Changes in distribution and abundance of antennal sensilla during growth of Locusta migratoria L. (Orthoptera : Acrididae)

The antenna of Locusta migratoria (Orthoptera : Acrididae) increases in length by the production of new annuli proximally and by elongation of existing annuli. The most distal annuli are fully differentiated at the time of hatching and no new olfactory sensilla are added to them at subsequent molts. More proximally, the differentiation of trichoid contact chemoreceptors precedes the development of olfactory sensilla. Sensillum differentiation proceeds from distal to proximal along the antenna and more distal annuli attain full development at each molt. The biggest increase in numbers of olfactory sensilla occurs at the final molt. On any one annulus, most olfactory sensilla are restricted to sensory fields on the anterior and posterior faces. Their spacing within these fields is consistent with the existence of separate but interacting chaetogens regulating the differentiation of basiconic and coeloconic sensilla.     

Ultrastructure of the chemosensitive basiconic single-walled wall-pore sensilla on the antennae in adults and embryonic stages of Locusta migratoria L. (Insecta,Orthoptera)

Summary The structure and embryonic development of the two types (A, B) of basiconic sensilla on the antennae of Locusta migratoria were studied in material that had been cryofixed and freeze-substituted, or chemically fixed and dehydrated. Both types are single-walled wall-pore sensilla. Type-A sensilla comprise 20–30 sensory and 7 enveloping cells. One enveloping cell (thecogen cell secretes the dendrite sheath); four are trichogen cells, projections of which form the trichogen process during the 2nd embryonic molt. The trichogen cells form two concentric pairs proximally. Two tormogen cells secrete the cuticular socket of the sensillum. The dendritic outer segments of the sensory cells are branched. Bifurcate type-A sensilla have also been observed. Type-B sensilla comprise three sensory and four enveloping cells (one thecogen, two trichogen and one tormogen). The trichogen process is formed by the two trichogen cells, each of which gives rise to two projections. The trichogen cells are concentrically arranged. The dendritic outer segments of the sensory cells are unbranched. In the fully developed sensillum, all trichogen and tormogen cells border on the outer receptor lymph cavity. It is suggested that the multicellular organization of the type-A sensilla can be regarded as being advanced rather than primitive.Supported by the Dcutschc Forschungsgemeinschaft (SFB 4/G1)     

Effects of an imidazole derivative (KK-42) on development and ecdysteroid production in Locusta migratoria (Insecta, Orthoptera)

The imidazole derivative KK-42 was applied in various experimental conditions to larvae and adult females of Locusta migratoria. The effect of this compound was monitored on the development of larvae, on oocyte growth in adult females and on the development of eggs laid by these females. KK-42 had only minor effects on postembryonic development; anticipation of imaginal moult was never observed. In contrast oocyte and egg development were markedly affected by KK-42: this effect is however not related to modifications of the synthesis of ecdysteroids in the ovaries.     

Fine-structural characteristics of the antennal sensilla of Agrotis segetum (Insecta: Lepidoptera)

Summary The turnip moth Agrotis segetum possesses seven different types of sensilla: four single-walled (SW), one double-walled (DW), one terminal-pore (TP), and one poreless sensilla (NP).The SW 1 and SW 2 sensilla have the same external appearance, being long and slender, but differ in the branching pattern of the sensory processes: unbranched and branched in SW 1 and SW 2, respectively. The SW 3 sensilla are shorter, sickle-shaped, and contain a large number of branches from the sensory processes. These three sensillar types are innervated by 2–3 sensory cells. The SW 4 sensilla are raisin-shaped and possess three profusely branched sensory processes. The DW sensilla are short and have apical slit-like pores. This sensillar type has 5–6 sensory processes. The TP sensilla possess five sensory processes, one of them terminates basally in a tubular body, the others in the apical part of the long cuticular bristle. The NP sensilla are stout and have apical conelike structures. Two of the sensory processes terminate in the apical part, the third proximally. The third sensory process has a lamellar pattern. The fine structure indicates the following functions: SW and DW sensilla: chemoreception; TP sensillum: chemoreception and mechanoreception; NP sensillum: thermoreception and hygroreception.Supported by joint grants from the Swedish Council for Planning and Coordination of Research, the Swedish Natural Science Research Council, and the Swedish Council for Forestry and Agricultural Research     

Bewegungsmechanismus und gelenkrezeptoren der Antennen von Locusta migratoria L. (Insecta,orthoptera)

The morphological arrangement of the antennal joints and their significance for the complex antennal movements are investigated. Particular value is attached to the mechanoreceptors of the 2. segment because of their importance for the function of the antennae as air current sense organs.     

Effect of [His7]-corazonin on the number of antennal sensilla in Locusta migratoria

Abstract.  Certain types of antennal sensilla are known to be more abundant in solitarious individuals than in gregarious ones in the migratory locust, Locusta migratoria . We tested the hypothesis that injection of a neurohormone, [His⁷]-corazonin, into isolated-reared nymphs of this species mimics the effect of crowding on the frequencies of various types of antennal sensilla. One nmol of the hormone was injected into nymphs on two occasions, on the third days of the second and third stadia, respectively. Upon adult emergence, the numbers of different types of sensilla on the eighth antennal segment were compared with those of oil-injected controls. [His⁷]-corazonin did not influence the numbers of basiconic sensilla type A, basiconic sensilla type B and trichoid sensilla significantly compared to oil-injected controls. However, the number of coeloconic sensilla was reduced significantly by the hormone injections. Because the length of the antennal segment was not affected by the hormone injection, it appears that the hormone influenced the development of coeloconic sensilla. The results support the hypothesis tested and are consistent with the idea that [His⁷]-corazonin plays an important role in the control of phase polymorphism in L. migratoria .     

Involvement of ecdysone in the control of meiotic reinitiation in oocytes of Locusta migratoria (Insecta,orthoptera)

Following their biosynthesis in the follicle cells of vitellogenic ovaries, large amounts of ecdysteroids pass into the oocytes where they accumulate and persist during ovulation and egg-laying. The present paper shows that free ecdysone is unevenly distributed in the oocytes exhibiting the highest concentrations in the region of the posterior pole where the final sequences of nuclear maturation, including germinal vesicle breakdown (GVBD), occur. A correlative study indicates that the concentrations of free ecdysone in this region are particularly high (10 to 20 μM) during two periods of meiotic reinitiation observed in the oocytes: reinitiation I, leading from prophase I to metaphase I with GVBD; and reinitiation II, from metaphase I to the end of meiosis. In vitro incubations of oocytes in meiotic arrest (prophase I) in the presence of exogenous ecdysone demonstrate that complete reinitiation (including GVBD) can be triggered in a dose-dependent manner by this hormone.     

Prepulse inhibition of the startle reaction in the locust Locusta migratoria (Insecta: Orthoptera: Acridoidea)

A fast startle reaction of unrestrained sitting locusts (Locusta migratoria) can be elicited by sound pulses of steep rise time above 80 dB. The reaction consists of a fast jerky movement of legs and body with a mean latency of 35 ms and graded amplitude. The fast startle reaction did not result in any positional change; this was in contrast to acoustically induced escape reactions of flying Orthoptera. The startle reaction could be inhibited by pure tone stimuli of much lower intensity (60 dB) presented 160 ms before the startle-eliciting noise. This type of reflex modification is a striking convergence to the well-known prepulse inhibition of the mammalian startle response where it has been used to assess sensory thresholds. In the locust, prepulses between 3 and 20 kHz suppressed the startle reaction completely, with thresholds in the locust's hearing range as known from tympanal nerve recordings. No inhibition could be observed at prepulse frequencies of 40 kHz, although this frequency lies within the locust's hearing range. The presence of prepulse inhibition in an invertebrate preparation shows that it is not restricted to vertebrates.     

The fine structure of the antennal sensilla of the pine saw flyNeodiprion sertifer (Insecta: Hymenoptera)

Summary There is a considerable sexual dimorphism as regards the development of the antenna inNeodiprion sertifer. In the male this is manifested in a larger antennal surface area and the possession of a great number of long single-walled sensilla (SW 1), which are located on the antennal branches. In the female the antennal branches are vestigial, and the single-walled sensilla, of a second type (SW 2), are shorter and present in lower numbers. Both sexes have in addition four types of sensilla in common:viz. a third type of single-walled sensilla (SW 3), terminal pore sensilla, double-walled sensilla, and poreless sensilla. These four sensillar types are characterized not only by their external appearance but also by their internal morphology. Especially the cells enveloping the sensory cells vary in number and appearance.Supported by joint grants from the Swedish Council for Planning and Coordination of Research, the Swedish Natural Science Research Council, and the Swedish Council for Forestry and Agricultural Research.     

A broadly tuned odorant receptor in neurons of trichoid sensilla in locust,Locusta migratoria

Insects have evolved sophisticated olfactory reception systems to sense exogenous chemical signals. Odorant receptors (ORs) on the membrane of chemosensory neurons are believed to be key molecules in sensing exogenous chemical cues. ORs in different species of insects are diverse and should tune a species to its own specific semiochemicals relevant to their survival. The orthopteran insect, locust (Locusta migratoria), is a model hemimetabolous insect. There is very limited knowledge on the functions of locust ORs although many locust OR genes have been identified in genomic sequencing experiments. In this paper, a locust OR, LmigOR3 was localized to neurons housed in trichoid sensilla by in situ hybridization. LmigOR3 was expressed as a transgene in Drosophila trichoid olfactory neurons (aT1) lacking the endogenous receptor Or67d and the olfactory tuning curve and dose-response curves were established for this locust receptor. The results show that LmigOR3 sensitizes neurons to ketones, esters and heterocyclic compounds, indicating that LmigOR3 is a broadly tuned receptor. LmigOR3 is the first odorant receptor from Orthoptera that has been functionally analyzed in the Drosophila aT1 system. This work demonstrates the utility of the Drosophila aT1 system for functional analysis of locust odorant receptors and suggests that LmigOR3 may be involved in detecting food odorants, or perhaps locust body volatiles that may help us to develop new control methods for locusts.     

The structure of bimodal chemo-, thermo-, and hygroreceptive sensilla on the antenna of Locusta migratoria

Summary Coeloconic sensilla on the antenna of Locusta migratoria were investigated electrophysiologically, labeled and then examined under the scanning or transmission electron microscope. They can be categorized into two main morphological types: 1) sensilla with wall pores and two concentric cuticular walls (wp-dw sensilla), and 2) sensilla lacking wall pores (np-sensilla).Hygroreceptors were observed only in np-sensilla. Olfactory receptors were observed in wp-dw sensilla, but not in np-sensilla. Cold-sensitive units were found in both types. Wp-dw sensilla with a cold-sensitive unit also manifest a fatty acid olfactory receptor. This cold-sensitive unit appears to be less sensitive to drops in temperature than the cold receptor combined with hygroreceptors in np-sensilla. Recordings from both types revealed up to three or sometimes four units that could be distinguished on the basis of impulse amplitude.The number of np-sensilla on each antennal segment ranges from 0 to 6. These sensilla are pegs 4–5 m long with a terminal swelling. The dendritic outer segments of two sensory cells fit tightly into these pegs. The dendrite of the third sensory cell ends at its base. All three are up to 1 m thick, unbranched, and densely filled with microtubules. The cuticular wall and socket of the peg are characterized by (1) an electron-dense inner layer surrounding the dendrites and continuous with electron dense clusters at the base of the peg, and (2) clefts in the cuticle at the insertion of the peg.The present observations favor the hypothesis that insect sensilla combining the triad of moist air, dry air and cold receptors generally lack pores. Specifical, though still alternative roles in stimulus transduction can be suggested for substructures demonstrated in the present and previous electron microscopic investigations.Supported by the Deutsche Forschungsgemeinschaft (SFB 4:G1/D)     

Differences in numbers of sensilla on the antennae of solitarious and gregarious Locusta migratoria L. (Orthoptera: Acrididae)

It has been assumed that differences in the behaviour of solitarious and gregarious locusts arise from differences in the central nervous processing of information, and that no essential differences occur in the sensory systems. This investigation reveals that solitarious adults and fifth instar nymphs of Locusta migratoria (Orthoptera: Acrididae) have more olfactory sensilla on the antennae than comparable gregarious phase insects. Behavioural differences could thus arise as a result of different peripheral inputs. It is suggested that social facilitation, resulting from the gregarious habit, permits a decrease in individual sensitivity with an associated reduction in the numbers of olfactory sensilla. The numbers of contact chemoreceptors are proportional to the lengths of the antennal annuli and do not differ greatly between the phases.     

Ultrastructure of sensilla on the antennal pedicel of the brown planthopper Nilaparvata lugens Stal (Insecta: Homoptera)

Scanning- and transmission electron-microscope studies of the sensilla of the pedicel of Nilaparvata lugens have revealed an elaborate plaque organ and three structurally different types of trichoid hairs. Each plaque organ is innervated by 120-150 neurons arranged in groups. The porous sensory cuticle is folded into finger-like projections and is surrounded by protective non-sensory denticles. Trichoid sensilla differ in number of neurons, appearance of dendrites and arrangement of pores. Type I, innervated by 2 neurons, has pores along its length and is probably olfactory in function. Types II and III are innervated by one and five neurons, respectively, and the absence of pores along the hair shaft indicates a possible gustatory role. The importance of these sensilla is discussed with reference to the behaviour of this important insect pest.     

Campaniform sensilla of Calliphora vicina (Insecta,Diptera)

Summary A classification scheme of campaniform sensilla using morphological criteria was developed. All variations of the two most important outer structural elements, the cuticular cap and the cuticular collar, were taken into consideration: (a) the external shape of the cuticular cap; (b) the position of the cuticular cap in relation to the remaining cuticle; (c) the position of the cuticular collar in relation to the cuticular cap. This resulted in a classification of campaniform sensilla into 24 types. This typology was applied to the campaniform sensilla of Calliphora, which show considerable variations in their outer structures. According to SEM (scanning electron microscope) pictures and TEM (transmission electron microscope) sections we found only 9 out of 24 different types of campaniform sensilla in the fly.     

Structure and function of antennal pore plate sensilla of oryctes rhinoceros (L.) (Coleoptera : Dynastidae)

The antennae of the rhinoceros beetle, Oryctes rhinoceros (Coleoptera : Dynastidae), comprise 4 parts : the scape, the pedicel, a funicle, and a club of 3 lamellate segments. The inner surfaces of the lamellate club segments carry one type of trichoid sensilla, 2 types of sensilla coeloconica, and 3 types of multiporous pore plate sensilla. The total surface occupied by the sensilla on the antenna is 5.2±0.4 mm² in males (mean±SD) and 5.4±0.5 mm² in females. With a measured density of 8665±1254 sensilla per mm² in males and 8952±1642 sensilla per mm² in females, the total number of pore plate sensilla was estimated to be between 45,000 and 50,000. The structure of the 3 types of pore plate sensilla is described. SP1 are the most abundant type of placoid sensilla. They show a convex and rugged plate whose infoldings form a circle of irregular cavities. SP2 sensilla are characterised by a smooth and convex plate, surrounded by a furrow with a ridge. SP2 are localised on a wide band situated along the straight side of the lamella. The plate of SP3 is nearly flat and there is no furrow. SP3 are confined within a narrow margin along the convex edge of lamellae. The 3 types of pore plate sensilla house 2 neurones whose dendrites branch repeatedly under a plate of thin (0.2 μm) cuticle, which is pitted with numerous pores, 40 nm in diameter. Single sensillum recordings with tungsten microelectrodes revealed the firing activity of 2 neurones. These receptor neurones responded specifically to olfactory stimulus. Olfactory receptor neurones tuned to the male pheromone compound, ethyl 4-methyl octanoate, were found in male and female antennae. Other receptor neurones responded to plant volatiles. Morphological and electrophysiological data suggest the absence of a sexual dimorphism in the olfactory organs. The functional organisation of the olfactory organs is discussed in terms of their adaptation to the ecology of O. rhinoceros.     

Reduction of sensory cells in antennal sensilla correlated with changes in feeding behavior in the beetle Loricera pilicornis (Insecta,Coleoptera, Carabidae)

Summary The structure of the setae on the proximal antennal segments of the beetle Loricera pilicornis is described using electron microscopical methods. These setae are part of a prey-capturing apparatus and are inserted within flexible sockets. They have no central lumen.Four or five sensory cells are connected to each seta. One cell is characterized as a mechanoreceptor due to the presence of a tubular body and the location of its dendritic outer segment. The other sensory cells are of two types. One type shows the usual features of sensillar receptors except that the dendritic outer segments end beneath the seta within the cuticular sheath. In the other type all parts of the cell, including the perikaryon, appear undersized, and no axon was found. In a single case a sixth cell was found which lacks any process, although, due to its location, it belongs to the sensory cell group.The enveloping cells also deviate from the usual pattern. Trichogen and tormogen cells have no membrane folds nor microvilli. From the membrane of the thecogen cell, where it borders on the inner receptor lymph cavity, invaginations have developed which form voluminous membrane whorls. Portasomes are found on these membranes.On the basis of the structural features we hypothesize that the setae represent sensilla undergoing stepwise reduction, losing primordial gustatory units whilst the prey-capturing mechanism is optimized.Dedicated to Professor Dr. Dietrich Schneider on occasion of his 65th birthday     

Further experimental evidence for the involvement of ecdysone in the control of meiotic reinitiation in oocytes of Locusta migratoria (Insecta, Orthoptera)

Ecdysone has recently been shown to be able to trigger meiotic reinitiation in vitro in submature oocytes of Locusta. In the present study we have experimentally depressed (by 60-70%) ecdysone biosynthesis in the ovaries of adult females by rearing them on a diet with a modified sterol profile. Mature oocytes from such females fail to undergo normal reinitiation, but when incubated in vitro, can be induced to break their meiotic arrest by the addition of exogenous ecdysone. These results lend further support to the hypothesis that in Locusta, ovarian ecdysone is involved in the control of meiotic reinitiation.     

Characterization of three hydroxylases involved in the final steps of biosynthesis of the steroid hormone ecdysone in Locusta migratoria (Insecta, Orthoptera)

It is most generally accepted that the last three enzymatic reactions in the biosynthetic pathway of ecdysone are, in this order, the hydroxylations at positions C-25, C-22 and C-2. Using high specific activity tritiated ecdysone precursors (2,22,25-trideoxyecdysone, 2,22-dideoxyecdysone and 2-deoxyecdysone) we have characterized the hydroxylases involved in these reactions, in the major biosynthetic tissue of ecdysone, i.e. the prothoracic glands. We show that C-2 hydroxylase is a mitochondrial oxygenase which differs from conventional cytochrome P-450-dependent monooxygenases by its relative insensitivity to CO. In contrast, C-22 and C-25 hydroxylases appear as classical cytochrome P-450 monooxygenases; C-22 hydroxylase is a mitochondrial enzyme whereas our data point to a microsomal localization of the C-25 hydroxylase.     

Ultrastructure and early development of the pore plate sensilla ofGymnomerus laevipes (Shuckard) (Vespoidea,Eumenidae)

Summary The oval pore plates (approx. 17 m long) are separated from the antennal cuticle by a furrow, the inner wall of which is flexible. The thin perforated plates are strengthened by an encircling and a middle ledge, the latter of which branches into about 100 almost parallel rims. Each pore plate is innervated by about 20 sense cells. The dendrites fork into numerous branches occupying the outer receptor lymph cavity below the perforated plate. Each pore plate is associated with one thecogen cell, two trichogen cells, one tormogen cell and one envelope cell 4. A so-called additional cell surrounds the sensillum in the imaginal stage. The envelope cells in the later of the two pupal stages examined, have reached an arrangement which immediately precedes the secretion of the cuticulin layer. The surface of the duplicate trichogen cells is almost equal in area to the completed perforated plate. A dendritic sheath, entirely reduced in the imago, protrudes into the exuvial space, where it encloses a single dendrite.In the younger pupal stage the Sensillenanlage forms a crater, whereby envelope cell 4 overtops the other envelope cells. The distal ends of the trichogen cells are divided into several appendages that form the bottom of the crater.