35^ The Pedigree of Insects 



two. We have already seen (Chapter II, pp. 116-8) 

 how several stages of transition from the active nymph 

 to the passive pupa can be traced, thus bridging, at 

 least to some extent, the gap between incomplete and 

 complete metamorphosis. But it has recently been 

 pointed out that a deeper distinction than the nature 

 of the pre-imaginal instar marks off the metamorphic 

 insects from the lower orders ; in the nymphs of the 

 latter the wing-rudiments are always visible outside 

 the body, in the hirvs of the former they are 

 always concealed within, appearing only at the pupal 

 stage (69J. This distinction, however, becomes less 

 marked when we remember that in insects which 

 do not undergo a complete transformation the wing- 

 rudiments first arise beneath the skin of an early 

 instar though they soon appear outside the body. 

 It is not hard to conceive how, as the habits of the 

 young insect diverged more and more from those of 

 the parent, the appearance of the wing-rudiments 

 outside the skin would be put off to a later and later 

 stage. And the recent observation of visible wing- 

 rudiments occasionally appearing on the full-grown 

 grub of a beetle {Tenebrio) shows that in the ancestors 

 of the Coleoptera the wings grew outside the body 

 (79)' Starting then with no greater change than 

 the acquisition of wings after hatching, an opposite 

 modification of the wingless and winged stages in 

 the life-history independently of each other has led 

 to the startling transformations now passed through 

 by all the higher insects. And it has thus come to 

 pass that, in contradiction to the prevailing rule among 

 animals, the highest insects undergo the most marked 

 changes of form. 



Neuroptera and Coleoptera. — The above sugges- 

 tions as to the origin of metamorphosis among insects 

 point clearly to the Neuroptera and Coleoptera as the 



