Systematics, Ecology and Behaviour of Orthopteran Insects
The Order Orthoptera (crickets and grasshoppers) is one of the evolutionarily ancient orders of insects and is distributed world-wide, with several representative groups in both temperate and tropical regions of the Old and New World. The order may be broadly sub-divided into three sub-groups: the Ensifera (or crickets), the Caelifera (or grasshoppers) and the Gryllacridoidea (whose phylogenetic relationship with the other two groups is currently unclear). In contrast to the Caelifera and the Gryllacridoidea, in which many of the species do not produce sound, members of the sub-order Ensifera often exhibit acoustic communication. The acoustic signals are typically produced by adult males in the context of mate attraction. The acoustic signals or calling songs produced by the males are highly species-specific, with each cricket species producing a unique song.
The Ensifera may be further
sub-divided into two families: the Gryllidae (or true crickets) and the Tettigoniidae
(or bushcrickets). Both gryllids and tettigoniids produce sound by rubbing the
forewings together: a specialised structure called the plectrum (on the inner
margin of one forewing) scrapes against a row of pegs on the stridulatory file
on the ventral surface of the other fore wing to produce sound. The sound is
then amplified by special resonating areas of the forewing, such as the harp
and mirror. Most gryllid and tettigoniid species also possess tympanal ears,
which are located, one on each foreleg, at the proximal end of the tibia.
Cricket species of the sub-order Ensifera provide excellent systems to study the perception, function, diversity and evolution of animal communication signals because they use a repertoire of acoustic and non-acoustic signals for both long- and close- range communication in the context of mate-finding. Since they are very widely distributed over a range of habitats from suburban gardens, scrub and grasslands to tropical rainforests, and occupy a variety of micro-habitats ranging from the ground level to the canopy, they also offer opportunities for studies in community ecology. Both behavioural and ecological studies require, however, to be placed in an evolutionary context and the reconstruction of phylogenetic relationships between the focal species or genera therefore acquires significance.
Natural communities of
acoustically communicating orthopterans have rarely been studied in any depth in
tropical forests world wide and even less so in India. Most studies to date have
been short-term and carried out in a qualitative manner. Whereas communities of
birds and frogs have been studied in considerable detail, we know little about
the spatio-temporal patterning of orthopteran communities. We have recently
initiated studies in evergreen forests of the Southern Western Ghats aimed at
characterising the structure of communities of acoustically communicating
ensiferan insects. We are currently engaged in obtaining the calls and
identifying the species that constitute the acoustic insect community and aim in
the long run to understand the patterns of habitat selection and temporal
dynamics of these communities, as well as their response to habitat
fragmentation and degradation. These studies should provide information of
relevance to bio-diversity conservation.
The species-specific calling songs of crickets are used by females to identify and approach conspecific males. Females are thus tuned to the calling songs of males of their own species and tend to ignore heterospecific males that may be calling at the same time. The neural basis of recognition of conspecific song has been an important area of research in the field of neuroethology for several decades. Whereas the basis of tuning to spectral features of the song is fairly well understood, the neural basis of temporal pattern recognition continues to pose a challenge. In collaborative research with the University of Erlangen, Germany, a neural model for song syllable recognition in the duetting grasshopper Chorthippus biguttulus has been developed. We have also examined additive and synergistic interactions in the processing of different features of the signal within the female receiver.
The role of calling song as a pre-zygotic mechanism of species isolation is currently being investigated in two sympatric species of tree crickets of the genus Oecanthus, which are also synchronous in their seasonal breeding cycles. These two species are to be found co-existing in areas of natural vegetation (dry grasses and shrubs) on the campus of the Indian Institute of Science and are being studied in the field. The calling songs of the two species, Oecanthus henryi and Oecanthus indicus, are similar in several of their features, raising the question of the effectiveness of song in maintaining species identities in this case. Ethograms of the courtship behaviour of these species have also been developed in the same context. We aim to use these species in order to investigate the role of calling and courtship signals in species isolation and mate choice.
Centre for Ecological Sciences,
Indian Institute of Science,
Bangalore 560012, India
Ph: 3600985 Extn: 103