December 19, 1895J 



NA TURE 



153 



our own time written upon the origin of insect transform- 

 ations. It was, as they themselves tell us, Darwin's 

 " Origin of Species " which incited each of them to look 

 at the old facts in the light of a new theory. Fritz 

 Miiller, being at the moment specially occupied, not with 

 insects, but with crustaceans, threw out casually, as it 

 were, a number of general results of the greatest interest 

 and value, which he did not attempt to support nearly so 

 fully as his own knowledge of insects would have enabled 

 him to do. Brauer, a little later, travelled over the sub- 

 ject in a somewhat more leisurely way. The most im- 

 portant principles had already been indicated by Miiller, 

 but he was able to contribute many facts of special 

 interest, and to point out a definite and rather widely 

 distributed larval stage, of which much has since been 

 said in discussions on the phylogeny of insects. I need 

 not remind English naturahsts that, even before 1874, 

 Sir John Lubbock had won fame by his researches into 

 the life-history and habits of insects. He had also 

 written specially upon insect transformations before he 

 collected his matured thoughts into a book. His " Origin 

 and Metamorphoses of Insects " is now very widely 

 known. It is by far the most readable exposition which 

 we possess, and will long hold its place as an interesting 

 account in simple language of that part of the subject 

 which involves no special knowledge of insect anatomy. 

 Some day it should be supplemented by an account of 

 the mechanism of pupation, showing in detail how 

 various larva; are converted into winged insects. It is 

 on this side that insect transformations have been most 

 successfully attacked during the last thirty years. 



It is natural that after an interval of many years we 

 should find what seem to be deficiencies in the work of 

 those who have gone before us. The most notable 

 deficiency which I find in Lubbock's book is that he 

 does not remark the great distinction between insect 

 metamorphoses and those of most other animals.' They 

 occur, as I think, in a different part of the life-history, 

 and arise out of conditions which are different, or even 

 diametrically opposite. There are other points, too, 

 which seem to me to be passed over too briefly by 

 Lubbock and all previous writers. In particular, certain 

 aphorisms of Fritz Miiller seem to me to deserve a fuller 

 explanation than they have yet received. 



Such inevitable gaps in the expositions of our pre- 

 decessors render it possible to supplement even works 

 which have attained the rank of classics. Each genera- 

 tion for a long time to come will be able to add its 

 quota of facts and reflections without exhausting this 

 immense and difficult subject of inquiry. It is satis- 

 factory to note that Lubbock's account has been very 

 little disturbed as to matters of fact by later investiga- 

 tions, so that his readers, though they have plenty still 

 to learn, have very little to unlearn. 



Let me first attempt to justify my contention that the 

 transformations of insects are fundamentally unlike the 

 transformations of polyps, echinoderms, mollusks and 

 crustaceans. In the marine groups the minute animal, 

 just escaped from the egg, passes rapidly through its 

 changes, often before the yolk is absorbed. It may 

 complete them before it begins to feed, always long 

 before it has attained its full size. The insect, on the 

 contrary, undergoes its most striking change of form 

 after it has attained its full size. Moreover, the planula 

 of the polyp, the bipinnaria or pluteus of the echino- 

 derm, the Nauplius or Zocca of the crustacean, are 

 unknown except as transitory forms. But the less 



1 I am not aware that any one else has made more than a passing allusfon 

 to this distinction, but I may easily have overlooked some important 

 reference in the vast literature on insect transformations. Macleay has re- 

 marked that in insects the change of form takes place during their last two 

 or three st.iges, whereas " the metamorphosis of all other Annulosa only 

 occurs during the first or second moult after leaving the egg." {/llustr. 

 Zool. South Africa, p. 53). I do not find this distinction recognised as part 

 of ordinary biological knowledge in our best treatises on development, such 

 as Balfour, or Korschelt and Heider. 



specialised insect-larvae make a very close approach to- 

 certain lower, wingless insects in their adult state. The 

 insect-larva, before it gives place to the pupa, has not 

 only attained its full size, and acquired a form answer- 

 able to that of some perfectly adult insects, but it already 

 contains in many cases completely developed repro- 

 ductive products ; indeed, a very few insects are known 

 to be capable of reproduction as larvae. It seems to me 

 preposterous to say, as Harvey did in the 17th century,, 

 that the insect-larva is a kind of walking egg, or with 

 Quatrefages, that it is an embryo which leads an in- 

 dependent life. It would be far more truthfully described 

 as an animal which has attained the normal structure of 

 adult Arthropods, though it has still to undergo a peculiar 

 adult transformation. Where the insect larva falls short 

 of that adult structure, it is because abundance of food, 

 or some other external circumstance, has induced de- 

 generation, but we shall best understand insect-larvae in 

 general by comparing them to sexually mature myrio-. 

 pods, protracheates, scorpions and Thysanura. 



Adult transformation is rare among free-living animals, 

 though parasites furnish many examples. The Cteno- 

 phora, instead of settling down early, maintain a pelagic 

 life, and became specially modified thereto in a late 

 stage of development. The secondary sexual characters 

 assumed by some birds and mammals at the time of 

 sexual maturity, such as the train of the peacock, or the 

 antlers of the stag, are analogous facts. But the closest 

 parallel to the transformation of insects is to be found in 

 the Amphibia. Frogs and toads, having already as tad- 

 poles attained the full development of the more primitive 

 Amphibia, change to lung-breathing, tailless, land-travers- 

 ing animals. The motive is the same as that which led 

 to the acquisition of wings by insects. It is by virtue of 

 their adult transformation, that both the amphibian and 

 the insect are enabled to wander from the place of their 

 birth, to seek out mates belonging to other families, and 

 to lay their eggs in new sites. 



In those Amphibia which undergo transformation, the 

 stage added to the life-history of the more primitive 

 forms is not the tadpole, but the frog or toad. In those 

 insects which undergo transformation, complete or in- 

 complete, the winged state is the new addition. If a 

 pupa appears in the life-history, it results indirectly 

 from the acquisition of wings by the adult. Hence it 

 seems to me that in Amphibia and insects the peculiar 

 change, which renders possible all the rest, belongs to 

 the adult condition, i.e. these animals undergo an adult 

 metamorphosis. 



Transformation in the inhabitants of the shallow seas 

 is closely related to the crowded state and severe com- 

 petition of the area. The connection is two-fold. Unusual 

 risks imply numerous eggs, numerous eggs must be small, 

 and small eggs, with little or no yolk, hatch early, pro- 

 ducing very immature animals, unlike their parents. 

 Again the risks of the shore favour heavy-armoured 

 species, and it is well-known that a great proportion of 

 the invertebrate fauna of the sea-shore is defended in 

 this way. But heavy armour diminishes activity, and in 

 particular often renders swimming impossible. The dis- 

 persal of the species is therefore left to the young fry, 

 which are often specially organised for locomotion at 

 the surface of the sea. Marine animals which are not 

 armoured, such as the Cephalopoda, may undergo no 

 transformation at all. 



Certain general propositions concerning larval trans- 

 formation are disturbed if the adult transformation of 

 insects and Amphibia is included. Indeed, I was first 

 led to notice the distinction between adult and larval 

 transformation by observing that insects and Amphibia 

 do not conform to the general rule, that while the in- 

 habitants of the shallow seas go through transformations 

 in early life, fluviatile and terrestrial animals do not. 



Every animal and every plant has these two functions^ 



NO. 1364, VOL. 53] 



