4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



of her race, inserts her eggs into the stems of trees, suggestive that 

 formerly the young cicadas lived on the trees, as do young aphids and 

 scale insects. At present, the young cicadas simply drop off the trees 

 to the ground, a habit that may have been accidental at first, but gave 

 a better chance of survival. Here, then, is a case where the young 

 insects have deserted their parents, and fortunately became structurally 

 adapted to a subterranean burrowing life. Also in the ground they 

 were able to prolong their lives. 



From the case of the cicada it is only a step, though perhaps a 

 long one, to that of the dragonfly in which the young insect has become 

 entirely different from its parents in adaptation to living and feeding 

 in the water, but still it reverts to the adult structure at one moult. The 

 same is true of some other insects, such as the mayflies and the 

 stoneflies. 



As this process of independent juvenile specialization is carried 

 still farther in the higher insects, affecting not only the external form 

 but the internal organs as well, many of the larval tissues and organs 

 become so different from those of the adult that they have to be 

 destroyed. The corresponding adult parts are then newly built up, 

 in which case it can hardly be said that the larva is metamorphosed 

 into the imago. It now becomes necessary for a reconstructive stage, 

 or pupa, to intervene between the larva and the fully formed imago, 

 which is liberated by a final moult. The insect is now said to be "holo- 

 metabolous." The holometabolous insect becomes virtually two distinct 

 animals separated by the pupa, in which one is broken down and the 

 other newly constructed. 



Now the question comes up as to how does one egg produce two 

 individuals so different as the larva and the adult may be. This is 

 probably a question for the geneticists to explain, but so far as known 

 to the writer they have not done so. However, since gene mutations 

 affect adult structures, they should produce modifications also in 

 juvenile stages. If a mutation is beneficial to the adult, it can be 

 preserved ; when useful only to the young insect, it must be discarded 

 at the moult to the imago, but restored to the next larval generation. 



Experimentally it has been shown that the egg is potentially both 

 larval and adult, but the fact does not explain how it has come to be so. 

 We can surmise that the egg contains two sets of chromosomes or 

 two kinds of genes, but how did this condition arise only in cases 

 where the young insect could not lead the life of its parents and had 

 to be given a form of its own? In the course of normal development 

 the young animal naturally comes first, so in the case of double 



