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MATERIAL FOR STUDY. 



For general histology. — Any organs or tissues desired 

 for study should be dissected out of a specimen which has 

 been made insensible by chloroform or ether or cocaine or 

 which has just been killed by some quick process. The 

 point is to get the material in a fresh and perfectly normal 

 condition. The tissue must be alive at the time it goes into 

 the fixing fluid. A certain way of accomplishing this is to 

 kill the insect by dropping it into the fixing fluid. But that 

 the tissue may certainly be reached and affected by the fluid, 

 the specimen, unless small and with very weakly chitinized 

 integument, must be cut open. 



For studying anatomy or development.— It is often 

 desirable to make serial sections of the whole body of an in- 

 sect. In the case of insects too small to dissect this is the 

 only means of studying their anatomy. In the study of the 

 post-embryonal development of insects which have complete 

 metamorphosis, the advanced larval and the pupal stages 

 must be examined by such sections, because of the great 

 breaking down of many of the body-tissues. Specimens 

 should be killed immediately after moulting, when the outer 

 chitin wall is thinnest and most pervious to the fixing liquids. 

 By rearing the specimens, just moulted larvae, just formed 

 pupae, and just issued imagines may be obtained, and the use 

 of such specimens will be found to be immensely advan- 

 tageous. Strongly chitinized specimens, which must be sec- 

 tioned, may be carefully and cleanly cut into two or three 

 parts, say, head, thorax, and abdomen. This will enable the 

 fixing fluids to penetrate the body cavity, although the diffi- 

 culty of cutting the firm chitin wall with the microtome knife 

 still remains. Certain methods of softening the chitin are 

 used by some workers, but there is always danger in the 

 practice of injuring the other more delicate tissues. 



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