EYES OF INSECTS. 



561 



objects, for the display of their intimate structure, they should be 

 mounted in Canada balsam, after soaking for some time in tur- 

 pentine ; since this substance has a peculiar effect in increasing 

 the translucency. Not only the horny casings of perfect Insects of 

 various orders, but also those of their pupae, are worthy of this 

 kind of study ; and objects of great beauty (such as the chrysalis 

 case of the Emperor-moth), as well as of scientific interest, are 

 sure to reward any who may prosecute it with any assiduity. 

 Further information may often be gained, by softening such 

 parts in potash, and viewing them in fluid. The scales of the 

 wings of Lepidoptera, &c., are best transferred to the slide, by 

 simply pressing a portion of the wing either upon the slip of 

 glass or upon the cover ; if none should adhere, the glass may 

 first be gently breathed on. Some of them are best seen when 

 examined "dry," whilst others are more clear when mounted in 

 fluid ; and for the determination of their exact structure, it is 

 well to have recourse to both these methods. If these scales are 

 to be used as "test-objects," it is preferable to place them 

 between two pieces of thin glass, in the manner specified in 

 122. Hairs, on the other hand, are best mounted in balsam. 

 383. Parts of the Head. The Eyes of Insects, situated upon 

 the upper and outer part of the head, are usually very conspicu- 

 ous organs, and are frequently so large as to touch each other in 

 front (Fig. 283). We find in their structure a remarkable exam- 

 ple of that multiplication of similar parts, which seems to be 

 the predominating idea in the conformation of Articulated 

 animals; for each of the large protuberant bodies which we 

 designate as an eye, is really an aggregate of many hundred, or 

 even many thousand minute eyes, which are designated ocelli. 

 Approaches to this structure 

 are seen in the Annelida and FlG - 283 - 



Entomostraca ; but the number 

 of "ocelli" thus grouped toge- 

 ther is usually small. In the 

 higher Crustacea, however, the 

 ocelli are very numerous ; their 

 compound eyes being construct- 

 ed upon the same general plan as 

 those of Insects, although their 

 shape and position are often very 

 peculiar (Fig. 343). The indi- 

 vidual ocelli are at once recog- 

 nized, when the composite eyes 

 are examined under even a low 

 magnifying power, by the " fa- 

 cetted" appearance of the sur- 

 face (Fig. 283), which is marked 

 out by very regular divisions either into hexagons or into 

 squares: each facet is the cornea of a separate ocellus, and 



36 



Head and Compound Eyes of the Bee. show- 

 ing the ocelli in situ on one side (A), and dis- 

 placed on the other (B) ; a, a, a, stemmata; b, b. 

 antennae. 



