INSECTS AND ARACHNIDA. 



229 



the arrangement of light ordinarily used, they are seen to the best 

 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 turpentine; 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 Lepidop- 

 tera, etc. , 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 



FIG. 423. 



FIG. 424. 



Head and Compound Eyes of the Bee, 

 showing the ocellites -in situ on one side (A), 

 and displaced on the Other (o) ; a, a, a, stem- 

 mata, b, b, antennae. 



Section of the Composite Eye of 

 Melolontha vulgar is (Cockchafer): 

 a, facets of the cornea; 6, trans- 

 parent pyramids surrounded with 

 pigment; c, fibres of the optic nerve; 

 d, trunk of the optic nerve. 



recourse to both these methods. Hairs, on the other hand, are best 

 mounted in Balsam. 



626. 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 

 frequently so large as to touch each other in front (Fig. 423). We find 

 hi their structure a remarkable example 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 a ' compound ' eye, made up of many 

 hundred or even many thousand minute conical ocellites (B). Approaches 

 to this structure are seen in Annelida and Entomastraca; but the number 

 of 'ocellites' thus grouped-together is -usually small. In the higher 

 Crustacea, however, the ' ocellites ' are very numerous; and their com- 

 pound eyes are constructed upon the same general plan as those of In- 

 sects, though their shape and position are often very peculiar (Fig. 491). 

 The individual ocellites are at once recognized, when the ' compound 



