lowne (loc. cit.) recommends 

 that the head should be hardened in a 2 per cent solution of chromic 

 acid, and then imbedded in cacoa-butter; the sections must be cut very 

 thin, and should be mounted in Canada balsam. The following are some 

 of the Insects whose eyes are best adapted for Microscopic preparations: 

 Coleoptera, Cicindela, Dytiscus, Melolontha (Cockchafer), Lucanus (Stag- 

 beetle) ; Orthoptera, Acheta (House and Field Crickets), Locusta; 

 Hemiptera, Notonecta (Boat-fly); Neuroptera, Libellula (Dragon-fly), 

 Agrion; Hymenoptera, Vespidae (Wasps) and Apidae (Bees) of all kinds; 

 Lepidoptera, Vanessa (various species of Butterflies), Sphinx ligustn 

 (Privet Hawk-moth), Bombyx (Silk-worm moth, and its allies); Dip- 

 tera, Tabanus (Gad-fly), Asilus, Eristalis (Drone-fly), Tipula (Crane-fly), 

 Musca (House-fly), and many others. 



1 For minute details as to the structure of the Eyes of Insects, see the admira- 

 ble Memoir by Mr. Lowne. in "Phil. Trans.," 1878, p. 577. 



