A recent cladistic analysis based on 36 morphological features argues for a sister-group relationship between these two families of small to minute Neuropterans (Aspock et al., 2001; Aspock, 2002). However, earlier work came to very different conclusions (summarised in New, 1991), and molecular data have so far failed to support the hypothesised relationship (Haring and Aspock, 2004). These two families will be considered together because both include species which communicate acoustically.
Spongilla-flies are small, peculiar Neuropterans whose larvae are subaquatic predators of freshwater sponges. Although constituting only 50 described species, Sisyridae is nonetheless cosmopolitan in distribution (Pupedis, 1980, 1985). Reproductive behaviour has been observed in a handful of species. In North American Climacia areolaris (Hagen), courtship includes horizontal extension and rapid vibration of the wings on one side of the male's body, whereby he "fans" the head of the female and perhaps produces a faint sound (Brown, 1952). In contrast, Holarctic Sisyra nigra (Retzius) (S. fuscata [Fabricius]) foregoes wing fanning in favour of abdominal tremulation (Killington, 1936; Rupprecht, 1995). Here, males and females oscillate their abdomens erratically during courtship at frequencies between 100 and 450 Hz. Additionally, published sonograms show some evidence of percussive drumming during tremulation (Rupprecht, 1995, see figure). Tremulation in spongilla flies does not seem to be organised into discrete volleys of vibration.
The reproductive behaviour of the "dusty-wings" or "wax flies" (Plant, 1991) has been little studied, probably because they are so small. More than 300 species have been described, but courtship and mating have been seen in only seven (Withycombe, 1922; Collyer, 1951; Henry, 1976; Johnson and Morrison, 1979).
In his recent review of the subject, Devetak (1998) mentions "vibratory signals" in Coniopterygidae, but only precopulatory wing fluttering was actually documented in the cited study (Johnson and Morrison, 1979). Johnson and Morrison reported wing fluttering in males and females of the three California species they examined. Furthermore, fluttering appeared to be temporally structured into 1 to 2 sec "calls", delivered intermittently. It is therefore not out of the question that abdominal vibration accompanies or even causes wing fluttering, but was simply overlooked in such tiny insects.
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