EE Trans. Acad. Sci. of St. Louis 
long observation of the wasps at work, I have never 
seen S. caementarium use an old cell or C. caeruleum 
build a new one. Now since I have recently discovered 
how entirely different are the two species in their habits, 
even those pages will have to be revised. It is natural 
that these erroneous conclusions should be reached by 
a study of the nests in the laboratory; this perpetuation 
of an error is a weighty argument in favor of field ob- 
servations, for one turn to the nearest barn at almost 
any point in our country at any time from May to Oc- 
tober would have disclosed the truth about this common 
insect. 
The two species, Chalybion caeruleum and Sceliphron 
caementarium, formerly placed under the generic name 
Pelopoeus and regarded as two species of the same 
genus, are said by Rohwer to have certain anatomical 
characters which undoubtedly place them in two distinct 
genera. 
It is interesting to know that the Peckhams, in open- 
ing the nests, found some cells that had been used a 
second time, and concluded that since both species 
emerged from them, both species make new nests and 
that both species likewise sometimes use old nests. 
Close observation, both in the field and in the labora- 
tory, has convinced me that wherever the cocoons* are 
those of Chalybion the plug shows that the nest has been 
resealed, and that where the occupant is a young Sceli- 
phron the entire nest is new and the plug at the opening 
is still the original one. 
The Peckhams point to the fact that often the builder 
finishes the nest by sticking numerous round pellets over 
*In a former paper I have shown that while the cocoon of C. caeru- 
leum is similar to that of S. caementarium, it has in addition a white, 
webby silk cover which the former lacks. In ten years, I have never 
found this distinction to fail. 
