287] NOCTUID LARVAE— RIPLEY 45 



reasonably regarded as the expression of the two factors combined and is 

 referred to as recapitulative-adaptive. When these two forces conflict, 

 the effect of the recapitulative one is completely obscured, as will be demon- 

 strated later, the factor of adaptation to unequal function being dominant. 



Since certain animal structures do not grow as rapidly as others, they 

 are generally relatively larger in earlier developmental stages than in later. 

 The familiar instance already cited of the newly born mammal, with its 

 proportionately large head, is parallelled generally by insects, the heads 

 of the first instar being markedly large relative to the body. The ocellariae 

 and crochets of lepidopterous larvae are strikingly large in the first instar, 

 growing slowly in comparison with the surrounding structures. These 

 phenomena are ob\dously not an expression of recapitulation, but are most 

 probably to be explained by the relative rates of cell proliferation in differ- 

 ent kinds of tissue. This factor, like unequal function, may undoubtedly 

 operate either with or against the recapitulative force. When the effect 

 of the latter is obscured by that of the mechanics of growth, the resulting 

 change is classified as mechanical, whereas when these two forces exert 

 themselves in the same direction the change produced would be termed 

 recapitulative-fncchamcal, although no clearly defined instance of this 

 situation has been found. 



The compound types involving recapitulation have been already de- 

 fined. One instance noted is obviously the result of a combination of un- 

 equal function and mechanics of growth. This change is classified as 

 adaptive-mecliatiical. 



Of the possible combinations of these factors all have been actually 

 indentified as responsible for certain postembryonic changes in noctuid 

 larvae, except two, recapitulative-mechan-ical and recapitulative-adaptive- 

 mechanical. The first of these very probably finds expression in the mi- 

 gration of certain head-setae to be discussed later. The second type of 

 change possibly occurs also in these larvae. 



It should be noted that, unlike the other two factors, recapitulation is 

 to be regarded as a general law, which fails to express itself only when ob- 

 structed by other forces, which are dominant over it. 



Many postembryonic changes in structure are inexplainable in the dim 

 light of our knowledge of the factors involved. Our lack of adequate 

 knowledge of phylogeny is probably largely responsible for this situation, 

 since many changes such as those of the head-capsule of muscid larvae 

 figured by Nielsen, may prove to be of recapitulative significance, when 

 sufficient morphological work is done to establish the race-history of such 

 structures. An investigation, moreover, of the functions in different stages 

 of organs undergoing postembryonic changes will most probably reveal 

 many instances of adaptation to unequal function, while the determination 

 of the importance of the factor of the mechanics of growth awaits the re- 



