254 ENTOMOLOGICAL NEWS. [Oct., 'O2 



life of both was probably much the same ; but with metamor- 

 phosis once established the possibility of a still greater differ- 

 ence was present, in that the imago might adapt itself to a 

 different mode of life from that of the larva. This we find has 

 taken place to a marked extent in all the higher orders of 

 insects. 



With the establishing of metamorphosis a new factor was 

 introduced in the larval period besides that of growth, viz. that 

 change preparatory to metamorphosis. This change becomes 

 apparent externally in the presence of wing-pads. 



In primitive insects, with wing-metamorphosis once estab- 

 lished, this change, appearing early in the larva, took place 

 pari passu with that of growth, as is still the case in most of 

 the generalized orders of insects. On the other hand, in all of 

 the more specialized orders we find the tendency to seperate 

 metamorphic changes from that of growth, which becomes 

 especially marked in those orders where metamorposis implies 

 not only a change of wing-pads into wings, but also those of 

 mouth-parts, internal organs, etc. This leads to the formation 

 of a fourth stage in such insects the pupa stage, which ma} 7 

 be defined as the resting stage during which most of the meta- 

 morphic changes take place leading to the imago. This is a 

 secondary separation of the larval period, like that of the 

 imago, by which the period of growth is still more shortened, 

 and the metamorphic changes are concentrated to form a dis- 

 tinct period. 



An intermediate condition is found in a number of insects in 

 which the tendency to such a separation or change from that of 

 growth becomes apparent without being fully realized. In 

 these the wing-pads, with other metamorphic structures, first 

 become apparent as external organs with the penultimate 

 ecdysis. To these the term nymph or active pupa \vas first 

 applied. But as the mode of life of this so-called nymph is 

 usually the same as that of the larva, growth being carried on 

 without interruption, it can scarcely be considered to form a 

 distinct stage, but should rather be looked upon as an incipi- 

 ent or primitive pupa-stage not yet separated off from the 

 larva, The extension of the term nymph to the whole Iarv;i1 



