664 MODES OF PREPARING EYES OF INSECTS. 



resembling those of the Arachnida ; these are seated upon the top 

 of the head (Fig. 338, a, a, a), and are termed Stemmata. — It is 

 remai-kable that the Larvae of Insects which undergo a complete 

 Metamorphosis, only possess Single Eyes ; the Composite Eyes 

 being developed, at the same time with the Wings and other parts 

 which are characteristic of the Imago state, during the latter part 

 of Pupal life. 



517. 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. 338) ; 

 whilst the latter will best display one, when the eyes are situated 

 more at the sides of the head. For the minuter examination of 

 the 'Corneules,' however, these must be separated from the herni- 

 spheroidal mass whose exterior they form, by prolonged macera- 

 tion ; 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 en- 

 tire, the central portion must be thrown into plaits, so that its 

 Corneules overlap one another. 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. Such 

 preparations may be mounted either in Liquid, Medium, or Canada 

 Balsam ; the latter being preferable when (as sometimes happens) 

 the membrane is so horny as to be but imperfectly transparent. 

 Vertical Sections, adapted to demonstrate the structure of the 

 Ocelli and their relations to the Optic Nerve, can of course be 

 only made when the body of the Insect is fresh ; and these should 

 be mounted in Liquid or in Medium. The following are some of 

 the Insects whose eyes are best adapted for Microscopic prepara- 

 tions : — Coleoptera, Cicindela, Dytiscus, Melolontha (Cockchafer), 

 Lucanus (Stag-beetle) -,—Orihoptera, Acheta (House and Field 

 Crickets), Locusta; — Hemiptera, Notonecta (Boat-fly) ;— Neuropil ra 

 Libellula (Dragon-fly), Agrion ; — Hymenoptera, Vespidae (Wasps) 

 and Apidae (Bees) of all kinds ; — Lepidoptera, Vanessa (various 

 species of Butterflies), Sphinx ligustri (Privet hawk-moth), Bom- 

 byx (Silk-worm moth, and its allies);— Diplera, Tabanus (Gad-fly), 

 Asilus, Eristalis (Drone-fly), Tipula (Crane-fly), Musca (House-fly), 

 and many others. 



518. The Antenna, which are the two jointed appendages arising 

 from the upper part of the head of Insects (Fig. 338, b, b), present 

 a most wonderful variety of conformation in the several tribes of 



