ix] PAST AND PRESENT 119 



early rudiments to become sunk into the body, so 

 that the cuticle of the second, and, later, of the third 

 and succeeding instars, showed no outward sign of 

 their presence. This suggestion is confirmed by 

 Heymons' (1896, 1907) observation of the occasional 

 appearance of outward wing-rudiments on the thoracic 

 segments of a mealworm, the larva of the beetle 

 Tenebrio molitor, and by F. Silvestri's discovery 

 (1905) of a 'pro-nymph' stage with short external 

 wing-rudiments between the second larval and the 

 pupal instars of the small ground-beetle Lebia scapu- 

 lar is. Whatever may be the exact explanation of 

 these abnormalities, they show that in the life-story 

 of the higher insects outward wing-rudiments may 

 even yet appear before the pupal stage, confirming 

 our belief that such appearance is an ancestral cha- 

 racter. The inward growth of these wing-rudiments 

 may well have been correlated with a difference in 

 form between the newly-hatched insect and its parent. 

 As this difference persisted until a constantly later 

 stage, and the pre-imaginal instar became necessarily 

 a stage for reconstruction, the present condition of 

 complete metamorphosis in the more highly organised 

 orders was finally attained. 



To explain satisfactorily these complex life-stories 

 is however admittedly a difficult task. The acquisi- 

 tion of wings is, as we have seen, a dominating feature 

 in them all, but if we try to go yet a step farther back 



