INTRODUCTION 



II 



renewed and shed, the casting or '' moult " of the exo- 

 skeleton being called an ecdysis. Often the various instars 

 (the forms assumed by an insect in the successive stages 

 of its life-history) differ from one another, and this is, 

 to some degree, inevitable, since, while the vast majority 

 of insects are winged when adult ; no insect has wings 

 when first hatched or born. The wingless young may be 



Fig. 4. — Successive instars of Meadow Plant-bug (Leptopferna 

 dolobrata), Great Britain, from newly-hatched (A) to adult (F) ; in 

 C, D, and E can be traced the growth of the wing-rudiments (tv). X 5. 

 After A. Tullgren, 1919, and H. Osborn, jfourn. Agric. Res. (U.S.D.A.) 

 XV, 1918. 



Strikingly Hke the winged adult as in the case of grass- 

 hoppers and plant-bugs (Fig. 4), or to all outward seeming 

 most unlike it, as may be seen by comparing a butterfly or 

 moth with its caterpillar (Fig. 5), a bee with its grub, or a 

 bluebottle fly with its maggot ; in such cases the transfor- 

 mation exemplified in the insect's Hfe-history is profound. 



