400 ON nfSECT ANATOMY. 



after which the tissues shrink and become granulated and knotted 

 together so as to break under dissecting needles; besides which 

 they gradually get stained and opaque, so that they are not any 

 longer well seen under the microscope by transmitted light. "Water 

 should not be used when dissecting, but the object must be floated 

 in glycerine, and all fatty tissues removed as soon as possible. 



The magnifying power under which dissection is carried on 

 necessarily varies with the minuteness of the object — the lowest 

 power under which the parts can be distinguished should always be 

 chosen, because they can be kept better in sight and a firmer control 

 over the movement of the dissecting needles exercised. "With high 

 power, the object escapes readily oat of the field, and the needles 

 are not easily brought to bear upon it. The power should be either 

 a single lens, or a combination of lenses which magnifies without 

 inverting the image. 



Most insect preparations can be examined and made under a low 

 power (5 to 25), but it is well to examine them under higher power 

 during their preparation (50 to 75). "V\^hen histologic elements are 

 studied, still higher powers are needed. If, for instance, insect 

 muscle is the object to be examined, the striation may be readily 

 seen with quite low powers, but the arrangement of sarcous 

 elements can be demonstrated only when powers varying from 400 

 to 800 are used. And by far the most beautifal objects are 

 obtained when polarised light is used and advantage taken of the 

 different refracting power of the discs and intervening substance. 

 The ordinary striation of muscle fibre may be seen most perfectly 

 in the muscles of the Thorax which naturally split up into long 

 fibres, offering excellent specimens for study, {e.g. in the common 

 house fly). For minute analysis of the sarcous mass, the muscles 

 of mites (Trombidium) have been recommended, as the striation of 

 such muscles is remarkably coarse and distinct. 



Muscle must be taken from an insect immediately it is killed. It 

 may sometimes be advantageously treated with alcohol, or osmic 

 acid (weak solution) and prepared in glycerine. "When studying 

 the phenomena of contraction, which will be seen in various phases 



