Dependence of Pad Contamination on Wax Micromorphology

Analyzing the relationship between the contaminating ability and geometrical parameters of wax crystals, we found that the contamination positively correlated with both the largest dimension and the largest aspect ratio of crystals. Thus, larger, relatively longer, and/or relatively thinner wax structures should adhere to insect pads more easily than smaller and shorter ones. We suppose that large crystals with high aspect ratio are much more brittle and erodible compared with compact ones and can break readily into tiny pieces during contact formation (Figure 7.8B vs. 7.8E). These hydrophobic pieces can attach easily to the setal surface covered with hydrophobic pad secretions (Figure 7.8F).

Our data show the relationship between the degree of contamination and the type of wax projections (see Table 7.1). Thus, the presence of the tubulelike crystals led to almost clean or weakly contaminated insect pads, with the only exception being the very short tubules in P. domestica that nevertheless cause relatively strong contamination. After having contact with the substrates, covered with wax rodlets, pads were strongly contaminated. In the cases of membraneous and irregular platelets, strong or very strong contamination was recorded.

The crystals or at least their contours were clearly observed on the contaminated insect surfaces only in the case of platelets (except A. vera); they were not recognizable for other wax crystals such as tubules and rodlets. The homogeneous structure of the wax contamination and the absence of the crystals in the contaminating

Approach

Approach

Low aspect ratio

Contact

Low aspect ratio

Contact

High aspect ratio

High aspect ratio

Contact

Contact

F

FIGURE 7.8 Diagram explaining the effect of the aspect ratio of wax crystals on the degree of pad contamination. (A) to (C) Low aspect ratio. (D) to (F) High aspect ratio.

material may be explained by the wax dissolving in the secretory fluid produced by tenent setae (wax-dissolving hypothesis). It has been previously found that insect pads contain oily substances [36-39]. Since only very nonpolar solvents, such as benzene, hexane, and chloroform, have been previously reported to be able to dissolve epicuticular waxes of most plants [40], experiments focusing on the dissolution of various plant waxes in the pad secretion or its oily components should be performed to prove the wax-dissolving hypothesis.

It can be assumed that not only the type of the wax projection and the geometry of crystals, but also other characteristics of the wax coverage, may influence the pad contamination. When the plant surface is not uniformly covered with the crystalline wax and there are gaps free of crystals, weak contamination of insect adhesive pads is expected. However, this effect was observed only in B. vulgaris, whereas in most other cases, the contamination appeared to be strong (T. montanum and A. vulgaris) or even very strong (V vinifera and L. serriola). On the contrary, when the wax bloom is very dense and consists of several layers of crystals, relatively strong contamination can be expected. Since crystals from an upper layer of the composite wax system presumably detach more easily than those in direct contact with the subjacent surface, composite wax systems should lead to stronger contamination. Such an effect has been previously considered for the upper layer of wax platelets in pitchers of carnivorous plants from the genus Nepenthes to explain the antiadhe-sive properties of the waxy zone [5,20]. The compact network of crystals, especially when they have a high aspect ratio, should also cause a strong contaminating effect.

The delicate mechanical removal (by brushing) of threadlike crystals composing dense networks, and strong contamination of adhesive pads in the fly Calliphora vomitoria by these waxes have been previously described in carnivorous plants Brocchinia reducta and Catopsis berteroniana [23]. In our study, ribbonlike terete rodlets of A. negundo forming relatively compact networks showed a similar contaminating effect. Moreover, this wax can disable the attachment ability of beetles for some time [28].

Thus, our experiments with C. fastuosa beetles and various pruinose plant substrates differing in wax morphology showed that wax crystals in many plant species are easily detachable structures that can readily adhere to the surface of attachment organs in insects and contaminate them. The presence of recognizable crystals in the contaminating material found for some waxes provides additional evidence for the contamination hypothesis. The homogeneous contamination may be explained by wax dissolution in the pad secretion. To prove the wax-dissolving hypothesis, further experiments should be carried out. In a few plant species, waxes showed no contaminating effect; however, a decrease in the ability of insects to attach on these substrates has been previously recorded [28]. It may be assumed that in these cases other mechanisms, such as the reduction of the contact area or the absorption of the pad secretion by crystalline wax coverage, are involved in the adhesion force reduction.

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