CLASSIFICATION OF ANIMAL PREPARATIONS. 417 



adopted; if, on the contrary, the colours be at all deadened, 

 it should be mounted dry, either on a disc or in a cell, as 

 described at page 322. The elytra of some beetles, after 

 having been softened in caustic potash, may be mounted 

 between flat glasses, as ordinary objects, and in them the 

 arrangement of the trachese, the pits, and elevations on the 

 surface, and the short spiny or branched hairs, may be well 

 examined. 



Eyes of Insects, Crustacea, and Arachnida. — In the first 

 two the eyes differ in very many points from the same organs 

 in the higher classes of animals, each being composed of an 

 aggregation of many hundreds of minute lenses. In the 

 Arachnida or spiders each eye has only a single lens, and, in 

 order to compensate for this seeming want, the number of 

 eyes is increased from four to twelve in some species ; a few 

 genera of insects are provided with two or three single eyes 

 in the front of their heads. The shape of the lenses is always 

 such as to admit of being adapted to each other without loss 

 of space ; the more common form is hexagonal, but in some 

 Crustacea they are square. The external form of the eye 

 may be seen in situ in all insects when viewed as opaque 

 objects, but the layer of lenses requires the aid of maceration 

 and dissection to free them from a considerable amount of 

 pigment; these may be mounted either dry, in fluid, or in 

 balsam ; in the latter way the collection of lenses, if required 

 to be flat, must be made so whilst soft, by pressure, otherwise 

 they are liable to split. 



The subjoined list wiU serve to point out some of the insects 

 from which the most striking specimens of eyes may be taken : 



Feet of Insects, Sfc. — These may be examined as opaque 

 objects when mounted on discs, or by transmitted light when 

 placed in fluid or in Canada balsam; the latter method is, 

 27 



