46 Bulletin American Museum of Natural History. [Vol. XXIII^ 



eral consecutive days. Pieces of various ant-larv.ie, beetle-larvse, Liiliohius, 

 Scutigera, Oniscus, — all were served up to the ant-larv?e and partaken of 

 with great relish by the Dipteron larvte as well. There could be no doubt 

 that the latter were true commensals, — perhaps the most perfect commen- 

 sals, in the original meaning of the term, to be found in the whole animal 

 kingdom ! 



As one of the smallest Pachycondyla larvee, scarcely one-fourth grown^ 

 bore a very small Dipteron larva, it is, perhaps, safe to say that the ant-larva 

 acquires its commensal at a very early age. The two then grow up together, 

 so that there is always a certain relation between the two kinds of larvae — 

 large Pachycondyla larvae bearing large commensals, and vice versd. The 

 worker ants lick and cleanse the commensals at the same time that they are 

 caring for their own larvae. This is usually done after meals. Since, 

 during this operation of cleansing, the ants spend no more time over the 

 commensals than they do on a similar area of the body surface of their own 

 larvae, it would seem that they are not even aware of the existence of the 

 commensals. To these nearly blind ants, which must rely almost exclu- 

 sively on their senses of smell and touch, the larvae bearing commensals, 

 if distinguished at all from individuals without these satellites, would prob- 

 ably be perceived merely as having unusually protruding necks. But 

 there is nothing to indicate that these insects are really capable of perceiving 

 such differences in their environment. 



On the 5th of November I obtained satisfactory evidence that the Meto- 

 pina larva is not obliged to remain always with the same Pachycomlyla 

 larva. During the night one of the large larvae had moved and attached 

 itself to the first abdominal segment of an ant larva which already bore a 

 commensal around its metathoracic segment. The two larvae were oriented 

 in opposite directions, i. e., wuth their heads reaching around opposite sides 

 of their host. Subsequently one of these commensals moved to an unoccu- 

 pied Pachycondyla larva. I was not present w^hen the change occurred, 

 nor was I able to determine whether it was the originally stationary or the 

 adventitious larva that moved. Although this observation makes it certain 

 that the Metopina larvae can shift their position from one host to another, 

 I am convinced, nevertheless, that they must do this with great reluctance 

 and only under urgent circumstances, such as extreme hunger, the death 

 of the larva to which they are attached, or, perhaps, when fully mature and 

 about to pupate. 



As the days passed, the mature ant-larvae spun their brown cocoons 

 (PI. V, Fig. 69 u) one by one, and one by one the mature commensals dis- 

 appeared. Did they also pupate and for this purpose conceal themselves 

 in the soil of the nest? Or had the ants at last detected the villains and 



