ACOUSTIC SIGNALS IN INSECTS: A REPRODUCTIVE BARRIER AND A TAXONOMIC CHARACTER

摘要

In singing insects, the song is an important component of the specific mate recognition system (SMRS). In communities of sympatric singing species, there is a partitioning of communication channels, the so-called "acoustic niches". Within one community, the songs of different species always differ in temporal or frequency characters, i.e. occupy different acoustic niches. However, conspecific songs do not always act as an inter specific reproductive barrier, despite always being a SMRS component. The species that do not communicate acoustically due to either allopatry or a different timing of vocalization, or inhabiting different biotopes, or unmatched food specializations can sing similar songs, thus forming reproductively isolated communities. Individuals of the opposite sex not only need to recognize a conspecific mate, but also have to evaluate its "quality". A close-range signal (courtship song) provides more opportunities for a choice of the "best" male than does a distant signal (calling song). In many species of Orthoptera, courtship includes not only acoustic, but also vibration, visual, chemical and mechanical signals. An analysis of cricket songs showed that, on average, the courtship songs are more elaborate and more variable than the calling songs. At the same time, due to the difference in mating behaviour between grasshoppers and crickets, the acoustic component of courtship is used for an evaluation of mate quality to a greater extent in the fowler than it is in the latter group. Courtship songs of grasshoppers are generally more elaborate in temporal structure than cricket songs and, moreover, can be accompanied by visual displays such as movements of various parts of the body. Thus, song evolution in grasshoppers is more driven by sexual selection than that of crickets. According to the reinforcement hypothesis, a premating barrier between hybridizing species becomes stronger in response to reduced hybrid fitness. However, our behavioural experiments conducted on two groups of hybridizing grasshopper species did not confirm the reinforcement hypothesis. We explain this firstly, by a low level of genetic incompatibility between the hybridizing species and secondly, by high hybrid fitness when attracting a mate. A high competitive capability of hybrids might be accounted for by the attractiveness for females of new elements in hybrid courtship songs. When we divide similar forms on the basis of songs, we distinguish biological species using their reproductive isolation as a criterion. Acoustic differences between species are usually greater than morphological ones. Therefore, analyzing the song allows us to find out the real status of doubtful taxa of species rank, to distinguish species in a flock of sibling forms and to reveal cryptic species in the cases when morphological studies fail to provide a univocal result. At the same time, songs have intra-specific variability and its range can be different in various groups. Therefore, it is necessary to study which degree of difference corresponds to the species level before interpreting the status of some forms based on song comparisons. Besides this, song similarities cannot be evidence that acoustically isolated forms are conspecific. On the other hand, song differences between these forms prove that they are full-rank species.