Half of all insect species are dependent on living plant tissues, consuming about 10% of plant annual production in natural habitats and an even greater percentage in agricultural systems, despite sophisticated control measures. Plants are generally remarkably well-protected against insect attack, with the result that most insects are highly specialized feeders. The mechanisms underlying plant resistance to invading herbivores on the one side, and insect food specialization on the other, are the main subjects of this book. For insects these include food-plant selection and the complex sensory processes involved, with their implications for learning and nutritional physiology, as well as the endocrinological spects of life cycle synchronization with host plant phenology. In the case of plants exposed to insect herbivores, they include the activation of defence systems in order to minimize damage, as well as the emission of chemical signals that may attract natural enemies of the invading herbivores and maybe exploited by neighbouring plants that mount defences as well.

Insect-plant biology

This review surveys the organization of the olfactory and gustatory systems in the imago and in the larva of Drosophila melanogaster, both at the sensory and the central level. Olfactory epithelia of the adult are located primarily on the third antennal segment (funiculus) and on the maxillary palps. About 200 basiconic (BS), 150 trichoid (TS) and 60 coeloconic sensilla (CS) cover the surface of the funiculus, and an additional 60 BS are located on the maxillary palps. Males possess about 30% more TS but 20% fewer BS than females. All these sensilla are multineuronal; they may be purely olfactory or multimodal with an olfactory component. Antennal and maxillary afferents converge onto approximately 35 glomeruli within the antennal lobe. These projections obey precise rules: individual fibers are glomerulus-specific, and different types of sensilla are associated with particular subsets of glomeruli. Possible functions of antennal glomeruli are discussed. In contrast to olfactory sensilla, gustatory sensilla of the imago are located at many sites, including the labellum, the pharynx, the legs, the wing margin and the female genitalia. Each of these sensory sites has its own central target. Taste sensilla are usually composed of one mechano-and three chemosensory neurons. Individual chemosensory neurons within a sensillum respond to distinct subsets of molecules and project into different central target regions. The chemosensory system of the larva is much simpler and consists essentially of three major sensillar complexes on the cephalic lobe, the dorsal, terminal and ventral organs, and a series of pharyngeal sensilla.

The Organization of the Chemosensory System in Drosophila Melanogaster: A Review

https://www.mcs.anl.gov/uploads/cels/papers/sdarticle.pdf

A Chemosensory Gene Family Encoding Candidate Gustatory and Olfactory Receptors in Drosophila

Twenty-six strains of mice were tested for their reaction to four different sweet substances; saccharin, acesulfame, dulcin and sucrose. There was considerable strain variation in the degree to which they found the sweet substances preferable to water. The variation in preference for any one sweet substance is very highly correlated with the variation in preference for the other sweet substances. This is interpreted to mean that there is only one sweetness receptor, although an alternative explanation in terms of variation in psychological motivation is not discounted. The difference between C57BL/6Ty and DBA/2Ty is largely due to a single gene, Sac.

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The genetics of tasting in mice. VI. Saccharin, acesulfame, dulcin and sucrose.

Gustatory Perception and Behavior in Drosophila melanogaster

The sensilla on the proboscis and tarsi of Drosophila contain five neurons, four chemosensory and one mechanosensory. The sugar-sensitive neuron, designated S, carries independent acceptor sites for pyranose, furanose and trehalose. Two others, L1 and L2, respond to salts. The fourth neuron, W, is inhibited by salts and sugars, and is believed to mediate detection of water. We describe here a gene in which mutations alter the neurons in such a way that the S cell is excited by salts. As a result, the mutant flies are strongly attracted by NaCl at concentrations which are repellent to the wild type. To our knowledge, this is the first instance of a mutation which changes the specificity of the chemosensory neurons.

A gene affecting the specificity of the chemosensory neurons of Drosophila.

We have used mutants defective in gustatory behavior to investigate the neural basis of salt perception in the fruitfly Drosophila melanogaster. Salts elicit responses from two sensory neurons, L1 ...

Genetic investigation of salt reception in Drosophila melanogaster

We are studying the molecular basis of salt reception in Drosophila melanogaster. The gustatory sensilla of Drosophila contain two different neurons designated L1 and L2 which respond to salts. Previous work has shown that the L1 neuron responds principally to cations and carries at least two acceptor sites that distinguish different cations (Arora, 1985). We have further obtained biochemical and genetic evidence that at least two types of cationic sites are involved. One of these responds selectively to Na+ while the other is relatively non-specific and responds to a variety of cations. The response of L1 to salts can be inactivated by trypsin. Following inactivation, the response to Na+ and K+ recover with a different time course.

Swati Joshi

Papers

A gene affecting the specificity of the chemosensory neurons of Drosophila.

Genetic investigation of salt reception in Drosophila melanogaster

Cationic Acceptor Sites on the Labellar Chemosensory Neurons of Drosophila Melanogaster