712 MEDICAL ENTOMOLOGY 



is done by macerating the tissue in a solution of potash or soda, which 



dissolves out the cellular tissues without affecting the 

 Cleared preparations . . . . . .--''. i". 



chitmous parts. The specimen is then dehydrated in 



alcohol and mounted in balsam. 



The strength of the solution of caustic potash or soda to be used 

 depends upon the thickness of the exo-skeleton and on the temperature at 

 which it is to be applied. For ordinary use a two per cent solution of 

 the former is the most satisfactory, when the specimen is to be left in it 

 at room temperature. An ordinary object, such, for instance, as the 

 proboscis of the house fly when dissected from the head, will become 

 sufficiently cleared and transparent after two or three days in such a 

 solution. For more chitinous and more opaque objects, one can either 

 increase the strength of the solution or use a higher temperature, such as 

 that of the paraffin oven, which forms a convenient receptacle for the 

 specimen, or simply allow the solution to act for a longer period. For 

 rapid work specimens can be boiled in a ten per cent solution, but this 

 has the disadvantage of sometimes distorting the preparation through 

 the sudden expansion of the contained air ; there is also a considerable 

 risk of losing a small object if it is boiled in a test tube. 



There are two points to be noted in the manipulation. In whole 

 insects the communications between the exterior and the soft tissues 

 are very restricted, and it is advisable to make sure that the solution 

 will have free access to the interior by making a small cut in some 

 unimportant part of the integument, or by pulling off the legs and wings 

 if these are not required. Secondly, chitinous structures appear darker 

 when in a watery solution than when in balsam, and the clearing should 

 accordingly be stopped at a point when the specimen appears darker 

 than is really required. 



Soft tissues which have been fixed in alcohol take much longer to 

 dissolve than when fresh, and if the specimen is not well opened up 

 much of the pigment of the chitin will be dissolved before the internal 

 organs are got rid of. Fresh or dried specimens should accordingly 

 be used for making cleared preparations. 



To make a cleared preparation of any part, or of the whole of a small 

 arthropod, proceed as follows. Place the specimen in potash solution, 

 of a strength and at a temperature which are determined by the circum- 

 stances, till cleared. Then wash in tap water to get rid of the alkali. 

 A few minutes is sufficient for this ; then place in fifty per cent alcohol 

 for at least twenty minutes more does no harm then in ninety per cent, 

 and finally in absolute alcohol. Then transfer to clove oil, and after 



