GROWTH AND TRANSFORMATION 165 



those of the adult, we find, as a rale, more or less difference 

 in structure, and such difference involves transformation of 

 a greater or less degree in the course of the life-history. 

 For example, there are two families of sucking-insects — the 

 Fsyllidae and the Cicadidae — related to the aphids mentioned 

 above. Aphids undergo little or no change of form in the 



Fig. 43. — Apple Sucker {Psylla mali) . a, female, X 8; 6, egg, 

 X 80 ; c, first-stage larva (ventral view), X 100 ; d, nymph, fifth instar 

 (dorsal view with legs shown on left, feelers and wing- rudiments on 

 right), X 20. After G. H. Carpenter {Econ.Proc. R. Dublin Soc.i, 

 1909). 



process of their growth, and young or adult aphids usually 

 live, as we have seen, under much the same conditions. 

 But in the life-cycle of a psyllid or a cicad striking changes 

 of form are to be noted, the young of these insects living in 

 conditions quite different from those of the adult. Psyllids 



