315] NOCTUID LARVAE— RIPLEY 73 



what may prove to be a general zoological law, namely, When the expression 

 of the recapitulative law conflicts with the development in successive instars 

 of a series of adaptations to different functions, or to different degrees of the 

 same function, the latter is dominant . 



In species where either all or none of the instars spin silk it might be 

 argued that the recapitulative force would be allowed to express itself, since 

 the factor of unequal function would be eliminated. Instances are rare in 

 the Noctuidae where the larvae of all stadia spin silk in approximately pro- 

 portionally equal amounts, as in the tent-caterpillar, Malacosoma americana. 

 Sidemia devastatrix furnishes the only instance known in the Noctuidae 

 where this habit appears to be equally developed thruout larval life, and 

 the data in this case are not conclusive, since live larvae of only the first 

 and last three stadia have been seen by the author. The first instar spins 

 silk threads during the feeding period, the fourth and fifth form slight 

 cocoons in which to molt and the last pupates within a cocoon. Since the 

 long ancestral spinneret has been preserved in this species, the expression of 

 recapitulation would not involve any postembryon'c change and so far as 

 known none occurs, the spinneret of all stadia examined being long. Type 

 IV presents the opposite condition where there is no silk-spinning in any 

 stadium. In this type the recapitulative law is not followed with respect 

 to the relative length of the spinneret, which remains approximately the 

 same thruout larval development. 



An analysis of the possibilities with regard to the original use of the 

 habit of spinning silk in the order and in the family reveals the fact that we 

 cannot reasonably expect to encounter an expression of the recapitulative 

 force in species where the factor of unequal function has been secondarily 

 eliminated, as it has in Type IV. There are at least three ways in which 

 this habit may have originated in the ancestral lepidopterous larva. It 

 may have developed originally in the first instar, functioning as a means of 

 dissemination by the wind, as it does in various existing species, or in some 

 other capacity. Apparently better grounded is the hypothesis that the 

 spinning of a cocoon by the fully grown larva represents the primitive 

 condition, the other instars having in certain forms subsequently developed 

 the habit of spinning threads. Perhaps most probable of all is the possi- 

 bility that this habit was originally equally developed in all stadia, as it is 

 now found in the case-bearers, tent caterpillars, borers which line their 

 burrows and miners which line their mines with silk. The frequent occur- 

 rence of this condition among larvae of the more generalized families lends 

 weight to this view, altho the limitation of the spinning of silk to cocoon- 

 spinning often met with thruout the order favors the conclusion that this 

 situation is the ancestral one. 



However this may be, the very exceptional occurrence among noctuid 

 larvae of the equal development of the silk-spinning habit in all stadia 



