44 ILLINOIS BIOLOGICAL MONOGRAPHS [286 



of this animal as the possessor of a proportionately large head. Nor does 

 our knowledge of the postembryology of the house-fly convince us that it 

 descended from an apodous insect with vestigial biting mouth-parts. We 

 do not look to recapitulation to account for such conditions. Such post- 

 embryonic changes may be described as non-recapitulative as opposed to 

 those of recapitulative significance, which apparently conform to the law. 

 This point can be determined with regard to a structure undergoing change 

 in ontogeny with a certainty proportional directly to our knowledge of the 

 phylogeny of the structure in question. Thus, if the changes undergone 

 by any structure in the course of its development recapitulate its race- 

 history, we regard that structure as of recapitulative significance, but if 

 its phylogeny be doubtful, our decision on this point must be proportion- 

 ately tentative. The successive instars of species of Leucaspis figured by 

 Lindinger reveal beyond any reasonable doubt the recapitulative signifi- 

 cance of the pygidial structure in these coccids, the postembryology re- 

 capitulating minutely their phylogeny, which has been well established by 

 the extensive morphological studies of MacGillivray. The recapitulative 

 significance of wing-venation in pupal postembryology has already been 

 mentioned. Of many parallel instances the case of Mantispa is perhaps 

 the most familiar, the larva of this insect passing through transitional 

 stages from a thysanuriform to an cruciform type, repeating the generally 

 accepted phylogeny of the latter form of larva. The taxonomic advantages 

 gained by the establishment of the recapitulative significance of a structural 

 change in postembryology will be demonstrated later. 



Many structures are adapted to the mode of life of a particular stage 

 or to a habit associated with a single point in the life-cycle. Such organs 

 function unequally or even diSerently in different stages of development, 

 frequently being used in only one stage. Lepidopterous pupae, notably 

 of the Sesiidae, frequently bear spines or projections used for breaking 

 the cocoon and for wriggling into the open. These belong distinctly to the 

 pupae. A parallel instance is furnished by the wings of insects, which 

 function only in adults and appear in earlier stages merely as developing 

 adult structures. Similarly caterpillars often spin silk in certain stadia 

 and not in others, and noctuid larvae frequently do not employ the first 

 one or two pairs of larvapods in the first stadium. This unequal function 

 of a structure in different stages is generally correlated with structural 

 differences, hence a non-recapitulative factor is introduced. Postembry- 

 onic changes which are the expression of this factor will be referred to as 

 adaptive to unequal function. It is evident that the two factors, recapitu- 

 lation and adaptation to unequal function may act in the same or in opposite 

 directions with reference to a particular postembryonic change. In the 

 former event it is impossible to ascertain to what extent each of these forces 

 has operated in the production of the change, which is consequently most 



