Bert Hölldobler.
A new exocrine gland in the slave-raiding ant genus Polyergus.
Psyche 91(3-4):225-235, 1984.

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A NEW EXOCRINE GLAND IN THE SLAVE RAIDING ANT GENUS POLYERGUS
Department of Organismic and Evolutionary Biology, MCZ-Laboratories, Harvard University,
Cambridge, Massachusetts
In his classical study of the workers and males of Myrmica rubra, Janet (1898, 1901) discovered a pair of clusters of a few glandular cells under the 6th abdominal tergite. Each cell is drained by a duct that penetrates the intersegmental membrane between the 6th and 7th abdominal tergites. Recent investigations have demonstrated that this gland is very common in ants, but it varies considerably in size and structure (Holldobler and Engel 1978). Since the gland is anatomically closely associated with the last exposed tergite in female ants (7th abdominal tergite = pygidium) Kugler (1978) sug- gested that it be called the pygidial gland. Recently Jessen and Maschwitz (1983) proposed the alternative name, Janet's gland, in honor of its discoverer Charles Janet.
The pygidial gland has been found in workers of representative species belonging to all subfamilies except the Formicinae (for liter- ature review see Holldobler and Engel 1978; Holldobler and Engel- Siege1 1982; Holldobler 1982).
1 report here the first discovery of a pygidial gland in Polyergus, a genus of the subfamily Formicinae. But the anatomy has unusual features that point to a possibly indepedent origin in evolution. Three species of Polyergus were investigated. The European spe- cies Polyergus rufescens was collected near Wtirzburg (W-Ger- many), the two North American species P. breviceps and P. lucidus were found near Portal (Arizona) and near Rocky Point in Suffolk County (New York), respectively. Virgin queens and males were *Manuscript received h,r the editor March 24, 1984,



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available only in the case of P. lucidus. For comparison two com- mon slave species of P. rufescens, Formica rufibarbis and F. fusca (from the Wurzburg area), as well as two species of the slave raiding Formica sanguinea group (collected in Massachusetts), were also studied.
For histological investigations life specimens were fixed in alco- holic Bouin or Carnoy (Romeis 1948), embedded in methylmetha- crylate, and sectioned 6-8 p thick with a D-profile steel knife on a Jung Tetrander I microtome (Rathmayer 1962). The staining was Azan (Heidenhain). The SEM pictures were taken with an AMR 1000 A scanning electron microscope.
Workers and queens of the three Polyergus species investigated possess a large, tergal complex gland. One part of it is located between the 6th and 7th abdominal tergites; the other part belongs to the 7th tergite (Fig. 1). Because of its anatomical location 1 pro- pose tentatively to call this organ pygidial gland, although it differs in several respects from the pygidial glands found in other ant subfamilies.
The pygidial gland of Polyergus consists of a reservoir, formed by an invagination of the intersegmental membrane between 6th and 7th abdominal tergites. Glandular cells (-80 cells in P. rufescens workers) are drained into this reservoir through ducts penetrating the intersegmental membrane. Ducts of a second group of glandular cells open into a series of cuticular cups located along the anterior margin of the 7th abdominal tergite (Fig. 1,2). This striking cuticu- lar structure is usually not visible, because it is covered by the poste- rior portion of the 6th abdominal tergite. It is, however, easily exposed by slightly pulling the two terminal abdominal tergites apart. Although only a few specimens of each species were investi- gated in detail, species-specific differences in the pygidial cup struc- ture are so conspicuous that this organ might be considered a valid taxonomic character.
The cup structure is most strikingly developed in workers of P. rufescens (Fig. 2). In this case several cups are often merged to form one large cup containing 2-5 glandular cell openings. I counted a total of approximately 250 duct openings on the pygidium of P. rufescens workers. In workers of P. breviceps the cup structure is



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19M] HO//i/obler - New exocrine gland 227 Fig. 1. Above:Schemaiical illuMralion ofthe Pdlyrr~w pygidialgland between 6th and 7th abdominal tcrgilc'i. Below: Longitudinal section through the pygidial gland ofa P. rg/e.+<rnv worker. CS; cup structure: CU: cup; GC; glandular cells; R: inteficgrnental reservoir




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228 Psyche [vol. 91
Fig. 2. SEM-photograph ofthe pygidiai cup structure of a P. rufe-nen.~ worker. Above: overview of the pygidium (= 7th abdominal lergite). Below: close-up of the cup structure. Note the duel openings inside the cups,



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19841 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. rufescens, as well as two representatives of the slave raiding Formica sanguinea group were found not to possess a pygidial gland (Fig. 6). Of a total of 17 formicine species investigated (the current study and Holldobler and Engel 1978) belonging to 6 genera (Acanthole- pis, Camponotus, Formica, Myrmecocyst us, Oecophylla, Polver- gus) only those in Polyergus 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"



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230 Psyche [VOI. 91
Fig. 3. SEM-photograph of the pygidial cup structure of a P. hrevireps worker. Above: overview of the pygidium. Below: dose-up of the cup structure.



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19841 HoUdobler - New exocrine gland 23 1 Fig. 4.
SEM-photograph of the pygidial cup structure of a P. twidus worker. Above: overview of the pygidium. Below: clobe-up of cup structure.



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Psyche pol. 91
Fig. 5, SEM-photograph of the pygidia! cup structure of a P. 1widu.s queen. Above: overview of the cup structure at the anterior margin of the 7th abdominal tcrgite. Below: close-up of the cup structure. Arrows point 01 the intersegmental duct openings.




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I 9841 Holldo bler - New exocrine gland 233 Fig. 6.
SEM-photograph of the pygidium of a worker belonging to the Formica sanguinm group. Note: no cup structure on the anterior margin of the 7th abdominal tergiie.
around the leader ant, often moving briefly ahead, but always returning to the leader. Myrmecologists have long noted this pecu- liar looping behavior at the front of the raiding columns of Pofy- ergus (see for example Schmitz 19061, but only the recent studies by Topoff have indicated that this appears to be due to the orgainizing rote and excitement emanating from the leader ant. These observa- tions suggest that the leader discharges not only a longer tasting trail pheromone but also an arousal signal, possibly from the pygidial gland.
Similar recruitment patterns have been observed in Carnponoius socius and Myrmecocystus mimicus, where secretions from the poi- son gland seem to function as arousal signals (HSlldobler 1971, 1981), as well as in Camponotus ephippium, where the arousal sig- nal appears to originate from the ctoacaf gland (Holldobler 1982). However, this hypothesis cannot be applied to the well developed pygidial gland in Polyergus queens. Wasmann (1 9 15) first proposed that a young Polyergus queen, in order to found a new colony, has to intrude a colony of a slave ant species, where she kills the resident



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234 Psyche [vo~. 91
queen and is subsequently adopted as the "replacement" queen by the slave ant workers. This mode of parasitic colony founding was later confirmed in field observations by Gosswald (1932). It is possible that the Polyergus queen achieves her acceptance in the foreign colony by emitting either a supernormal attractant or a disorganizing alarm substance (hence, some form of queen "propa- ganda" substance), possibly discharged from the pygidial gland. In fact, the pygidial gland secretions of Polyergus workers might also function as a kind of propaganda allomone when encountered by slave ants, very similar to the Dufour's gland secretions of the slave raiding species Formica pergandei and Formica subintegra which have a confusing propaganda effect on the raided slave ant colony (Regnier and Wilson 1971).
Further experimental work is required to determine which, if any, of these proposed speculations on the function of the Polyergus pygidial gland is correct.
I thank Hiltrud Engel-Siege1 and Ed Seling for technical assist- ance, and Howard Topoff and Linda Goodloe who generously pro- vided specimens of Polyergus breviceps and P. lucidus. This work was supported by NSF grant NBS 82 19060. GOSSWALD. K.
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H~LLD~BLER, B.
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Ethological aspects of chemical communication in ants. Advances in the Study of Behavior. 8: 75-1 15.
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1 9841 Holldobler - New exocrine gland
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