62 PHIL RAU AND NELLIE RAU 



How cheering it is to find that 972 of these, or 76 per cent, 

 gave forth good adults in the spring after wintering in the cells. 

 In 126 cases (10 per cent.) did the mothers err in filling the 

 cell or fail to oviposit; 27 cells (2 per cent.) were sealed empty, 

 but in some of these cases the sealing partition was so placed 

 that the room was so small that a growing larva could not 

 possibly have had room to develop anyway. We have never 

 yet noticed Sceliphron committing this blunder of constructing 

 her cells too small to permit the full development of her young. 

 There was only one cell that was unsealed containing a few 

 spiders; this case may well have been due to the sudden death 

 of the mother. In only 4 cases can death in the larval stage 

 be attributed to insufficient food, while 20 others died in the 

 larval stage despite the fact that much food remained untouched 

 in their respective cells. 



The deaths in the prepupal, pupal and adult stages were 

 37, 12 and 13 respectively, lower per cents than occurred in 

 the other species. 



These insects were preyed upon by some of the parasites of 

 Chalybion and Sceliphron but not to so great an extent; 26 cells 

 were parasitized by Melittobia and 14 showed that the Anthrenus 

 beetle had played havoc; 28 cells were divided between a Dip- 

 terous parasite and an Ichneumon belonging to the tribe Ophio- 

 nini, indicated in the table by "D" and "O." 



Thus elimination in all the stages of development is far less 

 in T. albitarsis than in the other species. 



So here we have three species so similar in morphology that 

 we cannot discern any differences which would give one or the 

 other the least advantage in the struggle for existence. Like- 

 wise their life history, their habits of living and their habits 

 of nesting are so similar that it would appear to us that they 

 have equal chances of surviving. And yet there exists this 

 remarkable difference in the survivability of these species, and 

 even more strange it is that the species which has the fewest 

 numbers in this vicinity is the one which thrives far better 

 than the others. Had the variation studies, for which the 

 material was first intended, been carried to a conclusion, we 

 might thereby have located some tendencies which would account 

 for the comparative vigor or weakness in these species, but I 

 think it more than likely that in pursuing variation problems 



