20 THE SOLITARY WASPS. 



over the whole. The others used only two or three lumps of 

 earth which they fitted neatly into the opening just below the 

 surface. Although it is usual for urnaria to leave her nest 

 closed while she is off searching for her prey, there is no invaria- 

 ble rule in the matter even for single individuals. Once hav- 

 ing seen a wasp dig her nest and close it up, we drew some radi- 

 ating lines from the spot, in the light dust that covered the 

 place, that we might find it again. When we returned, two 

 hours later, the same wasp had made a nest four or five inches 

 distant from the first one, and had left it wide open, while she 

 had gone off to search for her caterpillar. She had probably 

 been alarmed by the marks that we had made and had felt it 

 necessary to dig a new nest, but being in a hurry to lay her egg 

 had omitted the usual process of closing it. We witnessed the 

 storing of the caterpillar and the final closing. 



From Fabre we learn that A. argentata and A. sabulo- 

 sa, like our own urnaria, close the nest as soon as it has been* 

 made, at least when the provisioning is to be postponed until 

 the next day, while A. holosericea leaves it open until it is com- 

 pletely stored. He suggests an explanation for this variation 

 by dwelling upon the inconvenience that would result if it were- 

 opened every time that the wasp brought in a caterpillar, since 

 holosericca stores up five or six small larvae instead of one or 

 two large ones. But what, then, shall be said of A. yarrowi, 

 which, according to Dr. Williston, while it also stores a number 

 of small caterpillars, takes the greatest pains to close and con- 

 ceal the entrance every time that it comes out? We see the 

 same habit in other genera where the mother continually passes 

 in and out, as in Bembex and Oxybelus. 



Fabre thinks that A. hirsuta has the habit, unusual for Am- 

 mophila, of catching her prey first and then digging the hole in 

 which she bestows it. As she takes only one large caterpillar 

 she is thus relieved of the necessity of closing the nest more 

 than once. 



As has been said, urnaria usually hunts a long time before 

 she finds her caterpillar, and one or two days may pass before- 



