586 



THE MICROSCOPE. 



" Each hexagon forms the slightly horny case of an 

 eye.^ Theu^ margins of separation are often thickly set 

 with hair, as in the Bee ; in other instances naked, as in 

 the Dragon-tly, Honse-fly, &c. The numher of these lenses 

 has been calculated by various authors, and their multi- 

 tude, cannot fail to excite astonish- 

 ment. Hooke counted 7,000 in the 

 eye of a House-fly ; Leeuwenhoek 

 more than 12,000 in that of a 

 Dragon-fly ; and Geoffiy cites a 

 calculation, according to which 

 there are 34,650 of such facets in 

 the eye of a Butterfly." ^ 



The trunk is situated between 

 the head and the abdomen ; the legs 

 and wings are inserted into it. The 

 thorax is the upper jDart of the 

 trunk ; the sides and back of which 

 are usually armed with points or 

 hairs. The abdomen forms the 

 posterior part of the body, and is 

 generally made up of rings or seg- 



Fig. 263 — Breoihing - aper- 

 ture, spiracle, of silkvi07-m 

 (The cii'clo incloses the ob 



ments, by means of which the 



jectaboutthenaturaisize.) insect lengthens or shortens itself. 

 Eunning along the sides of the abdomen are the spiracles, 

 or breathing apertures, fig. 263, communicating directly 

 mth the internal respiratory organs. Pure air being thus 

 freely admitted to every part, and the circulating fluids 



(1) "Each of the eyelets, or 'ocelli,* which aggregated constitute the com- 

 pound eye of a Bee, is itself a perfect instrument of vision, consisting of two 

 remarkably formed lenses, namely, an outer -corneal' lens and an inner or 

 ' conical' lens. The 'corneal' lens is a hexahedral or six-sided prism, and it is 

 the assemblage of these prisms that forms what is called the ' cornea ' of the 

 compound eye. This ' cornea ' may easily be peeled off, and if the whole or a 

 portion be placed under the microscope, it will be seen that each corneal lens is 

 not a simple lens but a double convex compound one, composed of two plano- 

 convex lenses of different densities or refracting powers, joined together by their 

 ]ilane surfaces. The effect of this arrangement is, that if there should be any 

 aberration of the rays of light during their passage through one portion of the 

 lens, it is rectified in its transit through the other. It appears questionable 

 whether the normal shape of these lenses is hexagonal, or whether this form is 

 )i()t rather a ne(;essity of growth, (fee, tliat is, that tliey are normally round, but 

 Mssume the licxagonal shape during the process of development in consequence 

 of tlieir agglomeration." J. Samuelson and B. Hicks, "On the Eye of the 

 IBee," Journ. Micros. Soc. vol. 1. p. 51. 



(2) "Remarks on the Coniea of the Eye in Insects," by John Gorham, 

 Ji.R.C.S. Journ. Micros. Science, p. 70, 1S53. 



