THE BIOLOGY OF THE MUD-DAUBING WASPS 59 



that the mothers' habits or instincts relative to nest-building 

 are fairly constant at all times. 



In the group of those which died as larvae amid plenty of 

 food, the death-rate runs higher among the winter brood than 

 in the summer brood. At present I see nothing to account 

 for this unless it be that the cold of winter caught a few which 

 had not yet pupated. However, the per cent, is so small in 

 both cases that the difference might be little more than mere 

 chance. 



Those which died as prepupae show even a more marked 

 difference, 24 and 17 per cent, against 10 per cent. Now this 

 is the stage in the insects' development which is greatly pro- 

 longed in the winter brood to cover the period of dormancy. 

 Also it is the stage wherein the insects pass through the winter 

 cold and may succumb to it. Both of these factors cause the 

 probability of fatality to increase. But when we turn to the 

 pupal and adult stages, which are very similar in duration and 

 conditions in the two broods, we again find the per cents of 

 mortality practically identical. 



The greatest difference of all occurs in the proportion of 

 young wasps killed by parasites (or riddled by them after 

 their death from other causes. Melittobia had infested 34 

 per cent, of the Kansas winter brood, and 22 per cent, of 

 the Meramec Highlands lot, while it had reached only 7 per 

 cent of the summer lot. This is probably due to the prolonged 

 period of exposure to chance of attack during the prepupal stage. 

 Anthrenus was somewhat more destructive in the summer brood. 



Hence, we see that practically the whole difference in the 

 mortality in the two broods occurs in these three groups which 

 we have pointed out, where the mothers late in the fall failed 

 to deposit eggs in the cells, where the young died in the pro- 

 longed prepupal stage, and where they were the victims of para- 

 sites 



But this does not end the problems of regeneration of these 

 species. Some large factor is as yet undiscovered. Let us first 

 look at the winter crop of nests. If we may assume that each 

 nest stands for one mother, then the 643 females gave to the 

 next spring's population only 172 new wasps. Now supposing 

 one half of these were males, then the task of keeping up the 

 stock is left to 86 females. If they in turn build nests and 



