982 



INSECTS AND ARACHNID A 



portions it is well to choose those which have the brightest and 

 the most contrasted colours, exotic butterflies being in this respect 

 usually preferable to British ; and before attaching them to their 

 slides care should be taken to ascertain in what position, with the 

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

 vantage, 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 cases of perfect insects of various orders, but also of 

 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 prose- 

 cute 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 Lepido- 

 ptera &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 

 FIG. 731. Head and compound eyes of the methods. Hairs, on the other 

 bee, showing the ocellites in situ on one i* an j QT , a v. oc f 1T ,f or l ;,, 

 side, A, and displaced on the other, B ; n ' 

 a, a, a, stemmata; &, 6, antennae. balsam. 



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. 731). We find in their structure 

 a remarkable example of that multiplication of similar parts which 

 seems to be the predominating ' idea ' in the conformation of arti- 

 culated 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 ocelli (B). Ap- 

 proaches to this structure are seen in Entomostraca ; but the 

 number of ' ocellites ' thus grouped together is usually small. 

 In the higher Crustacea, however, the ' ocelli ' are very numerous ; 

 and their compound eyes are constructed upon the same general 

 plan as those of insects, though their shape and position are often 

 very peculiar. The individual ocelli are at once recognised when 

 the 'compound eyes' are examined under even a low magnifying 

 power by the 'faceted' appearance of the surface (fig. 731, A), 

 which is marked out by very regular divisions either into hexagons 



