1984] 
Holldobler — New exocrine gland 
229 
smaller (with a total of ~ 150 duct openings), but in this species as 
well several small cups frequently form one large cup (Fig. 3). In P. 
lucidus workers the pygidial cup structure is still smaller, with 
approximately 100 duct openings. In this case the cup structure 
consists primarily of single cups (Fig. 4). It is interesting to note, 
however, that in P. lucidus queens the pygidial cup structure is 
considerably larger (~200 duct openings), and many of the cups are 
merged (Fig. 5). 
External morphological studies of P. lucidus males indicate that 
they lack the organ. 
Formica rufibarbis and F. fusca, two common slave species of P. 
rufcscens, as well as two representatives of the slave raiding Formica 
sanguinea group were found not to possess a pygidial gland (Fig. 6). 
Discussion 
Of a total of 17 formicine species investigated (the current study 
and Holldobler and Engel 1978) belonging to 6 genera ( Acanthole - 
pis, Camponotus, Formica, Myrmecocystus, Oecophylla, Polyer- 
gus) only those in Polvcrgus possess the pygidial gland. This 
structure is a complex gland with one group of glandular cells open- 
ing into an intersegmental reservoir, and another group of cells 
being drained into cuticular cups in the 7th abdominal tergite. From 
these features and from its general anatomy I conclude that the 
pygidial gland of Polyergus is not homologous with the pygidial 
gland of other ant subfamilies, but has evolved independently, pos- 
sibly in connection with the highly specialized slave raiding behavior 
of Polyergus . 
Polyergus slave raids are organized by scout ants that deposit 
chemical trails (Talbot 1967). The precise origin of the trail phero- 
mones has not yet been established. In most formicine species secre- 
tions from the rectal bladder serve as trail pheromones (for review 
see Holldobler 1978). These function in many cases primarily as 
orientation cues and are often supplemented by excitement (or 
arousal) signals, which can be chemical or mechanical in nature. 
Recent observations suggest that recruitment to slave raids in 
Polyergus might be based on a similar mechanism. 
Topoff et al. (1984) found that individual scout ants lead the 
raiding party of nestmates to the slave ants’ nest. He observed that 
the ants, although following a chemical trail, tend to “swarm” 
