A good part of this thesis deals with insects’ exocrine glands and their secretions, thus a short overview of relevant terms appears to be useful. Exocrine glands by definition secrete onto the body surface or into a duct. They are of ectodermal origin and primarily in contact with the cuticle. A universally accepted classification of insects’ exocrine glands into three types is based on the type of the cells’ connection to the adjacent cuticle (Noirot and Quennedey, 1974, 1991; Quennedey, 1998).
Class 1 gland cells are in direct contact with the cuticle as it is found for common epidermal cells. The secretion of these cells therefore has to pass through the cuticle through small canals. In class 1 gland cells we often find an enlarged surface of the cell membrane, e.g. apical microvilli and basal invaginations. Class 2 gland cells are surrounded by epidermal cells which are covered by cuticle. The secretion is first transferred to the epidermal cells, which are themselves class 1 gland cells. Finally the secretion is delivered through the cuticle. The most derived gland cells are those of class 3 where a gland cell is penetrated by a canal cell with a porous cuticle that is surrounded by microvilli. The combination of canal and microvilli forms the so-called end apparatus and can be seen in the gland cells as an elongated slightly fuzzy area. The canal is in contact with the cuticle of the respective secretion organ or body surface. Frequently the class 3 gland cells are clustered in so-called ‘acini’
with the canals of the gland cells forming a bundle between an acinus and the cuticle [Fig. 1.1]. In this thesis we mostly deal with class 3 cells and also class 1 cells bordering gland reservoirs.
According to their numerous exocrine glands and their highly diverse gland products insects are referred to as “chemists par excellence” (Blum, 1985, p.536) or “walking glandular batteries” (Billen, 1991, p.67). Some examples for the functions of exocrine gland secretions are intra- and interspecific
Fig. 1.1. Class 3 gland cell units (acini) of mandibular gland of male P. triangulum. (A) SEM micrograph. (B) 3D reconstruction based on semithin sections (by Nathalie Czech). c3 – class 3 gland cells; cc – conducting canals; cd – collecting duct; nu – nucleus. scale bars = 20 µm
cd cc nu
c3
c3 nu
A B
cd cc
c3
c3
(e.g. Terra, 1990; Swart and Felgenhauer, 2003) as well as host detection (Isidoro et al., 1996; Bin et al., 1999), wax secretion (e.g. Gullan and Kosztarab, 1997; Muller and Hepburn, 1992), silk production (Sehnal and Akai, 1990; Fisher and Robertson, 1999) or the feeding of bacterial symbionts (Currie et al., 2006). In particular social hymenoptera (ants, bees, wasps) show a vast variety of exocrine glands and individuals can bear 20 or more different types located in all body parts from antennae to tarsal segments (e.g. Jackson and Morgan, 1993; Jeanne, 1996; Billen and Morgan, 1998).
Exocrine glands are frequently denominated according to the location of the gland itself (e.g. antennal, postpharyngeal gland (PPG)) or its openings (mandibular gland (MG)). The next paragraphs provide a short overview of the occurrence of the hymenopteran glands that are dealt with in this thesis and their already known functions. These are in particular two male cephalic glands (mandibular gland and postpharyngeal gland), female postpharyngeal glands and antennal glands. The results presented in this thesis refer to the respective glands and functions in our model species, the European beewolf and its congenerics.
1.1.1 Postpharyngeal glands and nestmate recognition
It is crucial for social insects to distinguish between nest-mates and foreigners which may threaten brood and food storages inside the nests. As within the dark nests visual cues are of limited use – most termites are blind anyway – social insects rely on olfaction for nestmate-recognition (e.g. Gamboa et al., 1986; Breed, 1998; Vander Meer and Morel, 1998). The nest-specific odour which identifies all members of a colony is composed of complex blends of more or less volatile hydrocarbons (HC) on the insects’ cuticle (e.g. Hölldobler and Carlin, 1987; Smith and Breed, 1995; Singer, 1998; Dani et al., 2001).
In ants (Formicidae) the formation and dispersion of the nest-specific ‘Gestalt’ odour is at least partly accomplished by the postpharyngeal gland (PPG) (e.g. Crozier and Dix, 1979; Hefetz et al., 1992;
Oldham et al., 1999; Boulay et al., 2004). Each ant takes up HCs from the cuticle of nestmates during allogrooming, mixes them in the PPG with HCs sequestered from its own hemolymph and again delivers the PPG content to other colony members (Hefetz et al., 1992, Soroker et al., 1994, 1995a,b, 1998; Vienne et al., 1995; Lenoir et al., 2001; Soroker and Hefetz, 2000). Hitherto the PPG had been described only in ants and authors referred to it as an idiosyncratic organ of Formicidae that evolved in response to the requirements of eusociality (Crozier and Dix, 1979; Billen, 1990; Hölldobler and Wilson, 1990; Lenoir et al., 1999; Eelen et al., 2006).
Recently, female European beewolves (Hymenoptera, Crabronidae) have been reported to use the secretions of a large cephalic gland to coat their paralyzed honey bee prey (Strohm and Linsenmair, 1995; Herzner and Strohm, 2007; Herzner et al., 2007). This female beewolf gland was referred to as PPG the first time in 2001 (Strohm and Linsenmair, 2001). However only a detailed morphological investigation of this putative PPG could confirm this assumption and provide a basis for phylogenetic considerations.
1.1.2 Male cephalic glands and pheromones
The exchange of information between conspecifics via volatile chemicals, so-called pheromones, is found in all taxa of insects (e.g. Blum and Brand, 1972; Tillmann et al., 1999). Basically pheromones might be involved in aggregation, dispersal, alarm and sexual behaviour (Shorey, 1973; Ayasse, 2001). Many pheromones are efficient over long distances, inconspicuous to most predators and could contain valuable information about the sender (e.g. Herzner et al., 2006; Kaltenpoth and Strohm, 2006;
Kaltenpoth et al., 2007). The origin of pheromones is even more diverse as their function and the respective exocrine glands could be located throughout the insects’ body (e.g. Landolt and Akre, 1979;
Attygalle and Morgan, 1984; Downing, 1991; Jackson and Morgan, 1993; Jeanne, 1996; Billen and Morgan, 1998).
In aculeate Hymenoptera males frequently use the secretions of mandibular glands to scent mark their territories (Apidae: Cane et al., 1983; Cane and Michener, 1983; Hefetz, 1983; Vinson et al., 1982;
Gracioli et al., 2004; Vespidae: Wenzel, 1987; Crabronidae: Evans and O’Neill, 1988). Male digger wasps of the subfamily Philanthinae (Hymenoptera, Crabronidae) use pheromones produced in their mandibular glands (MG) to attract females and to mark their territories (e.g. Evans and O’Neill, 1988;
McDaniel et al., 1987, 1992; Clarke et al., 2001; Schmitt et al., 2003; Kroiss et al., 2006; Kaltenpoth et al. 2007). A common morphological feature of males in the subfamily Philanthinae is a clypeal brush which is used to dispense the mandibular gland secretion onto surfaces inside their territories (Evans and O’Neill, 1988; Alexander, 1992). An exception is the non-territorial species P. albopilosus where males have been reported to possess only reduced mandibular glands and lack a clypeal brush (Evans and O’Neill, 1988; unpubl. data). The putative contents of mandibular glands in the subfamily Philanthinae were analyzed in a number of species (P. triangulum: Kaltenpoth and Strohm, 2006;
Kroiss et al., 2006; Schmitt et al., 2003; Schmidt et al., 1990; Borg-Karlson and Tengö, 1980; P.
basilaris/ bicinctus: McDaniel et al., 1987; Schmidt et al., 1985: P. crabroniformis/ barbatus/ pulcher:
McDaniel et al., 1992; Eucerceris conata/ montana/ rubripes/ tricolor: Clarke et al., 2001). However, knowledge about the morphology of mandibular glands in Philanthinae is only fragmentary (Ågren, 1977; Gwynne, 1978; Evans and O’Neill, 1988).
(1) the contents of the new gland are used for scent marking and that (2) the gland is a PPG (Kroiss et al., 2006). Detailed morphological investigations should reveal whether assumption (2) is true and how the MG and the putative PPG are involved in the process of pheromone production and storage.
1.1.3 Antennal glands in Hymenoptera
There is only a small number of descriptions of antennal glands in the order Hymenoptera, most of them were found in males where they play a role in male courtship and mating behaviour (Bin and Vinson, 1986; Isidoro and Bin, 1995; Isidoro et al., 1996, 1999, 2000; Felicioli et al., 1998; Bin et al, 1999; Guerrieri et al., 2001; Battaglia et al., 2002; Romani et al., 2003, 2005). In species with male antennal glands the mating behaviour frequently involves rapid antennal movement (antennation) and physical contact between antennae of both sexes whereby the antennal gland secretion is most likely spread onto the female antennal receptors (Felicioli et al., 1998; Isidoro et al., 1999; Romani et al., 2003, 2005).
In Hymenoptera female antennal glands have been found in the parasitoid Trissolcus basalis (Scelionidae) where the secretions are likely involved in host recognition by dissolving kairomones from the host eggs (Isidoro et al., 1996; Bin et al, 1999). In aculeate Hymenoptera female antennal glands have been described in queens and workers of four ant species (Formicidae) where the function is unclear (Isidoro et al., 2000; Romani et al., 2006). All the antennal glands of Hymenoptera described so far consist of only small aggregations of either class 1 or class 3 gland cells (according to Noirot and Quennedey, 1974, 1991) secreting directly onto the outer antennomere cuticle without conspicuous reservoirs or other modifications of the antennal morphology.
Already in the 1960s Rathmayer discovered that female beewolves exhibit antennal glands with unusual morphology, but his results had been unregarded until 1995 the secretion of these antennal glands was proofed to provide directional information for the cocoon alignment of beewolf larvae (Strohm, 1995; Strohm and Linsenmair, 1995). As a part of this thesis it turned out in 2005 that the beewolf glands in fact contain filamentous structures which resemble bacteria.