Mechanisms of Slipperiness

Epicuticular wax crystals are typically formed by pure lipid compounds when they are present as the predominant component in the wax mixture. In Macaranga, the threadlike crystals (Figure 8.2B) are composed of triterpenoids [45]. Plant surfaces covered with wax crystals are mostly slippery for insects (see, e.g., [46-52]). Several hypothetical mechanisms have been proposed to explain this phenomenon.

1. Wax crystals form substrates with a microscopic surface roughness. Surface roughness may effectively prevent attachment of insect tarsi if (a) the roughness height and width are too large for surface minima to be filled completely with adhesive secretion, (b) the roughness width is too small to allow the soft adhesive pad cuticle to deform and make sufficient contact between asperities (which leads to a strongly reduced area of real contact), and (c) the surface texture is too fine to allow the claws to interlock with surface protrusions [42]. These criteria are met for intermediate surface roughnesses in the micron and submicron range, which correspond to the order of magnitude found in natural waxy plant surfaces. Evidence in favor of microscopic roughness reducing insect attachment forces is given by Gorb [41].

2. Wax crystals can be easily detached so that insect tarsi become contaminated when walking on a waxy plant surface [47,48,50,53-55]. Wax crystal particles not only contaminate insect attachment structures, but they can form an amorphous substance on the surface of adhesive pads when partly dissolved by insect adhesive secretions [49,50]. Both forms of contamination impede attachment.

3. The hydrophobic microrough wax crystal substrates may absorb and deplete insect adhesive secretions necessary for capillarity-based adhesion [50].

In the Crematogaster-Macaranga system, both (1) surface roughness and (2) wax crystal detachment and pad contamination contribute to the slipperiness of waxy Macaranga stem surfaces. We measured in-plane detachment forces of Cremato-gaster (Decacrema) msp.2 wax runners on nonexfoliating aluminium oxide substrates of different roughness using the centrifuge method [56] (Figure 8.3). Forces were large on smooth and on coarse rough surfaces, but were minimal on the intermediate microrough substrate (0.05-^m particle size, Figure 8.3B). Second, tarsi of ants freshly collected after locomotion on waxy Macaranga stems were clearly contaminated with wax crystals (Figure 8.4). Similar to the interaction between fly tarsi and wax crystal surfaces of the carnivorous plants Nepenthes ventrata (Nepenthaceae) and Brocchinia reducta (Bromeliaceae) [49], the crystal material appeared to be partly dissolved and amorphous on the surface of the arolia but less so on the claws and other parts of the tarsus (Figure 8.4).

Photoelectric barrier

Video

Video

Photoelectric barrier

Glass M. hypoleuca 0.05 ^m Surface (b)

FIGURE 8.3 (A) Centrifuge method (Federle et al., J. Exp. Biol., 203, 505, 2000) used to measure in-plane attachment forces of ants on artificial substrates and waxy Macaranga stems. (B) Attachment forces of Crematogaster (Decacrema) mspp.2 and 4 ants on substrates of different surface roughness. For each ant, the mean of three consecutive force measurements was used (n = 16 to 41 ants per species and substrate). The labels "0.05 ^m" and "12 ^m" on the x axis denote rough aluminium oxide substrates with the given particle sizes.

FIGURE 8.4 Tarsus of Crematogaster (Decacrema) msp.4 after climbing on a vertical waxy M. hypoleuca stem for 5-8 min. The ants were anaesthetized by cooling at 4°C and prepared for scanning electron microscopy (SEM) by freezing and drying overnight in a lyophilizer. (A) Lateral view of the tarsus. (B) Frontal view of the pretarsus. Note the threadlike wax crystal material typical of Macaranga (Figure 8.2B) on the claws and on the hairs at the ventral side of the tarsus. In contrast, the crystal particles are apparently dissolved by adhesive secretion on the arolium surface. Scale bars: 50 ^m.

FIGURE 8.4 Tarsus of Crematogaster (Decacrema) msp.4 after climbing on a vertical waxy M. hypoleuca stem for 5-8 min. The ants were anaesthetized by cooling at 4°C and prepared for scanning electron microscopy (SEM) by freezing and drying overnight in a lyophilizer. (A) Lateral view of the tarsus. (B) Frontal view of the pretarsus. Note the threadlike wax crystal material typical of Macaranga (Figure 8.2B) on the claws and on the hairs at the ventral side of the tarsus. In contrast, the crystal particles are apparently dissolved by adhesive secretion on the arolium surface. Scale bars: 50 ^m.

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