THE PROBLEMS OF TRANSFORMATION 249 



organized class among the Arthropoda. It is also abundantly 

 clear that within the class of the insects, the most specialized 

 members undergo the greatest changes ; primitive and com- 

 paratively lowly insects like bristle-tails and cockroaches are 

 much like their parents when hatched, but how different it 

 seems, with the butterfly's caterpillar, the bee's grub, or the 

 blue-bottle's maggot. We are therefore justified in regarding 

 the transformations of insects as presenting us with a problem 

 of remarkable interest. 



In looking for light on this problem it is well to recall the 

 warning (p. 65) that the difference between an insect and its 

 larva may easily be exaggerated. From the brief survey given 

 above of differences in the life-history among various classes 

 of the Arthropoda, it should be realized that no arthropod 

 is hatched in anything approaching the primitive condition 

 that is seen in the gastrula larva of a starfish, the trochophore 

 of an annelid or even the veliger of an oyster. The young 

 nauplius, the simplest type among crustacean larvae, is stamped 

 already as an arthropod because it bears jointed limbs ; it is 

 stamped as a member of the crustacean class because some 

 of those limbs are two-branched. This consideration warns 

 us that transformation among the Arthropoda, though often 

 superficially striking, is far less profound than among several 

 of the great, predominantly marine phyla. 



When we pass on to compare insect transformations with 

 the changes undergone by Crustacea and other Arthropods, 

 we notice that while in the latter there is usually necessity 

 for the growth after hatching of many segments and appen- 

 dages (as in the case of those Crustacea with the nauplius 

 larva, the millipedes, or the centipede Lithobius), in a typical 

 insect larva, such as a ground-beetle grub or a caterpillar, 

 the segmentation and the limbs of the adult are already fully 

 represented. The larva is built on the same general plan as 

 the adult ; it resembles its parent fundamentally, however 

 widely it may differ in details of form. Where, as in such 

 extreme cases as that of the blue-bottle and its maggot, the 

 difference is more striking, more profound, we have seen that 

 the divergence may be interpreted as a case of degradation 

 undergone by the larva, and that the modification of the larva 

 is a specialized even if a degenerate condition accompanying 



