Adhesive Exocrine Glands in Insects: Morphology, Ultrastructure, and Adhesive Secretion

Abstract

A literature survey is provided summarizing the available information on exocrine epidermal glands that produce adhesive secretions in insects. The focus is on both the ultrastructure of the gland cells and the identity and function of the chemical secretion produced by them. Insects employ adhesives for various functions such as tarsal attachment during locomotion, resisting external detachment forces, mating, phoresy and parasitism, egg anchorage, retreat building, self-grooming, prey capture, and active and passive defence. The available studies on the ultrastructure and the secretion of adhesive insect glands cover a broad spectrum of developmental stages and higher taxa, i.e., the Elliplura, the Ephemeroptera, the Polyneoptera, the Acercaria, the Coleoptera, the Amphiesmenoptera, the Hymenoptera, and the Diptera (Table 8.1). Based on this diversity of biological contexts and systematic groups, adhesive structures are found at various tagmata of the body, mainly at the head, the abdomen, and the legs, but also within the thorax in the form of the metapleural glands of ants. Class 1 epidermal cells are the predominant glandular cell type among the adhesive gland systems in insects. With respect to their ultrastructure, the adhesive class 1 cells show features (in terms of their provision with endoplasmic reticulum, Golgi system, free ribosomes, and secretion vesicles and granules) that are either indicative of predominant non-proteinaceous (lipid) or protein secretion. In class 1 cells that are employed in locomotion (i.e., reversible tarsal adhesion to natural substrates such as plant surfaces), lipoidal secretion seems to prevail (although these secretions often appear to be complex mixtures of lipids with proteins and carbohydrates), whereas in the contexts of more permanent body or egg anchorage and of retreat building, protein-based secretion dominates. Oenocyte-like class 2 adhesive gland cells have hitherto only been found in the defence systems of Aphidoidea and Tingidae (both Hemiptera). Adhesive class 3 glands are almost always bicellular, consisting of a terminal secretorily active cell and an adjacent canal cell that surrounds the cuticular conducting duct. The constituents found in insect adhesives belong to aliphatic compounds, to carbohydrates, to phenols, to isoprenoids, to heterocyclic compounds, and to amino acids, peptides, and proteins. Insect adhesives do not consist of one compound only but are highly complex (often emulsion-like) structural and chemical mixtures whose chemical and micromechanical functions are often poorly understood. The possible functional aspects of such mixtures include (1) the polar and nonpolar interactions of aliphatic compounds with the substratum, (2) the in situ differentiation of alkanes and alkenes at ambient temperatures forming colloid suspensions of solid wax crystals within a liquid matrix, (3) the non- Newtonian rheological behavior of colloid- and emulsion-like adhesive fluids, (4) the lipoid shields that prevent the aqueous fraction of an adhesive from desiccation and sticking to the walls of the outlet ductule, (5) those hydrocarbon properties (e.g., chain length and degree of unsaturation) that are decisive for the adhesive performance, (6) the rapid hardening of triglyceride- based adhesives caused by processes other than polymerization, (7) the water attractance of the large carbohydrate component of glycoproteins, (8) the quinone tanning induced by protein-polymerizing quinones, (9) the increased wetting properties toward lipophilic surfaces caused by monoterpenes combined with dissolved diterpenes that retard the rapid vaporization of the monoterpenes, (10) the production of aqueous lipid- glycoprotein- mucopolysaccharide mixtures, (11) those glues based on hydrophilic proteins (e.g., sericin with high Ser levels) coupling adhesion with high levels of extension and showing extensive hydrogen bonding, ester linkage, and/or ionic linkage, and (12) the proteinaceous underwater glues with their high levels of Cys (forming disulfide bonds) and charged amino acids. Those adhesives that work mechanically might comprise high-molecular compounds containing proteins, terpenes (resins), mixtures of long-chain hydrocarbons and mucopolysaccharides, or waxes. However, defensive adhesive secretions in particular not only function mechanically, but also concomitantly develop a chemical irritant function caused by reactive substances of a low-molecular weight that are mixed within the sticky secretion to produce {\textquotedblleft}toxic glue{\textquotedblright}. Table 8.1: Survey of studies that deal with the histology / ultrastructure of insect epidermal adhesive glands and / or the chemical nature of the adhesive secretion produced by them ( locomotion tarsal adhesion to various surfaces, dashes mark no data available). Nomenclature of epidermal gland cells is according to Noirot and Quennedey (1974, 1991) and Quennedey (1998). The extensive literature on special wax glands of insects (e.g., scale insects) is not considered, although the extensive waxy secretions of certain insects might entangle the sensilla and mouthparts of potential predators and parasitoids. In the column next to the last ({\textquotedblleft}Details on secretion{\textquotedblright}) larger substance classes (as defi ned in Sect 8.4) are separated by semicolons