Neural processing of chemosensory information from the locust legs

Abstract

Tarsal chemoreceptors in phytophagous insects can provide information on the chemical nature of the leaf surface, and are thus potentially important in host plant discrimination. Ovipositing Pieris brassicae, for example, drum the surface of leaves with their tarsi and tarsal chemoreceptors have been shown to respond to host-specific glucosinolates (Ma and Schoonhoven,1973). The classic work of Dethier on the blowfly, showed that chemical stimulation of the tarsi of Phormia regina led to proboscis extension, which was necessary to allow subsequent stimulation of additional chemoreceptors on the labellum and oral papillae, which finally led to drinking. This work was conducted in order to investigate the reflex of the pro and mesothoracic legs in Locusta migratoria L under control of their chemoreceptors in responses to be stimulating media. The study was conducted utilising different stimulating media namely : salt (10mM and 100mM NaCl), sugar (50mM sucrose and 50mM glucose), acid (25mM and100mMcitric acid) and alkaloid (50mM NHT) solutions, in addition to a repellent agent (10mM and 100mM quinine hydrochloride). The central projection of basiconic sensilla, canal sensilla and meshanoreceptors sensillum were studied by using neurobiotin backfilling and single cell recording. Results indicated that sensitivity of the prothoracic leg and its reflex are faster in comparison to the mesothoracic leg which could be explained by the dense spiking of the sensory neurones of the basiconic sensilla in the prothorecic ganglia controlling the prothoracic leg as compared to the spiking of the sensory neurones of the mesothoracic leg. Denser spiking of the sensory neurones of both canal and meshano sensilla in the prothoracic ganglia were also recorded in comparison to less spiking in mesothoracic ganglia . The basiconic sensilla shows responses to one or more of the tested stimulators indicating a partial specificity of basiconic sensilla . Results on the responses of motor neurones and their effect on the basiconic sensilla in relation to tested stimulus indicated that not all the motor neurones controlling the depressor muscles show similar responses. Some responded only to salt solution others only to sugars solutions, however, some responded to both stimuli at the same time. Local interneurones were also found to responded differently to stimulating media. One Local interneurones was identified for both salt(NaCl) and sugar(Glucose) solutions and a second was identified for the repellent agent (quinine hydrochloride). Based on the pervious result it could be concluded that chemoreceptors and specially basiconic sensilla on the tarsus, pro and mesothoracic legs play an essential role in identifying the nature of surrounding media which is intern translated as reflexes by the pro and mesothoracic legs.