30 I THE ARGENTINE ANT. 



EESEMBLANCE TO OTHER ANTS. 



There is little difficulty in distinguishing IriAomyrmex humilis 

 Mayr from its nearest American relative, Iridomyrmex analis Ern. 

 Andre. The latter species is quite common in cotton fields and other 

 situations in the South, is much lighter in color than humilis, and 

 possesses a very disagreeable odor which is entirely lacking in the 

 case of Jiumilis. The clearly marked trails of the Argentine workers, 

 when on their foraging expeditions or when moving from place to 

 place, have no counterpart in the case of analis, the workers of which 

 in large measure forage independently of each jother. I. analis 

 constructs inverted cone-shaped mounds or craters on the surface 

 above the underground nests, while what little dirt is excavated by 

 humilis is scattered about the entrance to the nest in promiscuous 

 fashion, the ants evidently desiring to rid themselves of the exca;- 

 vated pellets as expeditiously and conveniently as possible. The 

 "wet- weather sheds" of the Argentine ant, constructed only during 

 or just after prolonged rainy spells, bear no resemblance whatever 

 to the craters of analis; but on the contrary are more or less flat, 

 composed of fine particles of earth, unstable in structure and supported 

 by grass or leaves. 



However, the superficial resemblance of /. humilis to several spe- 

 cies of other genera is even closer than to I. analis and is sufficient 

 to make positive identification of humilis well-nigh impossible ex- 

 cept by one skilled in detecting the characters used by myrmecolo- 

 gists for classification. Among the southern forms most likely to 

 be mistaken for 7. humilis, and vice versa, may be mentioned the 

 ''crazy ant" (Prenolepis longicornis Fab.) and Dorymyrmex pyra- 

 micus Roger. The workers of both these species are of practically 

 the same size and color as those of humilis and the workers of all three 

 travel and forage i.i much the same way. Prenolepis is distinguished 

 from I. humilis by its camponotine characters, particularly the shape 

 of the gizzard, by the cloacal orifice being round rather than slit- 

 shaped, and by the presence of stifle, erect hairs upon the body. Dory- 

 myrmex is easily distinguished by the conical or pointed elevation 

 upon the epinotum (last dorsal segment of the thorax), a structure 

 that is entirely lacking in Iridomyrmex, the epinotum of which is 

 evenly convex. 



The resemblance of 7. humilis to still other species is sufficient to 

 be confusing at times, but one can, by a process of eliminating certain 

 easily observed characteristics, determine with reasonable probability 

 whether a colony of living ants belongs to this species or not. First 

 to be noticed is the size of the ants under suspicion. The workers of 

 the Argentine ant are from 2.2 to 2.6 mm. in length, the largest indi- 



