COMPOUND EYES OF INSECTS. 



661 



advantage, and to fix them there accordingly. — Whenever portions 

 of the Integument of Insects are to be viewed as transparent 

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

 mounted in Canada Balsam, after soaking for some time in Turpen- 

 tine ; since this substance has a peculiar effect in increasing their 

 translucence. 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 such as 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 press- 

 ing 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 § 154. Hairs, on the other hand, 

 are best mounted in Balsam. 



516. Parts of the Head. — The Eyes of Insects, situated upon 

 the upper and 

 outer part of the 

 head, are usually 

 very conspicuous 

 organs, and are fre- 

 quently so large as 

 to touch each other 

 in front (Fig. 338). 

 "We find in their 

 structure a re- 

 markable example 

 of that nmltipli- 

 cation of similar 

 parts which seems 

 to be the predom- 

 inating ' Idea' in 

 the conformation 

 of Articulated ani- 

 mals; for each of 

 the large protube- 

 rant bodies which 

 we designate as an 



Eye, is really an aggregate of many hundred, or even many thou- 

 sand minute eyes, which are designated Ocelli. Approaches to 

 this structure are seen in the Annelida and Entomostraca; but 



Head and Compound Eyes of the Bee, showing 

 the Ocelli in situ on one side (a), and displaced on 

 the other (b) ; a, a, a, Stemmata ; b, b, Antennae. 



