32 PHIL RAU AND NELLIE RAU 



mud when it is completed, thereby hiding its artistic architec- 

 ture, but it, like the other two species, smooths the interior 

 of each cell carefully. The holes at regular intervals in the 

 nest are made by the emerging adults, and the white spots are 

 the hardened chalky substance which the insects emit from 

 their bodies immediately after they emerge. This white sub- 

 stance is emitted by 5. caementarium and C. caeruleum also, 

 but in the form of many minute pellets discharged before 

 emergence. 



The larva of T. albitarsis spins a very light web about the 

 walls of its cell; just inside this it constructs its cocoon, black 

 very strong and brittle. This it probably makes of the excre- 

 ment which it clears from its alimentary tract after feeding 

 is completed, and utilizes for a cocoon by throwing it all over 

 itself. This is then kneaded into shape by dextrous movements 

 of the body; it then hardens and makes a very comfortable 

 abode. 5 



These nests are usually built against some flat surface; some- 

 times the back side is lined with mud, and sometimes the board 

 on which it is plastered serves as a back wall. Here in fig. 2, 

 we have a view from the rear of one such nest built without a 

 back wall, showing cells, partitions, pupal cases, spiders, empty 

 cells and two cells in course of construction. In this nest are 

 three instances of the emerging insect opening its way into an 

 adjoining cell instead of to the outside; if the adult belonging 

 in the latter cell had already emerged, this wasp could leave 

 by its exit, but if not it must certainly die imprisoned, because 

 instinctively the insects can open their way through only 

 one wall. 



In contrast to this flat form I have seen about a dozen nests 

 built on hanging corn-husks. Here without a substantial 

 foundation the cells attained a perfectly cylindrical shape, as 

 thick on the back as on the front. They afforded sufficient 

 protection in all respects, however, for they normally brought 

 forth healthy adults. 



On only one occasion have I been able closely to observe 

 the details of the building operations of T. albitarsis in progress. 



5 1 have tried to observe this process by placing larvae about to pupate in vials. 

 But it seemed they were unable to work on the smooth glass, for they produced 

 only thick ribbons of the black material, but no pupal case. 



