THE MICROSCOPE. 89 



choosing the subjects, of taking types of form, must be carefully 

 borne in mind. So long as they are good "types," the commoner 

 the better. It is desirable to either accompany or precede the 

 dissections by an example of the object in its entire state ; in some 

 cases a good coloured drawing may be the only means of prac- 

 tically doing this. Some good work should, in all cases, be taken 

 as a guide, and the assistance of a friend with knowledge and 

 experience will be invaluable, especially at the outset. The con- 

 tributor speaks of the spiracle as if there were but one ; that 

 usually sold under the name, and which he doubtless intended, is 

 the second or meta-thoracic, situate at the base of the balancers. 

 Besides this, there is one on the pro-thorax, a little above the attach- 

 ment of the first limb, and in addition, one on each side of the 

 five abdominal segments, besides one to nearly each segment of 

 the overpositor. So that, taking the Blow-fly as the type of the 

 Diptera, there are ten spiracles on each side in the male, and eight 

 in the female {Lowjie on the Blo7v-fiy). The explanation of this 

 discrepancy doubtless has reference to the greater activity of the 

 male in pursuit of the opposite sex, and consequent need of more 

 highly developed respiratory organs ; the females being often more 

 sluggish in their habit. 



Gizzard of the Green Weevil (PI. II., Figs. 1-3). — This slide 

 differs from others which I have seen, only in minor particulars. 

 The " teeth," which have their points directed towards the mouth, 

 are seated on elevated ridges ; in all the examples from weevils 

 which I have seen, they are thin chitinous plates, curiously resem- 

 bling the scales of Lepidoptera, both alike having longitudinal 

 ridges running out at their free extremities into spines, with shorter 

 transverse ridges. The more or less horny membrane on which 

 they are seated (the " gizzard bag,'' it might be called), is sur- 

 rounded by an inter-lacement of muscular fibres, whereby its 

 effective action is produced. The gizzard of the Cockroach offers 

 a favourable subject of examination of the muscular structures ; 

 they are the parts which the contributor of this slide calls " the 

 skin," and which it is generally preferable to leave in situ, when 

 not so thick as to interfere with the examination. 



