Host Plant Traits

8.2.4.2.1 Hidden Food Presentation

In many Macaranga host plants devoid of wax crystals, food bodies are presented in a secluded space under the recurved, succulent stipules, whereas waxy hosts offer them openly accessible on the plant surface. Similarly, extrafloral nectaries (EFN) are largely absent in nonwaxy Macaranga hosts but not in many waxy members of the genus [27]. The correlation of open food presentation and wax barriers is supported both by traditional and phylogenetically based comparative methods (independent contrasts [37]). Thus, the hidden food presentation in nonwaxy Macaranga host plants probably represents an adaptation to protect the specific ant partners against competition [27].

8.2.4.2.2 Prostomata

The prostoma, a preformed thin-walled zone of the stem where ants preferably chew their entrance holes, represents a further host plant trait that correlates with the presence and absence of wax barriers. In most Macaranga myrmecophytes devoid of wax crystals, the hollow internodes possess a longitudinal prostoma above the leaf insertion [38]. Many waxy host plants, however, lack prostomata and their domatium walls are uniformly thin on all sides of the internode. Absence of pros-tomata and presence of wax barriers were found to be significantly correlated when Macaranga ant-plant species were treated as independent samples [38]. When the distribution of prostomata is mapped on a phylogenetic tree of Macaranga, however, it can be seen that most prostoma-containing species in the genus section Pachy-stemon may have originated from a single ancestor. Even though prostomata are therefore unlikely to have evolved as an adaptation to the absence of wax barriers, they could function as filters promoting only the specialized Crematogaster (Deca-crema) ants capable of finding the prostoma. Crematogaster (Decacrema) workers recognize the position of the prostoma from inside the domatia by using incident daylight as an orientation cue, a capacity that may be absent in many generalist stem-nesting ants that chew entrance holes into hollow twigs [38].

Exclusion filters such as wax barriers, hidden food presentation, or prostomata have also been reported from other myrmecophytic systems. For example, the round or flattened prostomata in the ant-plant genus Leonardoxa facilitate hole boring for specific ants [39]; long and dense trichomes in a variety of ant-plant genera inhibit the movements of larger ants [40]. The selective effect of all these plant traits is based on more or less specialized behaviors or morphological properties of the ant partners.

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