1959 ] 
Wilson and Pavan — Chemical Releasers in Ants 
73 
Dufour’s gland caused a weaker response in S. getnmata , well below 
the intensity caused by a geminata gland and approximately the same 
as that caused by a saevissima gland (see Wilson, i960). These 
results are interpreted as indicating the chemical identity, or near- 
identity, of the Monacis bispinosa and Solenopsis saievissima Dufour’s 
secretions. Curiously, the Solenopsis use the secretion as a trail sub- 
stance, but the Monacis do not. The function of the secretion in 
Monacis remains unknown. 
Some further comment is needed concerning the organ we are here 
referring to as the ventral organ. The gland was discovered in 
Iridomyrmex humilis by Pavan ( 1955) and described in greater detail 
by Pavan and Ronchetti (1955). It is a paddle-shaped organ resting 
on the ventral body wall just over abdominal sternite VI (sternite 
IV of gaster) . It opens posteriorly via a neck-like constriction between 
the VI and VII abdominal sternites. The opening is fitted with an 
unusual beak-shaped deformation of the posterior border of sternite 
VI, a structure seemingly designed to guide the effluence of the gland 
contents. Until the present experimental work, the function of the 
gland remained unknown, although Pavan and his associates suggested 
that it produced either the trail substance or a secondary defensive 
secretion. Additional morphological studies by Miradoli Zatti and 
Pavan (1957) disclosed the presence of a closely similar gland in the 
primitive dolichoderine genera Aneuretus and Leptomyrmex. In the 
course of subsequent work we have noted its presence in Monacis 
bispinosa and Hypoclinea doriae Emery as well. The ventral organ 
appears to be peculiar to the Dolichoderinae and Aneuretinae and 
hence can serve as a valuable diagnostic character for these two closely 
allied groups. If it serves as the source of the trail substance in all of 
the dolichoderine tribes (and in the ancestral aneuretines) , as the 
preliminary evidence suggests, it can be interpreted as a remarkable 
example of a “social organ” evolved de novo with the specific func- 
tion of mediating worker communication. 
Alarm Substances 
When colonies of Tapinoma sessile , Liometopu?n occidental , and 
Monacis bispinosa are disturbed by opening the nest, workers release 
considerable quantities of secretions from their anal glands which can 
be easily smelled by the human observer. When the anal glands are 
removed from freshly killed workers of these species and their con- 
tents released in the presence of groups of resting live workers, the 
latter are thrown instantly into a typical alarm frenzy, indistinguish- 
