1 00 Psyche [June 



thorax and petiole, as well as the vertex, occiput and posterior corners of the head 

 and the upper surface of the gaster beset with small, rather blunt tubercles. These 

 are largest on the posterior comers of the head. Antennal scapes and legs covered 

 with smaller and much less distinct tubercles. 



Hairs and pubescence fulvous, the former hooked, uniformly distributed over 

 the body, scapes and legs, suberect on the body, scapes and legs, more reclinate on 

 the legs. Pubescence very short and delicate, confined to the antennal funiculi. 



Body ferruginous red; mandibles, antennae and legs somewhat paler; clypeus 

 darker, mandibular teeth, frontal area and a longitudinal mid-dorsal streak on the 

 gaster, black. 



Described from nine specimens taken Nov. 24, 1910 a few 

 hundred yards from the Carnegie Desert Botanical Laboratory 

 near Tucson, Arizona. 



This species is readily distinguished from all our other North 

 American species of Trachymrmex by its smaller size and the 

 conformation of the pro- and mesonotum. Seen from above this 

 region presents three successive pairs of bluntly angular projec- 

 tions of much the same size and shape, whereas in all our other 

 forms at least the first pair is long, rather slender and pointed and 

 differs considerably either in shape or size from the succeeding 

 pairs. 



T. desertorum was first seen on the banks of a dry arroyo that 

 skirts the rocky hill on which the Desert Botanical Laboratory is 

 situated. Here in the feeble shade of the Parkinsonia and Acacia 

 trees and in the very hard, pebbly, desert soil, two nests were 

 located by following single workers that were returning home laden 

 with caterpillar excrement or with green or yellow Acacia leaflets. 

 These nests were so inconspicuous that they could not have been 

 found in any other way, for the entrance to each was merely a 

 circular opening only l/l6 of an inch in diameter among the 

 pebbles, with a few dead leaves forming a small and very indis- 

 tinct crater around it. The entrance gallery descended perpendicu- 

 larly into the soil. As I did not discover the ants till late in the 

 afternoon and was obliged to leave Tucson the following morning, 

 I was unable to excavate the nests. These are probably similar in 

 structure to those of T. turrifex which I have described in detail in 

 my paper on the North American Attii (loco citato p. 753). 



T. desertorum is one of three fungus-growing ants I have found in 

 the dry arroyo near the Desert Botanical Laboratory. As these 

 insects will be within very convenient reach of the botanist who 



