GUESTS AND PARASITES OF HALICTUS. 



The most conspicuous of the smaller Hymenoptera that fre- 

 quent these grounds is a little species of Loxotropa. Time and 

 again this insect was observed crawling stealthily over the nest- 

 colony, tapping its antennae on the ground as it moved. During 

 this deliberate progress it covers an inch in four seconds, but as 

 soon as its nears a selected opening its movement slows down to 

 an almost imperceptible advance. Still holding its long and 

 clubbed antennae extended straight forward, their tapping now 

 reduced to a slight nervous vibration, it gradually insinuates itself 

 into the nest, even beneath the very jaws of the gatekeeper. 

 Often after crawling so far into the nest that only the tip of its 

 abdomen is visible, it finds the nest unsuitable. Then it deliber- 

 ates no longer, but makes a hasty exit, leaving the astonished 



FIG. 5- Loxotropa ruficornis Ashm. Halictits, $ . Pliora cata, sp. nov. 



sentinel to reach in vain with questioning antennae for its bold 

 and impudent disturber. 



As interested an observer of the incoming bees as is the Pliora, 

 is a tachinid fly. This species hovers over the breeding ground 

 and suddenly circles over a particular hole. Is it attracted to 

 the nest by the hollowness of the sound of its vibrating wings as 

 it flies over an opening, or does it discern the state of advance- 

 ment of the household below by an instinct less mechanical ? 

 Like its relatives, this species chooses the moment when the 

 incoming bee pauses at her threshold quickly and quietly to 

 oviposit on her pollen mass and thus infect her offspring. 



