48 BuUetin Avierican Museuvi of Natural History, [^'ol. XXIII, 



modation of the commensal. I am inclined to believe, however, that the 

 distortion may be produced by the Metopina larva while attaching itself 

 just before pupating to the newly woven and still plastic cocoon. 



Reflection shows that the position of the puparium in the posterior pole 

 of the cocoon, though the reverse of the position of the larval commensal 

 with respect to its larval host, is the only one which can be maintained by 

 the commensal with perfect safety. Like other ants, the Pachycondyla 

 leaves its cocoon through a rent in the anterior pole. This rent is certainly 

 made by the mandibles of the hatching ant, and it is possible that the callow 

 insect may succeed in making its way out of the cocoon without any assistance 

 from the workers. I have hitherto failed, however, to surprise one of these 

 ants in the act of hatching. But even if the obstetrical aid of the workers 

 is necessary, as it is in the more highly specialized Camponotinae, any 

 position for the commensal puparium, except at the posterior pole of the 

 cocoon, might be fatal, for the struggling jaws and legs of the emerging ant 

 and the jaws of the assisting ants would certainly be very liable to cut into 

 so delicate an object attached to the anterior or median walls of the cocoon. 



At this point my first set of observations on the Metopina ended. The 

 Phorid puparia were kept for several weeks in what I supposed to be the 

 proper conditions of warmth and moisture, but to my intense disappointment 

 they failed to hatch. During the autumn and winter frequent and diligent 

 search was made for more of the commensals in all the PacJtijcondyla nests 

 I could find, but in vain. Finally, during the latter part of May of the 

 following year, I discovered in a very different locality two Pachycondyla 

 nests which contained a few Metopina larvae. This discovery proved that 

 the Phorid is double-brooded like its host. But the larvae were very small 

 and attached to such very young ant-larvfe that I despaired of being able 

 to raise them in my artificial nests as far as the imaginal stage. The search 

 for Pachycondyla nests containing Metopina; was again renewed in the fall. 

 Two colonies were found, and from one of these Mr. Brues succeeded in 

 rearing several of the imaginal flies. These measure 2 mm. in length, are 

 black with yellowish brown legs and antennae and clear, hyaline wings. 

 As yet M. pachycondylce is the only known North American species of the 

 genus. 



It is not difficult to imagine the circumstances under which the Metopina 

 hatches and manages to lay its eggs in the same or other Pachycondyla nests. 

 Undoubtedly the ant is the first to hatch and to leave the cocoon. Now 

 very soon after this occurs, the useless cocoon is always carried by a worker 

 and placed on the refuse heap, which in the natural nest is often almost 

 entirely made up of the empty cocoons of from one to several broods of ants, 

 and lies in a rather dry and well- ventilated spot immediately beneath the 



