512 THE MICROSCOPE. 



motion, and produces the well known merry chirp ; the 

 intensity of which is increased by a hollow space, called 

 the tympanum, acting as a sounding-board. It must be 

 stated, however, that various other explanations of the 

 origin of the stridulating noise produced by these insects 

 have been given. 



In Thysanura there is a remarkable diversity of struc- 

 ture ; they undergo no metamorphosis, and have no 

 wings. This order contains two families, Poduridce, Spring- 

 tails, and Lepismenm. In the former, the caudal appen- 

 dage has the form of a forked 

 tail (Podura, fig. 2 67), which 

 is bent under the body 

 when not in use, by its 

 sudden extension the insect 

 causes itself to spring to a 

 Fur. wr.-podura 'piumbea. (The small very great distance, in corn- 

 circle encloses the insect of life-size.) parison with its size. The 



body is covered with numerous minute scales, mostly of a 

 beautiful silvery or pearly lustre, and curiously striated. 

 Some of the genus are found jumping about on the surface 

 of the water, whilst others are met with in profusion upon 

 snow and ice. 



Podura piumbea, Lead-colour Springtail, are generally 

 found in wine-cellars, amongst the sawdust, leaping about 

 like fleas, and are very difficult to take. The following 

 is generally the plan resorted to : sprinkle a little oatmeal 

 on a piece of black paper, near their haunts ; after a few 

 hours, remove it carefully to a large glazed basin, so that 

 when they leap from the paper, as they will when brought 

 to the light, they may fall into the basin, and thus become 

 separated from the meal. The best way of separating the 

 very fine scales from their bodies has been previously 

 explained (see page 58), the markings on which cannot be 

 seen with a lower power than a quarter-inch object-glass. 

 Under a power of 500 diameters, the surface appears to be 

 covered with extremely delicate longitudinal and wavy 

 lines. The smaller scales are much more difficult to 

 resolve than the larger, and these form a good test of the 

 defining power of a l-8th or 1-1 2th object-glass. No. 8, 

 Fig. 249, is a portion of a large scale. In Fig. 248, 



