v SOCIAL HOMES 165 



they construct a wall, often in the hollow of a tree, 

 filling up the crevice with the exception of a 

 narrow orifice. One species, still more determined 

 to secure safety and privacy, in addition to blocking 

 up the gap, fashions a neat tubular gallery of clay 

 outside its doorway; the tube has a trumpet-shaped 

 mouth, and the particles are kneaded together with 

 some substance of viscid consistency. At the entrance 

 a number of the pigmy owners are always stationed, 

 apparently acting the part of sentinels. That these 

 bees appoint sentinels to guard their gates is a fact of 

 frequent observation. They sedulously watch the out- 

 goings and incomings of their fellows, and seem now 

 and then to be relieved on duty. A relief-guard pre- 

 sents itself every twenty-four hours according to the 

 Mexican cultivators, but no unreasonable doubts may 

 be entertained of such regularity. At all times the 

 sentinel-bee occupies the hole leading to the interior, 

 and as often as a worker desires to enter or to quit the 

 nest, momentarily withdraws within a small cavity on 

 the side of the aperture, resuming its station with sur- 

 prising alacrity whenever the individual has passed in 

 or out. In Jamaica, where probably the species are simi- 

 lar, if not identical with those of Mexico and the Brazils, 

 no fewer than three sentinels have been* seen oppos- 

 ing the entry of the black ants that infest forest 

 trees. The assistants stood behind their principal, 

 heads downwards, and, clinging to the upper arch of 

 the entrance, they gazed upwards, steadily scanning 

 several ants within the crevices of the bark, prepared 

 to rush in if the guards remitted their vigilance for one 

 moment. The active ants paced upwards and down- 



