1959] Wilson and Pavan — Chemical Releasers in Ants 71 
smears from other body parts proving consistently inactive. This re- 
sult was not unexpected, since humilis workers can be clearly seen 
to drag the posterior portion of the abdomen over the ground while 
in the act of laying trails. 
Five organs in the abdomen are capable of emptying glandular se- 
cretions to the outside: (1) the hind-gut; (2) the paired “true” 
poison glands, with a large reservoir, the poison vesicle; (3) Dufour’s 
gland (also called the accessory gland) ; (4) the anal glands, opening 
just above the anus; (5) the glandular “ventral organ” recently dis- 
covered by Pavan ( 1955). In a series of experiments, the five organs 
were dissected out of freshly killed workers, washed in insect Ringer’s 
solution, and bioassayed with artificial trail tests. The results, sum- 
marized in table 1, show that the bulk, and perhaps the entirety, of 
fhe trail substance resides in the ventral organ. The occasional weak 
Table 1. Results of artificial trail assay of selected abdominal organs of 
Iridomyrmcx humilis workers. 
Organ 
Number 
of tests 
Number of 
positive 
responses* 
Number of workers 
responding : range 
(with mean) 
Duration of re- 
sponses in minutes: 
range (with mean) 
hind gut 
6 
t) 
0-2 (M< 1 ) 
0-1/2 (M< 1/2) 
true poison 
glands, plus 
reservoir 
6 
0 
0-2 (M<1) 
0-1/2 (M< 1/2) 
Dufour’s 
gland 
8 
2 
0-30 (M = 8) 
0-4 (M = l) 
anal gland 
S 
1 
0-15 (M = 6) 
0-1 ( M — 1 ) 
ventral 
organ 
7 
? 
65-339 
(M=134) 
2-9 (M — 5) 
* Positive responses are defined, arbitrarily, as the crossing of the trail mid- 
point by 10 or more workers. 
responses to the contents of the Dufour’s and anal glands may not be 
biologically significant, i.e., may not indicate the presence of sub- 
stances normally secreted into the trail, since responses of comparable 
magnitude have been obtained with pure methylheptenone, a sub- 
stance not known to occur in humilis workers. 
In a second series of experiments, the ventral organ and anal glands 
of Iridomyrmcx pruinosus (Roger) 4 were assayed. As in humilis , 
the ventral organ smears caused strong trail following while re- 
Tollected at Baton Rouge, Louisiana. 
