BIOLOGY OF PARACOPIDOSOMOPSIS. 297 



III. SUGGESTED EXPLANATIONS OF MIXED BROODS. 



On the basis of the data here presented, how is one to explain 

 the origin of mixed broods in Paracopidosomopsis? There are 

 at least two possible explanations that may be suggested: (i) 

 that a fertilized and an unfertilized egg had been deposited in the 

 egg of the host; (2) that a fertilized egg had given rise to males 

 as well as to females. 



One finds no difficulty in offering the first of these suggested 

 explanations to account for some of the mixed broods, especially 

 for the larger broods in which males and females are found in 

 about equal numbers. But if the same explanation is to be 

 applied to all mixed broods, it would be necessary to assume 

 that in the majority of cases the development of the unfertilized 

 egg is interfered with by the presence of the developing fertilized 

 egg. In the light of Lillie's ('16) recent discovery concerning 

 the origin of the "free martin" in cattle, this assumption would 

 not seem to be untenable. In the case of these insects it is 

 conceivable that a fertilized egg might generate "hormones" 

 which would inhibit the development of an unfertilized egg in 

 such a way that only a few of the male embryos reach maturity. 



One difficulty which at first seemed to stand in the way of the 

 application of the hormone theory is the fact that the vast 

 majority (about 85 per cent.) of all Paracopidosomopsis broods 

 are mixed. It did not seem possible that eighty-five per cent- 

 of the moth eggs, widely scattered as they are in the cabbage 

 patch, would each be visited and parasitized by a fertilized and 

 an unfertilized female insect. This raised the question as to 

 whether the female lays more than a single egg at each ovi- 

 position, and if so, whether the fertilized female has the power 

 to lay both kinds of eggs, as is the case of the queen bee. Sil- 

 vestri states that the female of Litomastix truncatellus lays but a 

 single egg at each oviposition. We have studied this question in 

 Paracopidosomopsis and find that the female, whether fertilized 

 or not, frequently deposits two eggs at one oviposition. In 

 one experiment, a single unfertilized female made seventeen 

 successive ovipositions in as many eggs. The time of ovi- 

 position varied from one minute and thirty seconds to three 

 minutes, with an average of two minutes for each egg. An 



