1921] Wheeler: Some Social Beetles 79 



attained a sufficient size, i.e. have produced at least three or four 

 fully developed leaves. 



Still neither the beetles nor the coccids are permitted to 

 live in perfect security till the obligate ants take possession of 

 their quarters. Where competition among insects is so very 

 keen as it is in the Neotropical jungle it is not surprising to find 

 that several predators and parasites are continually gaining 

 access to the petioles and decimating or even completely destroy- 

 ing their occupants. The greatest enemy of the beetles is the 

 small thief-ant Solenopsis altinodis Forel (See p. 48 and Zoologica 

 III, No. 4, p. 154), and the coccids have at least three formidable 

 enemies. All of these insects enter the petiole through the open- 

 ings made by the beetles and must therefore elude their watch- 

 fulness. We should expect the beetles to keep one of their num- 

 ber constantly on guard at the entrance, but they are neither suffi- 

 ciently constant in this behavior nor sufficiently discriminating 

 to keep out all intruders. When the petioles are taken into the 

 laboratory the beetles are often seen to remain for hours with 

 their heads in. the entrances and their bodies at right angles to 

 the longitudinal axis of the cavity, and even after the petioles 

 have been split longitudinally and placed in vials the insects still 

 exhibit this behavior, though it is now absurdly futile, since their 

 domicile is wide open. But not infrequently even the single 

 opening of a petiole may remain unguarded for long periods, 

 and when the petiole has several openings some of them are 

 apt to have no sentinels, so that predators and parasites small 

 enough to pass the narrow orifices, have no great difficulty in 

 gaining access to the colony. Moreover, a beetle that is guard- 

 ing an opening may fight off certain intruders but back away 

 and allow others to enter. On several occasions I held a beetle 

 with its head to a guarded entrance. The sentinel at once 

 grasped the stranger's head with its mandibles and pushed it 

 away. But when I placed a worker Solenopsis altinodis in the 

 same position, the beetle beat a hasty retreat and the ant climbed 

 into the petiole. From these experiments we may infer that 

 the beetles are more intent on keeping alien beetles of their own 

 species than dangerous pests like the Solenopsis out of their 

 nests. More probably some peculiar odor of the ant induces the 

 beetle to withdraw. Thus while it seems to be probable that alien 



