726 MEDICAL ENTOMOLOGY 



delicate and will not stand rough handling, and as the process may 



have to be carried out many hundreds of times, much 

 The examination of , . ... . , ,. , , ,, , . . , 



tissues for parasites trou ble will be saved and more reliable results obtained 



by adopting a regular routine. 



The method of dissection has already been described, both in the 

 general remarks embodied in this chapter and in the systematic accounts 



of the various forms, and the technique may be taken 

 Preliminary exami- . . , . , . , , , 



nation U P at * e P m * a * which the part to be examined has 



been isolated on the slide. Clear away all that is not 

 required, such as fragments of the exo-skeleton and fat body, and then 

 place a hair beside the dissection ; lower on to it a coverslip, and examine 

 first with a low power. The use of the hair is to prevent the pressure of 

 the coverslip from distorting the tissue, or driving out the contents. It 

 saves trouble to keep a small collection of bristles of different thicknesses, 

 from cattle, buffaloes, or pigs, in a tube for this purpose. When the 

 coverslip has been lowered it will probably be necessary to add more 

 saline, to expel any air bubbles which may have been included. Never 

 attempt to drive them out by pressure, but use excess of saline, sub- 

 sequently sucking up what is not required with a pipette or filter paper. 



With a low power the general appearance of the gut should be noted 

 carefully. With some practice one can learn to distinguish infected 

 gut and Malphigian tubes by their greater opacity, and to pick out the 

 pieces which require further examination with a high power. If neces- 

 sary the coverslip can be ringed with vaseline for a more prolonged 

 examination. A No. 6 objective, with a high power eyepiece, will 

 generally suffice, but an oil immersion lens can be used, if the precaution 

 is taken to have a thin coverslip. A high power dry objective is most 

 useful. The preparation may be examined at this stage with a dark 

 ground illumination. 



Having ascertained the presence of the parasite in a particular piece of 

 tissue, the next step is to isolate that piece for a closer examination, and 



for staining. Remove the coverslip by flooding the 

 Isolation of ,. , . , .. . . , 



infected part slide with saline and inserting the point ot a needle 



under one corner of it, pipette off the excess of saline, 

 and place the slide under the dissecting microscope. Then with a pair 

 of sharp needles cut out the part required, and remove it to a clean 

 slide, with a small drop of saline. It can either be lifted up with a 

 needle, or with a pipette. A needle is the best, as there is always a risk 

 of the tissue sticking to the glass of the pipette. The needle should be 

 perfectly clean and well smoothed down on a soft stone, or the tissue 



