THE BIOLOGY OF THE MUD-DAUBING WASPS 29 



May or June. Here at St. Louis we have both summer and 

 winter broods, and I have a faint suspicion that those nests 

 which harbor the winter brood of 5. caementarium and C. caeru- 

 leum are built more massive than the summer cells. This 

 question is worth investigation to determine whether the wasps 

 are endowed with the instinctive power of apprehending the 

 approaching season. When one sees the enormous thickness of 

 some of the walls (e.g., figs. 7, 9 or 10) in contrast with the thin 

 walls and partitions often found in the summer nests (fig. 20), 

 one is almost tempted to attribute this to something other than 

 individual traits, although we have found an enormous amount 

 of individuality expressed in the work of 5. caementarium. One 

 finds, for instance, much variation in the careful or slipshod 

 manner in which load after load of mud is applied to the nest. 

 Fig. 4 shows how precisely each mouthful is sometimes applied 

 to form half a ring on the cell, while fig. 8 reveals the careless 

 way in which another mother applied her plaster, without any 

 regard for size, shape or security. I am sure that a nest made 

 as loosely as this would be an easy mark for Hymenopterous 

 parasites of the genus Melittobia 1 or even Anthrenus larvae 

 when minute, had not this mother made up for her carelessness 

 by thickly daubing mud over the outside of the nest. This 

 photograph shows the details of the contours after the loose 

 reinforcement was carefully scraped off. The nests of 5. caemen- 

 tarium and C. caeruleum usually have additional mud daubed 

 over the outside, no doubt for the purpose of strengthening 

 the structure and increasing its warmth. Some are decorated 

 on top of this with whole pellets stuck here and there over the 

 surface — not flattened or spread as usual (figs. 3, 18). This 

 serves no utilitarian purpose, so far as we can see, and is not 

 generally done, but the very fact that it is sometimes done, 

 shows that individual differences in manipulation exist, and to 

 account for the origin of this difference leads one into fanciful 

 speculation. It certainly seems farfetched to say that some 

 females have developed an aesthetic taste. Better would it 

 be to say that this habit of decorating is a vestige from the time 

 when one or both species, living in milder climate, made a crude 

 nest entirely of small round pellets carelessly stuck together. 

 Later when climatic changes or migration northward occurred, 

 3 They can pass through holes .013-inch in diameter. 



