104 THE SOLITARY WASPS. 



slightly larger species of Diodontus which acted in every way 

 like americanus. In spite of all our efforts we were unable to 

 find the holes of this second and larger species. They were 

 present in large numbers and it seemed improbable that they 

 could have nested in territory so familiar to us as the garden 

 came to be without being discovered. After close watching we 

 concluded that these wasps flew over the fence into the woods 

 while americanus settled down close by. 



The month of July is evidently the working season of the 

 Diodonti since they were very active from the seventh of the 

 month, when we first saw them, until the first of August. From 

 that time on we saw them less and less, although a few of them 

 were still at work three weeks later . 



On July twenty-fifth we first saw the males of this species^ 

 which, Mr. Ashmead tells us, had been unknown before. On 

 this and on succeeding days we saw them mating with the fe- 

 males. As the females had been laying their eggs for about 

 three weeks the males had probably been present all the time, 

 but had escaped our attention. 



The parasitic Chrysis fly, Omalus corruscans, is always in at- 

 tendance upon americanus, both on the cherry bushes and on 

 the ground near the nests. Brilliant and beautiful but full of 

 evil intentions, she watches their comings and goings. There is 

 no opportunity for her to lay her egg on the aphis outside of 

 the nest. Holding it closely under her body and not exposing 

 any part to attack, the wasp, without making the least pause, 

 flies into her open doorway. When she comes out again the 

 enemy is still lurking near, but no instinct warns her to cover 

 her treasure. The door stands wide open as she takes her de- 

 parture, leaving her young exposed to the foe. Perhaps the 

 danger is not so great as it seems. The fly certainly penetrates 

 into the nest, since we have found it in the gallery when exca- 

 vating; and in one case we found a strange larva feeding on the 

 aphides along with that of americanvs; but it may be that the 

 supply of food is so ample as to cover the needs of both. At 



