Tremulation is found in all four families of Megaloptera and Raphidioptera (Figure 10.1). In every case, it includes "bouts" or volleys of abdominal vibrations which initiate and accompany courtship, sometimes in both sexes. These bouts may be accompanied by fluttering of the wings, which can produce faint but audible airborne sounds. In those species in which both sexes "sing", duetting between males and females is usually present.
MEGALOPTERA: SIALIDAE AND CORYDALIDAE
Courtship and mating within Megaloptera have been described for just one genus of Sialidae (alderflies) and five of Corydalidae (dobsonflies and fishflies). A detailed overview of megalopteran mating systems can be found in Henry (1997).
Best studied of all Megaloptera are two European alderflies, Sialis lutaria (Linnaeus) and Sialis fuliginosa F. Pictet. In S. fuliginosa, Killington first described tremulation as a mutual "twitching of abdomens upward at intervals" in both sexes (Killington, 1932, p. 67). Much later, Rupprecht (1975) distinguished two types of tremulation signals in both species, which he described as "rhythmic" vs. "prolonged, unstructured".
"Rhythmic" vibrations are relatively short volleys of low frequency abdominal tremulation repeated at regular intervals by both sexes. These signals "allow mutual approach and recognition of species and sex" (Rupprecht, 1975, p. 305). Volley characteristics are similar in both species. The duration of each volley is several hundred milliseconds, and carrier frequency gradually decreases (i.e. is modulated) within each volley from a mean of 200 Hz at volley onset to about 120 Hz at the finish. Volley period ranges from 250 msec to 2 sec, depending on the behavioural context. Both species use only rhythmic tremulation signals during heterosexual duets. However, duetting in Sialis is not precise: the stimulus produced by one courting insect does not trigger a predictably timed response from its partner.
Rupprecht's "prolonged, unstructured" vibrations were found only in males of the two Sialis species he studied. These signals do not show consistent temporal or frequency characteristics. They appear to reflect a general state of sexual receptivity in males, and often segue into periods of organised volley production, courtship and copulation (Rupprecht, 1975, Figure 4).
Rupprecht also noted percussive signals in Sialis, produced by "tapping of abdomen and wings on the ground" (Rupprecht, 1975, p. 305). These he interpreted as providing encouraging feedback from one partner to the other during the early stages of courtship. Drumming occurs only in males of S. lutaria, but in both sexes of S. fuliginosa. Females of the latter species can drum very rapidly, at a rate of nearly 20 strikes/sec (Rupprecht, 1975, Figure 7).
If acoustic signals have evolved in the context of premating reproductive isolation, one expects them to be significantly different in closely related, sympatric species, whether because of stabilising selection on different mate recognition systems (Paterson, 1985; Butlin, 1995) or because of reinforcement and reproductive character displacement (Butlin, 1989; Howard, 1993; Liou and Price, 1994). Yet few consistent differences are found between the vibratory "songs" of S. lutaria and S. fuliginosa, suggesting that alternative evolutionary dynamics have been at work.
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