PROCURING OBJECTS. 91 



of the disease, a red spot or line communicating with it ; this 

 part, and not the pustule, must be probed, and the insect, if 

 present, be turned out. It is often, however, difficult to de- 

 tect its haunts. 



To obtain the Entozoon fotticulorum, which is a parasite 

 occurring in the sebaceous follicles of the skin of the forehead, 

 nose, &c., squeeze the neighborhood of the little black spot 

 or pustule, so as to force out the sebaceous or oily matter. 

 This should be laid on a slide, and a small quantity of oil 

 added to separate the insects from the nidus in which they are 

 imbedded. They may then be transferred by a pencil-brush 

 to a clean slide, covered with thin glass, and mounted. 



Another species of Acarus, the harvest-bug or tick, A. au- 

 tumnalisj is a very painful source of irritation to the skin 

 wherein they may have insinuated themselves. They may be 

 dislodged with a needle, and mounted in fluid or balsam. 



Tracheae, and Spiracles of Insects. The respiratory system 

 of insects will be described in the chapter on Dissections, to- 

 gether with their nervous, digestive, and circulatory systems. 

 The manner of mounting them has been described. 



StingSj Ovipositors, &c., frequently require considerable care 

 in dissection. They may be mounted in fluid or balsam. In 

 order to prepare them, the abdomen should be laid open by a 

 slit along the back of the insect, in order to obtain a view of 

 the relations of the various parts ; then the posterior segment 

 should be fixed to a loaded cork under water and dissected be- 

 neath a shallow magnifier. The dissection should be made 

 from right to left, proceeding from without towards the interior, 

 as far as the median line, when it should be continued from 

 within outwards. This mode of dissection may also be advan- 

 tageously employed to display or procure other objects of inte- 

 rest. See the chapter on Dissecting Objects. 



SHELLS OF MOLLUSCA. The structure of shell has only 



