1907.] Wheeler, The Polymorphism of Ants. 49 



stone covering the nest. Along with the cocoon is carried the Metopina 

 puparium still adhering to the wall at its unopened posterior end. Thus 

 after a privileged existence as free pensioner and bedfellow to a generous 

 host, it is unwittingly carried away in the worn-out bedclothes and consigned 

 to the family rag pile. Here the small and very active Dipteron hatches, 

 leaves by the wide-open front door of the cocoon, and, after mating, either 

 returns to lay a few eggs in the galleries of its former host, or flies away to 

 oviposit in some other Pachycondyla nest. Thus the simple fact that the 

 Metopina hatches later than the ant renders it unnecessary for the fly to 

 possess some peculiar means of perforating the tough wall of the cocoon, 

 and also accounts for the position of the puparium in the posterior pole, 

 where it would be completely concealed from the workers even after the 

 escape of the callow ant. 



In conclusion attention may be directed to certain particulars of special 

 interest in connection with the life history of the Pachycondyla commensal: 

 First, the peculiar habits of the Phorid show clearly that the Ponerine method 

 of feeding the larvae with comminuted insects is not only the typical but the 

 only method employed by these ants, for such a commensal would certainly 

 starve if the Pachycondyla larvge were carefully fed like the larvae of Cam- 

 ponotus and Formica, by regurgitation of liquid from the mouths of the 

 workers. The Phorid profits by a peculiarity in the behavior of its host, 

 and thereby demonstrates — by one of Nature's experiments — that Pachy- 

 condyla harpax does not feed its young by regurgitation. Second, in the 

 peculiar symbiotic relationship existing between the Phorid and the ants, 

 the adaptations are all on the side of the former, whereas the latter pursues 

 its ancient and well-established mode of life uninfluenced by and apparently 

 in complete ignorance of the very existence of its little guests. Even the 

 distortion of the cocoon may be entirely due, as I have suggested, to the 

 activity of the Metopina. These strikingly unilateral adaptations are prob- 

 ably to be explained on the ground that the Phorid is so careful and con- 

 servative of the life and welfare of its host. The small amount of food 

 consumed by the little commensals can hardly be a serious drain on the pro- 

 visioning instincts of the Ponerinse, at least under ordinary conditions. The 

 larvae bearing the commensals were certainly as large and healthy as any 

 others in the nest, and produced perfectly normal pupae, which in the cases 

 observed all lacked the imaginal disks for the wings and were therefore of 

 the worker type. 

 [January, 1907.] 4 



