THE BIOLOGY OF THE MUD-DAUBING WASPS 31 



coming and going. But when I proceeded to gather them, they 

 all proved to be cells from previous years, and I found that the 

 good new nests were only on the ceiling, just out of reach. It 

 seems that the wasps built their nests wherever their fancy 

 dictated so long as they were unmolested, but when people 

 occasionally moved about in the room they promptly chose 

 sites higher up, out of the range of disturbance. 



The two species of wasps, Sceliphron caementarium and Chaly- 

 bion caeruleum make mud nests which are very similar in appear- 

 ance. The species of the builder is ascertained with accuracy 

 only by finding the dead pupae or adults in the cells or by noting 

 the species of the adults as they emerge. Occasionally the 

 nests of both are decorated with pellets. The only constant 

 distinction which I have been able to discover lies in the differ- 

 ence in the structure of the cocoon; that of 5. caementarium is 

 smooth, glossy and brittle while that of C. caeruleum is the 

 same but covered with a webby mesh. 4 The occasional occur- 

 rence of both species of wasps emerging from one colony of 

 cells does not necessarily mean that an erring mother has de- 

 posited her egg in the wrong nest, although it would seem to 

 us almost impossible for a mother to find her own nest among 

 hundreds of others as we sometimes see them massed in the 

 lofts of large barns. (In our barn, 643 were gathered and about 

 three times as many remained). The phenomenon is easily 

 explained by. the fact that the mud-dauber's nest occasionally 

 occurs on top or along side of the pipe-organ nests, and on 

 several occasions we have found them plastered to paper wasps' 

 nests. In fact one curiosity which we have is the nest of these 

 two species of mud-dauber and a pipe-organ nest all subjoining 

 a large paper nest. We can see that probably the mother seldom 

 commits the error of laying her egg in another's nest, but when 

 choosing a site on which to build she sometimes regards the 

 architecture of her sisters the same as the side of a barn. 



NIDIFICATION OF T. ALBITARSIS 



Trypoxylon albitarsis makes the Pipes of Pan, or as they 

 are more frequently called the pipe-organ nests. Fig. 5 shows 

 the nest as it usually occurs; the short tier is in course of con- 

 struction. T. albitarsis does not daub the nest all over with 

 4 Details in Psyche, Vol. XXII, p. 62-63. 



