1885.] Entomology. 305 
Dr. 
ground, where it divides into two branches, fifty-seven and a-half 
inches in circumference.” 
ENTOMOLOGY. 
THE FLIGHT OF THE ROBBER FLIES DURING ConNECTION.—Both 
fly vigorously though their heads are in opposite directions, the 
female, by her greater size, controlling the course. The difference 
in pitch of the two sets of wings is very striking —¥. E. Todd. 
Notes on THE Mounps OF THE OcciDENT ANT.—The mounds 
built by the Pogonomyrmex occidentalis can hardly fail to attract 
the attention of any observing traveler. 
neatly smoothed bare spot, from three to five feet in diame- 
ter is found in the grass and in its center is a mound of quite sym- 
metrical form. This mound is frequently covered entirely with 
Coarse sand ,or fine gravel, even when the surrounding surface 
shows little of this material. ; 
It would seem, at first, that the ants, guided by some instinct, 
selected the material which would best secure their structure from 
the wind and the rain. 
During the past season the writer has found opportunity to test 
this point. A little watching showed that the excavation below 
the surface and the building of the mound are carried on by two 
different sets of workers, one continually bringing out particles 
from within and dropping them upon the court or bare space 
around the mound, the other picking up particles from the surface 
of the court and depositing them upon the mound. The coarse- 
ness of the material seems to'be determined by the desire of a 
worker to find as large a particle as he can conveniently carry. 
For instance, when, as in the case observed, the crust of the 
ground was broken by the tramping of a horse, medium sized 
pieces were deposited upon a mound, but none of the smallest and 
_. Of course none of the largest, as ants have not learned to wor 
otherwise than individually. When particles of broken glass were 
scattered upon a court they were quickly found and in spite of 
their angular and polished surface were soon carried and dropped 
upon the mound. The coarseness and uniformity of the material, 
VOL, XIX.—NO, II, 20 
