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

All insects are colonized by microorganisms on the insect exoskeleton, in the gut and hemocoel, and within insect cells. The insect microbiota is generally different from microorganisms in the external environment, including ingested food. Specifically, certain microbial taxa are favored by the conditions and resources in the insect habitat, by their tolerance of insect immunity, and by specific mechanisms for their transmission. The resident microorganisms can promote insect fitness by contributing to nutrition, especially by providing essential amino acids, B vitamins, and, for fungal partners, sterols. Some microorganisms protect their insect hosts against pathogens, parasitoids, and other parasites by synthesizing specific toxins or modifying the insect immune system. Priorities for future research include elucidation of microbial contributions to detoxification, especially of plant allelochemicals in phytophagous insects, and resistance to pathogens; as well as their role in among-insect communicatio...

/pdf/multiorganismal-insects-diversity-and-function-of-resident-4gprjzjhpr.pdf

Multiorganismal Insects: Diversity and Function of Resident Microorganisms

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

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

日本には約110種の蚊が産する。その中で医学的に重要なのは, 幼虫(ボウフラ)が人里近くの水域に発生し, 雌成虫がヒトから好んで吸血する種である。西日本ではアカイエカ(南西諸島ではネッタイイエカにおきかわる), チカイエカ, コガタアカイエカ, ヒトスジシマカなどが最も重要である。これらの種は生態がことなるので, その被害に対する効果的対策もことなる。雌成虫が吸血源となる動物を発見して完了するまでの過程は複雑で, わかつていない点もある。

Biology of mosquitoes.

SUMMARY There is a resurgence of interest in the Drosophila midgut on
account of its potential value in understanding the structure, development and
function of digestive organs and related epithelia. The recent identification
of regenerative or stem cells in the adult gut of Drosophila has
opened up new avenues for understanding development and turnover of cells in
insect and mammalian gastrointestinal tracts. Conversely, the physiology of
the Drosophila gut is less well understood as it is a difficult
epithelial preparation to study under controlled conditions. Recent progress
in microperfusion of individual segments of the Drosophila midgut, in
both larval and adult forms, has enabled ultrastructural and
electrophysiological study and preliminary characterization of cellular
transport processes in the epithelium. As larvae are more active feeders, the
transport rates are higher than in adults. The larval midgut has at least
three segments: an anterior neutral zone, a short and narrow acid-secreting
middle segment and a long and wider posterior segment (which is the best
studied) that secretes base (probably HCO 3 – ) into
the lumen. The posterior midgut has a lumen-negative transepithelial potential
(35–45 mV) and a high resistance (800–1400 Ω.cm 2 )
that correlates with little or no lateral intercellular volume. The primary
transport system driving base secretion into the lumen appears to be a
bafilomycin-A 1 -sensitive, electrogenic H + V-ATPase
located on the basal membrane, which extrudes acid into the haemolymph, as
inferred from the extracellular pH gradients detected adjacent to the basal
membrane. The adult midgut is also segmented (as inferred from longitudinal
gradients of pH dye-indicators in the lumen) into anterior, middle and
posterior regions. The anterior segment is probably absorptive. The middle
midgut secretes acid (pH<4.0), a process dependent on a
carbonic-anhydrase-catalysed H + pool. Cells of the middle segment
are alternately absorptive (apically amplified by ≈9-fold, basally
amplified by >90-fold) and secretory (apically amplified by >90-fold and
basally by ≈10-fold). Posterior segment cells have an extensively dilated
basal extracellular labyrinth, with a volume larger than that of anterior
segment cells, indicating more fluid reabsorption in the posterior segment.
The luminal pH of anterior and posterior adult midgut is 7–9. These
findings in the larval and adult midgut open up the possibility of determining
the role of plasma membrane transporters and channels involved in driving not
only H + fluxes but also secondary fluxes of other solutes and water
in Drosophila .

Epithelial ultrastructure and cellular mechanisms of acid and base transport in the Drosophila midgut.

Sensilla lining the inner walls of the sacculus on the third antennal segment of Drosophila melanogaster were studied by light and transmission electron microscopy. The sacculus consists of three chambers: I, II and III. Inside each chamber morphologically distinct groups of sensilla having inflexible sockets were observed. Chamber I contains no-pore sensilla basiconica (np-SB). The lumen of all np-SB are innervated by two neurons, both resembling hygroreceptors. However, a few np-SB contain one additional neuron, presumed to be thermoreceptive. Chamber II houses no-pore sensilla coeloconica (np-SC). All np-SC are innervated by three neurons. The outer dendritic segments of two of these neurons fit tightly to the wall of the lumen and resemble hygroreceptor neurons. A third, more electron-dense sensory neuron, terminates at the base of the sensillum and resembles a thermoreceptor cell. Chamber III of the sacculus is divided into ventral and dorsal compartments, each housing morphologically distinct grooved sensilla (GS). The ventral compartment contains thick GS1, and the dorsal compartment has slender sensilla GS2. Ultrastructurally, both GS1 and GS2 are doublewalled sensilla with a longitudinal slit-channel system and are innervated by two neurons. The dendritic outer segment of one ofthe two neurons innervates the lumen of the GS and branches. On morphological criteria, we infer this neuron to be olfactory. The other sensory neuron is probably thermoreceptive. Thus, the sacculus in Drosophila has sensilla that are predominantly involved in hygroreception, thermoreception, and olfaction. We have traced the sensory projections of the neurons innervating the sacculus sensilla of chamber III using cobaltous lysine or ethanolic cobalt (II) chloride. The fibres project to the antennal lobes, and at least four glomeruli (VM3, DA3 and DL2-3) are projection areas of sensory neurons from these sensilla. glomerulus DL2 is a common target for the afferent fibres of the surface sensilla coeloconica and GS, whereas the VM3, DA3 and DL3 glomeruli receive sensory fibres only from the GS.

Fine structure and primary sensory projections of sensilla located in the sacculus of the antenna of Drosophila melanogaster.

Earlier studies using Golgi silver impregnations from the labellar sensilla of adult Drosophila melanogaster revealed seven types of sensory axons projecting into the suboesophageal ganglion of the brain. These sensory terminals were designated as coiled fibres (type-I), shrubby fibres (type-II), ipsilateral ventral fibres (type-III), ipsilateral dorsal fibres (type-IV), contralateral ventral fibres (type-V), contralateral dorsal fibres (type-VI), and central fibres (type-VII). The present study identifies the projections of sensory neurons present in a single labellar taste-sensillum, using the neuronal marker horseradish peroxidase (HRP). Although the taste sensillum in question has five neurons, in a given experiment only one or at the most two neurons are labelled. The type of neuron labelled was usually specific to the stimulant solute (sucrose, sodium chloride or potassium chloride) present in the HRP solution. Although type-II fibres get labelled most of the time, irrespective of the stimulant present in HRP solution, type-IV fibres are labelled when attractants (0.1 M sucrose or ≤0.1 M sodium chloride) are used as stimulants in HRP solution. Type-VI fibres are labelled when the stimulant is 0.1 M potassium chloride, a repellent. HRP dissolved in distilled water revealed type-I coiled fibres. Besides revealing projections of sensillar neurons to the brain the present technique also inferred their possible function. Incubation of whole-brain tissue with 0.04% 3,3′-diaminobenzidine tetrahydrochloride in presence of 0.06% hydrogen peroxide suggested that the glomerular organization is also present in the taste-sensory region as it is in olfactory neuropile.

Functional implications of the projections of neurons from individual labellar sensillum of Drosophila melanogaster as revealed by neuronal-marker horseradish peroxidase

The chordotonal or scolopophorous organs located in the femoral segment of all the legs of both male and female Drosophila melanogaster (Diptera : Drosophilidae) were examined by light and transmission electron microscopes. The femoral chordotonal organs (FCO) are arranged in 3 groups: one large group consists of about 32 aligned scolopes whose distal ends extend and terminate at the distal epicuticular surface of the femur; the other 2 central groups together contain about 42 scattered scolopes, and distally they terminate into the membranes connecting the muscles of the femur. No sexual dimorphism is evident either in the numbers of scolopidial groups or the total number of scolopidia in both sexes. Each scolopidium is innervated by 2 neurons of which one is less electron-dense than the other. The dendrites of these neurons bear sensory cilia. The fine structure of these chordotonal sensilla suggests that they probably respond to stretch or flexion. Proximally in the femur, the axons from FCO form a novel glomerular organization. These axons show lateral extensions and form different morphological types of synapses among themselves. From the presence of a large number of putative chemical synapses in the legs, it is evident that some degree of information processing is taking place in D. melanogaster at the periphery before being relayed to the central nervous system.

Ultrastructure of the femoral chordotonal organs and their novel synaptic organization in the legs of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

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.

Shubha R. Shanbhag

Papers

Epithelial ultrastructure and cellular mechanisms of acid and base transport in the Drosophila midgut.

Fine structure and primary sensory projections of sensilla located in the sacculus of the antenna of Drosophila melanogaster.

Functional implications of the projections of neurons from individual labellar sensillum of Drosophila melanogaster as revealed by neuronal-marker horseradish peroxidase

Ultrastructure of the femoral chordotonal organs and their novel synaptic organization in the legs of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

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