Diplopoda — Integument

CHAPTER 3 DIPLOPODA — INTEGUMENT BY SLOBODAN E. MAKAROV INTEGUMENT The integument of millipedes protects internal organs and tissues, represents a barrier against pathogens, parasites, predators and water loss, and enables communication with the external world. The integument consists of two distinct parts: cuticle and epidermal layer. The cuticle is secreted by epidermal cells and covers the entire body surface. The cuticle is composed of several layers but most of the mechanical properties are due to the exo- and endocuticle layers. A calcified cuticle is characteristic for all diplopod taxa except penicillates. A characteristic feature of integumental origin found in some millipede orders is a pair of glands in many diplosegments that are known as repugnatory, repugnatorial or defen- sive glands, or ozadenes. When disturbed, diplopods discharge odorous secretions from these glands, indicating a role in chemical defence. These glands may also have additional functions, such as antimicrobial or antifungal protection, and possibly intraspecific communication. The chemistry of millipedes defence secretions represents a source of unique, still largely undiscovered, natural compounds, and an important pool of novel characters for diplopod taxonomy, phylogeny and ecology. Thus, one of the main parts of this chapter reviews data gathered so far in the semiochemistry of the Diplopoda. Cuticle Data on the ultrastructure of the millipede cuticle are limited to the following species: Polyxenus lagurus (Seifert, 1967), Glomeris marginata (Ansenne et al., 1990; Compère et al., 1994, 1996a, b), Polyzonium germanicum (Wegensteiner, 1982), Ommatoiulus sabulosus (Blower, 1951), Ophyiulus pilosus (Thorez et al., 1992), Pachyiulus varius (Carmignani & Zaccone, 1977), Tachypodoiulus niger (Blower, 1951; Hicking, 1979), Aulacobolus excellens, Cingalobolus bugnioni (Rajulu & Krishnan, 1968), Orthoporus ornatus (Walker & Crawford, 1980), Gonoplectus malayus (Shrivastava, 1992), and ‘Spirostreptus’ asthenes (Subramoniam, 1974). © Koninklijke Brill NV, Leiden, 2015 Myriapoda 2 (3): 69-99 70 S. E. MAKAROV The outermost layer of millipede cuticle is the multi-stratified epicuticle including cement, wax, and cuticulin layer, as well as the inner epicuticle (Fig. 3.1A; Thorez et al., 1992; Compère et al., 1996a, b). The epicuticle covers the procuticle, which is subdivided into exocuticle and endocuticle. All layers are transversed by pore canals and dermal gland ducts (Fig. 3.1A, E). According to the dual-function model of arthropod cuticle (Compère & Goffinet, 1992), the upper part of the cuticle (superficial layers and cuticulin layer) in diplopods mainly contributes to cuticular permeability, while the lower part (inner epicuticle and procuticle) is responsible for the mechanical properties of the exoskeleton. The cement layer represents an outer protection consisting of wax-impregnated pro- teins, while the wax layer consists of stabilized lipid compounds (mostly unsaturated) (Compère et al., 1996a) (Fig. 3.1B, C). Both layers probably represent integumental adaptations to a terrestrial mode of life. The cuticulin layer is multi-laminated and is de- posited first during the moult (Fig. 3.1B, C). Between this layer and the fibrous procuticle there is the thickest epicuticular layer, the inner epicuticle (Fig. 3.1B, C). This consists of the lipoprotein matrix surrounding proteinaceous elements and chitin-protein microfibres in peculiar organization with helicoidal twisted arrangement of the exocuticle fibres (Ansenne et al., 1990; Compère et al., 1996a). Microfibres are absent from the inner epicuticle of membranes between tergites. Both exocuticle and endocuticle (Fig. 3.1E, F) have lamellate structure, due to a chitin-protein microfibre arrangement similar to most other arthropods (twisted plywood system) (Thorez et al., 1992). Moreover, Compère and co-workers found great similarity in structure between the epicuticle and procuticle microfibres in G. marginata which have the same origin and morphogenesis (Compère et al., 1996a). In O. pilosus and G. marginata the pore canals form a twisted ribbon-like vertical pathway extending from the epidermis up to the epicuticle (Fig. 3.1D, E) (Ansenne et al., 1990; Thorez et al., 1992). Vertical fibres form a twisted sheath around the canal lumen of the axial pore. Dermal gland ducts run through the whole cuticle and open at the surface. Their diameters are larger in the exocuticle than in the endocuticle. These glands probably secrete cement material (Ansenne et al., 1990). Pore canal originate from epidermal cell process. Their lumen persists as hollow unmineralized tubes in the endocuticle and regress after mineral deposition. In some millipedes cuticular specialization provide tolerance or resistence to submer- sion in water even for few months. Such adaptations have been reported for a few mem- bers of the Polydesmidae, Pyrgodesmidae, Blaniulidae, Julidae, and Siphonotidae (Adis et al., 1998). In Aporodesminus wallacei all tergites are covered with a thick secretion layer called cerotegument. This layer is externally hydrophilous and supported by small secretion pillars which are located between the tubercles of the tergite and the cones of the sternite (Adis et al., 1998). The cerotegument covers a hydrophobic cavity filled with air that enables plastron respiration (see Chapter 6). The cuticle of millipedes, except for penicillates, is mineralized. A few studies showed that Ca and Mg (mainly from carbonates) in the tergites represent approximately 40-70% of the total cuticle dry weight (Thorez et al., 1992; Vohland et al., 2003). In O. pilosus.

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