OUR HOUSEHOLD INSECTS 



FIG. 109. Scale of Le- 

 pisma. 



detached by a touch. These are the cause of the silvery 

 appearance, and produce a very slippery surface, which, 

 combined with its extraordinary 

 agility, makes the silver fish a diffi- 

 cult creature to catch and hold. 



Like other animals belonging to 

 the Arthropodous (jointed-legged) 

 division of the animal kingdom, the 

 silver fish sheds its skin periodically, 

 thereby providing for an increase 

 in size; but apart from the slight 

 changes produced by this regular 

 renewal, its appearance is not altered 

 throughout life. When hatched 

 from the egg, it has six fully-deve- 

 loped legs, as when mature, and 

 is similar in form, differing only in the proportions 

 of the parts and in depth of colour. As there is no 

 quiescent stage corresponding to the pupa of other 

 insects, and no acquisition of wings, the silver fish may 

 be truly said to undergo no metamorphosis. And the 

 same remark applies to all the members of the group to 

 which it belongs ; none of them ever possess wings or 

 exhibit that succession of changes of form to which the 

 name " metamorphosis " is applied. We have here, then, 

 quite a different case from that of the bed-bug or book- 

 louse. These were apterous members of groups which 

 are normally winged in the adult stage. The silver fish, 

 on the other hand, belongs to an order which is normally 

 apterous throughout life. This order is called Thysanura, 

 and, as restricted by Sir John Lubbock (i.e., without the 

 Collembola, or springtails, small jumping insects found 

 in profusion under stones, logs of wood, &c.), contains 

 only a very few British insects. Three only are likely 



