SCALES OF LEPISMA. 629 



markings on the scale are better seen when an achromatic 

 condenser is employed with a good objective. 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 l-12th object-glass. 'No. 5 a, fig. 274, is a 

 portion of a large scale. Fig. 273, No. 5, the longitudinal 

 markings are shown under a lower power. " But the 

 transverse striae on the scale of the speckled Podura are 

 rendered more distinct when the central rays are stopped 

 out. Any error in the correction of the lenses, whether 

 in the manufacture or in the adjustment of the thin 

 covering glass, is immediately detected by the peculiar 

 appearance which these markings present." 



Lepisma saccharina has a spindle-shaped body covered 

 with silvery scales, the sides of the abdomen being fur- 

 nished with a series of appendages or false feet, with 

 long-jointed bristle-like organs at their extremities. The 

 head is concealed under a pro-thorax ; the eyes are usually 

 compound, and generally occupy the greater part of the 

 head. The antennae are very long, and composed of 

 numerous joints ; the maxillary palpi, which are from five 

 to seven jointed, are very conspicuous. These insects are 

 also inhabitants of moist places. The Lepisma saccharina 

 is commonly found about houses, in sash-frames, old sugar- 

 casks, &c. ; from the latter circumstance it derives its 

 name. The scales (fig. 273, No. 8) have long been 

 favourite objects, and much used for testing the power of 

 'penetration and definition of object-glasses. The scales 

 should be mounted under thin glass covers ; oblique light 

 shows some portions of the scale to advantage ; other 

 parts are rendered more distinct when the central rays of 

 the achromatic condenser are stopped out. 



The metamorphosis is complete in the Suctoria, or 

 Siphonaptera, a wingless family — the larva, pupa, and 

 imago of which are very distinct in their apjDearances — 

 the well-known Flea is the best example of this small 

 group. By many authors these insects have been arranged 

 with the Diptera : this is most decidedly incorrect, since 

 they difi'er in many particulars. The external covering of 



