28 PHIL RAU AND NELLIE RAU 



two species are so similar as to be indistinguishable (figs. 3,4, 6, 

 18). 



Some authors have placed these two wasps in the same genus, 

 probably chiefly because of their similar habits of nesting, but 

 I have found that there is some difference in the spinning of 

 the pupal case by the larvae of the two species, and Mr. Rohwer 

 writes to me that he believes that the two species "should be 

 retained in different genera for besides a difference in coloring 

 there is a shortening of the petiole in C. caeruleum and certain 

 other differences which indicate that it is of a different group." 

 Thus we have convergence of habit of nest-building in three 

 distinct genera. 



NIDIFICATION OF S. CAEMENTARIUM AND C. CAERULEUM 



During the sunny days of summer one may see many of these 

 mud-daubers coming to the edges of streams and puddles, criti- 

 cally selecting mud of exactly the right consistency and literally 

 standing on their heads biting out chunks of it and carrying 

 it to some distant shelter. There they fashion it into the familiar 

 cells illustrated in figs. 3, 4, 6, 18. The size and shape of these 

 pellets which they carry may be seen in fig. 13. These were 

 dropped by insects taken at such sources, and are exact size. 

 The smaller ones however are probably incomplete balls dropped 

 by wasps which were interrupted during the gathering. It is 

 generally thought, and all the treatises (excepting the Peck- 

 hams' 2 ) state that this mud is mixed with the saliva in the mouth 

 of the insect. I do not wish to discredit this statement, for I 

 have no proof to the contrary, but since it is generally made 

 on supposition, I think it would be permissible for us here to 

 suppose also that the amount of saliva used by an insect in a 

 day would be many times the size of the little body producing 

 it, especially when the wasp sometimes constructs a whole cell 

 in an hour and a half, or on other occasions when she is building 

 incessantly, whole days at a time, or reinforcing the nest, as 

 shown by the thick walls in figs. 7, 9 and 10. 



In these two species and in T. albitarsis as well, the young 



from eggs deposited in the early fall feed and go through their 



metamorphosis during the winter and emerge as adults the next 



2 Instincts and Habits of Solitary Wasps, p. 178: "The wasp adds nothing to 

 the mud, depending upon its drying for the necessary firmness, and if by some 

 accident the rain strikes it the whole becomes soft and falls to pieces." 



