airborne sound I | abdominal tremulation L..Z.J tremulation & airborne sound putative stridulatory structures

Chrysopidae (after Brooks 1997, Winterton & Brooks 2002, etal.)

FIGURE 10.2 Phylogenetic hypothesis for the relationships of the subfamilies and tribes of Chrysopidae, including the major genera of tribe Chrysopini. Taxa with some type of acoustic communication are indicated. Relationships within Chrysopini are those hypothesised by Dr. Stephen Brooks (personal communication, July 2004; see Acknowledgments).

sexes in each), Brinckochrysa Tjeder (all 13 species, both sexes), and Chrysocerca Weele (one of five species, males only). He suggested independent evolutionary origins for these structures in each of the three genera, based upon fundamental differences in their anatomy and placement. That conclusion is also supported by phylogeny (Figure 10.2) — the three genera are not particularly close relatives and they do not trace their origin back to a more distant common ancestor possessing a "stridulatory" apparatus.

Circumstantial evidence for stridulation in green lacewings is compelling, even though sounds have not yet been recorded. Based on morphology, such signals are likely to be faint, of relatively high frequency and probably sexually dimorphic because the femoral pegs are usually larger in males than females and the form of the abdominal striae differs between the sexes (Brooks, 1987). The presence of tremulation in several close relatives of "stridulating" genera is also significant because abdominal vibration could have provided the initial behavioural basis for stridulation (Adams, 1962).

A stridulatory function has also been ascribed to interacting patches of microtrichia, located at the point where the folded forewings contact the metanotum (Riek, 1967; Eichele and Villiger, 1974). However, these "organs" are widely distributed in other Neuropteran families, Trichoptera and micro-Lepidoptera, and are more likely to function in wing positioning than sound production (Henry, 1979; Brooks, 1987).

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