42 ILLINOIS BIOLOGICAL MONOGRAPUS [284 



of this brood also attain a large size, they become greatly reduced in num- 

 bers by wilt disease and parasites, so that the midsummer -brood is usually 

 not large. The small individuals of the midsummer-generation yield moths 

 of low fecundity, accounting for the usual inconspicuousness of the third 

 brood, altho infrequent outbreaks have been observed in September. It 

 seems probable that this principle may prove to be an important one to be 

 considered in the prediction of these outbreaks. 



The fact that fecundity is hereditary in animals has been well es- 

 tablished. Geneticists have found that the tendency to bring forth twins 

 and triplets is hereditary in mammals. By artificial selection, Pearl and 

 Surface have greatly increased the egg-laying propensities of a certain 

 strain of Plymouth Rock fowls. It has been well established that fecundity 

 in Drosophila is an hereditary trait. Individual variation in fecundity is 

 considerable within the Noctuidae. Since those strains, in a species of 

 this family, with a tendency to lay many eggs must transmit this trait to 

 many more Individuals than would those inclined toward low fecundity, 

 it seems evident that in general species must increase fecundity in the 

 course of evolution up to a point where it is checked by some sort of barrier, 

 mechanical or physiological. The only possibility for a non-prolific strain 

 to untimately persist would involve necessarily its accompaniment by in- 

 heritable, advantageous properties not possessed by prolific strains, such 

 advantages offsetting their low fecundity. We have no evidence indicating 

 that this latter, seemingly unlikely possibility has taken place within the 

 Lepidoptera. 



In the light of the correlations demonstrated, it seems evident that the 

 amount of growth, or the number of molts, would act as a barrier to an 

 increase in fecundity, furnishing mechanical limits, which would prevent 

 further expression of this tendency. An hereditary increase in the number 

 of molts, such as the one cited with regard to a single individual of Cirphis 

 unipunda, would allow the individuals possessing this trait to attain a 

 larger size and consequently a higher fecundity. This would, therefore, 

 be transmitted to a larger number of offspring than would the tendency 

 toward a lesser number of molts. The persistent variation, then, would be 

 the one with the largest number of stadia. 



On the basis of this theory, the largest number of larval stages is the 

 most specialized condition in this family. This conclusion is supported by 

 all the other evidence available. As shown by Dyar, the great majority 

 of lepidopterous larvae undergo ecdysis four times, five times frequently, 

 and three, six, seven, eight, nine or ten times rarely. According to our 

 data the molting five times appears to be the general condition throughout 

 the Noctuidae, four molts occurring only in the two species of Phytometra, 

 and in the male of Cacnurgia erechtea, and six being found in but three 

 species. We should be justified apparently in regarding the passing of 



