Cooperation in insect societies
Introduction
Like all other social insects, ants live in social groups. This has lead to the evolution of several unusual adaptations of which a highly sophisticated chemical communication system is one. Very much like the cells within an organism, individual worker units signal various types of information so that a colony can often seem just as integrated as an individual organism. In social insects, pheromonal signals indeed often serve social functions like alarming colony members for predators, recognising colony members or signalling food availability (cooperative hunting involving trail formation or recruitment to newly discovered food sources). Of course, all the usual pheromones seen in solitary insects are also present, like those used to attract mates (sex pheromones).
Research on cooperation in insect societies mainly involves study of this elaborate pheromonal communication system. Such studies generally involve three steps: (1) identification of a putative pheromone secreting gland (morphological research), (2) study of the chemical composition of the secreted pheromones (gas chromatography-mass spectroscopy) and (3) performing bio-assays with glandular extracts or artificially synthesized pheromones to establish its function. Currently, study of cooperation and chemical communication are the main research topics of Johan Billen and Eric Schoeters.
Morphological research
Research aims to study the structural organisation of pheromone secreting glands using light microscopy and transmission and scanning electron microscopy. This can often yield useful hints regarding their function. For ultrastructural studies we are in the possession of a modern Zeiss EM900 transmission electron micropscope; for scanning electron microscopy we use the facilities of the 'Instituut voor Natuurwetenschappen' (KBIN, Brussels).
Presence or absence of particular glands can also often provide useful phylogenetic information (e.g. Billen 1990, Wenseleers et al. 1998). In this respect, study of the ontogenetic development of each gland can often settle issues of homology. Currently, over 70 distinct exocrine glands can be distinguished in the social insects (at least 45 in ants, 21 in bees, 14 in wasps and 11 in termites), several of which were discovered in this laboratory. The picture on the right shows some common exocrine glands that occur in ants.
The exocrine system of ants.
Image analysis - 3D reconstruction
Along different lines of research, morphological research is applied to study the detailed structure of the reproductive system of ants and to study the occurrence of pathogens that disrupt their reproduction. Image analysis and 3D reconstruction techniques can be used to gain deeper insight into the spatial relationship among morphological structures. Below is an example involving three dimensional reconstruction of the structure of the spermatheca of an ant queen based on serial sections. This work is part of a collaboration between J. Billen, B. Gobin , C. Peeters (Jussieu) and F. Ito (Takamatsu).
Three dimensional reconstruction of the spermatheca of an Odontomachus ant queen. Many social insects are unusual in having extremely long lifespans (often up to twenty years). During this extremely long period sperm is stored and kept alive in the queen's spermatheca. What adaptations this requires are currently under study.
In red: spermatheca, magenta: spermathecal glands, cyan: ducts releasing sperm and spermathecal gland product, green: oviduct.
(Serial sections were converted into a 3D model using SURFdriver and ray-tracing rendered in the Asymetrix 3D F/X package.)
Chemical research
Pheromones used in social insect communication are usually highly volatile, and their chemistry can be analysed using gas chromatography and mass spectroscopy. This type of research is done in collaboration with external laboratories, mostly Keele (E. D. Morgan and G. Jones) and Bayreuth (M. Kaib). Main attention has been directed towards trail- and alarm pheromones (mandibular gland, poison gland, Dufour's gland) and pheromones used in colony recognition (postpharyngeal gland and cuticular hydrocarbons).
Joint studies with Prof. E. D. Morgan from Keele University (U.K.), have for example lead to the discovery of both glandular source and chemistry of trail pheromones of the aenictine army ants (see fig. below).
Discovery of the trail pheromone of the african army ant (Aenictus). Aenictus army ants form massive trails in the search for food (left). The right figure shows a gas chromatogram of the contents of the postpygidial gland of six workers. Coupled mass spectroscopy (GC-MS) has shown that the two peaks correspond to methyl anthranilate (major peak on the right) and methyl nicotinate (minor peak on the left). Behavioural experiments have then established that these two chemicals are indeed the active compounds used in trail following. Left figure: reproduced with permission from D. Agosti's website
Right figure: reproduced from Oldham, Morgan, Gobin & Billen (1994)
Behavioural physiology
The final step in every chemical or morphological study of a particular exocrine gland is to do bio-assays to determine the effect of either whole gland extracts or artificially synthesized pheromones. This approach has been used for example to show that trail following in Aenictus army ants involves the secretion of two components, methyl anthranilate and methyl nicotinate, and that both components are necessary in eliciting a behavioural trail following response (see above).
References
Billen J. (1990) - Phylogenetic aspects of exocrine gland development in the Formicidae. In : Social Insects and the Environment (G.K. Veeresh, B. Mallik & C.A. Viraktamath, Eds.), Oxford & IBH Publishing Co, New Delhi, p. 317-318.Billen, J. & B. Gobin (1996) - Trail following in army ants (Hymenoptera, Formicidae). Neth. J. Zool., 46 : 272-280.
Billen J. & Morgan E.D. (1998) - Pheromone communication in social insects - sources and secretions. In : Pheromone Communication in Social Insects : Ants, Wasps, Bees, and Termites (R.K. Vander Meer, M.D. Breed, K.E. Espelie & M.L. Winston, Eds.), Westview Press, Boulder, Oxford, pp. 3-33.
Oldham N.J., Billen J. & Morgan E.D. (1994) - On the similarity of the Dufour gland secretion and the cuticular hydrocarbons of some bumblebees. Physiol. Entomol., 19 : 115-123.
Oldham N.J., Morgan E.D., Gobin B. & Billen J. (1994) - First identification of a trail pheromone of an army ant (Aenictus species). Experientia, 50 : 763-765.
Soroker V., Hefetz A., Cojocaro M., Billen J., Franke S. & Francke W. (1995) - Structural and chemical ontogeny of the postpharyngeal gland in the desert ant Cataglyphis niger. Physiol. Entomol., 20 : 323-329.
Wenseleers T., Schoeters E., Billen J. & Wehner R. (1998) - Distribution and comparative morphology of the cloacal gland in ants (Hymenoptera : Formicidae). Int. J. Insect Morphol. & Embryol., 27 : 121-12.
Copyright 1999, Katholieke Universiteit Leuven
Information provider: Katholieke Universiteit Leuven
Comments for the authors: Diane Allard
Page design: Tom Wenseleers. Last update: 20/12/04
URL: http://www.kuleuven.ac.be/bio/ento/cooper.htm
