264 MICROSCOPY FOR BEGINNERS. 



from the middle of the rear margin. One form of this 

 curious projection is shown in Fig. 176, a female by 

 Fig. 171, the projection varying in shape and size in 

 the different species. The males seem much less abun- 

 dant than the females ; they are, at least, less frequently 

 captured by the microscopical fisherman. 



On the ventral surface, behind or before, or on both 

 sides of the ventral plates, will be observed one or more 

 very small dark spots never bordered by a plate. These 

 are the external openings of the tracheae or air-tubes, 

 which extend through the body and supply it with oxy- 

 gen. As the mites are not known to come to the sur- 

 face for a supply of air, as so many aquatic animals do, 

 the tracheae are supposed to be able to absorb it directly 

 from the water. The tracheal openings are not an im- 

 portant aid in ascertaining the name of the creature, but 

 the beginner must not mistake them for the aperture 

 bordered or covered by the ventral plates. In some 

 mites they are not well marked, and may be overlooked, 

 and there is still another dark spot usually present near 

 the posterior part of the ventral surface which must not 

 be confounded with the ventral opening since it is in 

 the median line. This is the external opening of the 

 intestine. It is never bordered by plates, and is always 

 behind the ventral orifice, but it is not always con- 

 spicuous. 



Equally important to the student are certain eleva- 

 tions of the ventral surface which appear to cover the 

 attached ends of the legs. These are called the coxa, 



