PERMANENT MOUNTS 183 



that the small larvae may belong to another species, and they should not be 

 neglected. Anopheles larvse, which are surface-feeders, will not thrive in the 

 isolation tubes above described. They must be isolated in broad dishes or 

 saucers, which should be kept uncovered, in order that the dust from the air 

 may settle continuously upon the water. By properly watching the cultures 

 daily, the skin can be preserved at pupation and the pupa transferred to a cotton- 

 stoppered isolation tube for the emergence of the adult. 



SLIDE MOUNTS. 



The above method of preserving the larval skins suffices for temporary pur- 

 poses, and the skins when destined for dissection of the mouth-parts should be 

 kept in the alcohol and glycerine mixture. For examination the skins may be 

 placed on slides in a drop of glycerine; but for permanent preservation they 

 should be mounted in balsam. The skin should be treated with weak alcohol to 

 remove the glycerine, then in strong alcohol, absolute alcohol, oil of cloves and 

 finally Canada balsam, which should be heated after the cover glass has been 

 applied. Often sufficiently satisfactory results may be obtained by drying the 

 skin on a slide and applying balsam directly ; but if glycerine has been used this 

 must first be removed. Skins thus mounted are liable to become distorted, but 

 the method is more rapid and often suffices for ordinary determination. 



The male genitalia are important not only for specific identification, but also 

 for general study of systematic relations. For proper examination, the genitalia 

 must be mounted on a slide as microscopic objects. The tip of the abdomen 

 with the male organs attached can be readily broken off from a dried specimen by 

 the use of a pair of fine forceps. They are then placed in a solution of caustic 

 potash ; about one part of the saturated solution to ten parts of water will give 

 satisfactory results. The genitalia are left in this solution until only the 

 chitinous parts remain and all the details are readily visible. The time required 

 will vary with the temperature and with the size and chitinization of the speci- 

 men, very fresh specimens requiring much less time than old ones. The speci- 

 men should be examined from time to time while in the potash, as, if left too 

 long, it will become too soft and transparent and the details will then be very 

 difficult to see. Usually, if put in the potash late in the afternoon, the specimens 

 will be ready to mount on the following morning. If one desires to study a speci- 

 men immediately, it should be boiled in the potash over an alcohol lamp or 

 Bunsen burner. In this case, care must be taken, as the potash in boiling is 

 likely to spatter and the specimen thus be lost. Moreover the specimens treated 

 by the slower process make better mounts. 



When sufficiently cleared the specimens must be thoroughly washed with water 

 to remove all traces of the potash, then dehydrated with absolute alcohol, cleared 

 with oil of cloves and finally mounted in balsam. Before placing the cover glass, 

 the specimen should be examined to see that it is in the proper position for study. 

 Sometimes, when the parts are folded together, they can be spread by pressure 

 on the cover glass directly over them. This is especially true of those forms in 

 which the abdomen is depressed and the claspers prominently exserted, such as 



