Werner Gnatzy

Ultrastructure and mechanical properties of an insect mechanoreceptor: Stimulus-transmitting structures and sensory apparatus of the cereal filiform hairs of Gryllus

Summary1.The following features of the cercal filiform hairs of the cricket Gryllus were investigated: (a) the ultrastructure and geometrical peculiarities of the various auxiliary structures in the region of the hair base, as well as those of (b) the stimulus-receiving outer segment of the dendrite (including the tubular body), and (c) the mechanical properties (directionality and linearity and frequency dependence of mobility) of the hair.2.When stimulated by vibrations of the medium, the filiform hairs show regular or irregular oscillations depending on stimulus intensity. At higher stimulus intensities (ξ>∼-100μm at 100 Hz) the hairs flutter irregularly in various directions, at somewhat lower intensities preferentially in the plane of best mobility in even lesser intensities in the plane of stimulus vector. In the plane ob best mobility the maximal angle of deflection from the resting position is 5.3±1.4°.3.The dependence of hair mobility on stimulus frequency was tested in the range 20–1000 Hz. Best mobility was found in the range 100–200 Hz.4.The directional characteristic of hair mobility has the form of a figure eight. Hairs can be grouped into three classes on the basis of direction (with respect to the long axis of the cercus) of best mobility: parallel (L-hairs), transverse (T-hairs), and diagonal (D-hairs).5.The plane of best mobility corresponds with the plane of symmetry of the hair base. The hair can be deflected furthest from the resting position in the direction of a cuticular peg at the hair base, which projects toward the lumen of the hair and marks the flat side of the tubular body within the terminal dendrite segment. Deflection of the hair shaft in the opposite direction is limited by a fibrous cushion, which exerts a counter-pressure. When the hair is deflected, the cuticular peg causes deformation of the tubular body.6.The direction of best mobility of the hair is the direction in which the sensory cell is depolarized; the direction of depolarization can thus be determined entirely by morphological criteria.

Campaniform sensilla of Calliphora vicina (Insecta, Diptera)

SummaryLight and scanning electron microscopic investigations were carried out to map the topography, number, size and configuration of all campaniform sensilla in the exoskeleton of the blowfly Calliphora vicina. We counted a total of about 1,200 campaniform sensilla; sexual dimorphism was not found. The shape, i.e. cap and collar, of most campaniform sensilla is elliptical; only 24 circular sensilla were found. The occurrence of campaniform sensilla is limited to the antennae (pedicels), legs, wings and halteres. Due to their configuration we defined: (a) ‘single sensillum’, (b) ‘sensilla in groups’ and (c) ‘sensilla in fields’. Single sensilla (n=86) occur on all loci mentioned. Sensilla in groups (about 350, in 52 groups) occur on the legs and forewings. The largest group had 32, the smallest 3 sensilla. All sensilla in fields (about 730, in 12 fields) occur on the halteres except for one on the tegula of the wing. A total of about 670 campaniform sensilla, which are more than 55% of all sensilla, are localized in 10 fields on the halteres.

Cricket combined mechanoreceptors and kicking response

Summary1.Only those filiform hairs on the cerci ofGryllus, which are coupled with campaniform sensilla, show a) a thickening of the hair shaft at the height of the upper ring lamella of their sockets and b) a thin cuticular membrane, which surrounds their sockets. While thefiliform hairs themselves are deflected either parallel to the long axis of the cerci or perpendicular to it, thesockets of the filiform hairs may be deflected preferentially in the proximal and distal direction.2.Spike potentials can be recorded from the sensory cells of the filiform hairs as long as these are deflected in a weak air current, but not during permanent deflection in strong air streams when they touch the inner wall of their sockets (Fig. 5). The sensory cells of the campaniform sensilla respond to deflection of the sockets in a phasic manner. All of the investigated campaniform sensilla respond to a deflection of the sockets in either the proximal or the distal direction.3.Air currents of a speed, strong enough to deflect the sockets of the filiform hairs and thus to excite the campaniform sensilla, result in an increase of kicking responses from about 10–20% to nearly 100%. We conclude that the campaniform sensilla on the cercus ofGryllus trigger the kicking response.4.Through the functional coupling of filiform hairs with campaniform sensilla the working range of these combined organs is considerably extended. Up to a speed of air currents of 1.9 m/s only filiform hairs respond. Stronger stimuli of up to 2.3 m/s (stimulation parallel to the long axis of the cerci) or of up to 3.6 m/s (stimulation perpendicular to the long axis of the cerci) deflect the sockets and result in a response of the campaniform sensilla.

Are the funnel-canal organs the ‘campaniform sensilla’ of the shore crab, Carcinus maenas (Decapoda, Crustacea)?

SummaryThe funnel-canal organs on the dactyls of the shore crab, Carcinus maenas, are innervated by 3–24 sensory cells with unbranched dendrites, which attain a length of 500–1400 μm. The outer dendritic segments are enclosed in a dendritic sheath and pass through the cuticle within a canal. Two dendrite types can be distinguished according to ultrastructural criteria: Type I has a long ciliary segment, A-tubules with an osmiophilic core and arms, and a thick ciliary rootlet. Type II possesses only a short ciliary segment and a thin ciliary rootlet. Each funnel-canal organ contains two type-I dendrites. Their ciliary bases appear a few μm distal to those of the type-II dendrites (1 to 22 in number). Two inner and two to eight outer enveloping cells belong to a sensillum. The innermost enveloping cell contains a large scolopale. In the second enveloping cell single scolopale rods are present. Thus, the funnel-canal organs are characterized by structural features typical for mechano-sensitive scolopidia, on the one hand, and for chemoreceptors, on the other. Therefore, the funnel-canal organs are very likely bimodal sensilla (contact chemoreceptors). A comparison with other arthropod sensilla shows that cuticular mechanoreceptors of aquatic crustaceans generally exhibit a ‘scolopidial’ organization.

Digger wasp against crickets. II. An airborne signal produced by a running predator

SummaryFemales of the digger wasp Liris niger Fabr. hunt crickets to provide food for their offspring by running with high velocity on the ground (>20–50 cm/s). Crickets are able to detect the running wasps by the air particle movement generated by the predator. We measured signals produced by running wasps using a microphone sensitive to air particle velocity. The wasps generated single air puffs with peak air particle velocities of 1–2 cm/s measured close to the running wasp. We measured frequency spectra of the signals containing only components below 50 Hz, with increasing intensities towards lower frequencies, especially below 10 Hz.We measured the air particle movement generated by artificially moved wasps, crickets or a styrofoam dummy of similar size to investigate the effect of velocity and shape of the moving object upon the composition of the signal. The velocity of movement appeared to be important for the intensity and frequency composition of the air particle movement. The shape of the moved body had an influence on the intensity but only little effect on the frequency spectrum. Measurements with a thermistor anemometer showed that a moving object caused air currents lasting longer than 100 ms after passing or approaching the probe. The air particle movements generated by hunting wasps are entirely sufficient with respect to intensity and frequency range to be registered by the filiform hair sensilla upon the cerci of crickets.

Cuticle: Formation, Moulting and Control

The relative rigidity of the arthropod exoskeleton makes it impossible for body size to increase continuously during the postembryonic development of these animals. Once they have hatched from the egg, they grow in steps, passing through a variable number of (larval) stages (Fig. 1 a). Apart from a few exceptions, there are between 3 and 10 such stages in the arachnids, 3–20 in the crustaceans, and 3–10 in the insects. In many cases a metamorphosis stage intervenes (some crustaceans; holometabolous insects) (Fig. 9b, c).

The antennal feathered hairs in the crayfish: a non-innervated stimulus transmitting system

SummaryMechanical stimulation of feathered hairs on the crayfish antenna elicits spike activity in nerve bundles running in the flagellum. Electron microscopical studies showed, however, that these hairs are not innervated. Instead these hairs can be coupled mechanically with nearby innervated hairs of a different type, which perform the sensory transduction.

Sensitivity of an insect mechanoreceptor during moulting

ABSTRACT. The fine structure and the behavioural threshold for vibration sensitivity of the eight thoracic filiform hairs of Barathra brassicae caterpillars were investigated through an intermoult/moult cycle. Associated with each filiform hair is one bipolar sensory cell and three enveloping cells. The outer dendritic segment terminates in an ecdysial canal in the hair base and a tubular body lies at its distal end. Shortly before apolysis the dendrite elongates. By this means the connection between the sensory cell and the old cuticular apparatus is maintained while the epithelium and the old thoracic cuticle are separating. The new cuticular apparatus of the filiform hair is formed in the second half of the larval stage by the three enveloping cells. A second tubular body in the elongated outer dendritic segment is formed at the base of the replacement hair 10 h before next ecdysis, so that the new hair functions as soon as ecdysis is completed, the old cuticular apparatus with the old tubular bodies being torn away with the exuvia during ecdysis. Sensitivity to a 300 Hz tone was tested in the standing wave of a Kundts tube. Throughout most of the larval instar the threshold was 2.0 ± 0.3 μm particle displacement amplitude until 1–2h before ecdysis when it rose to 6.8 ± 1.3 μm and at 10–30 min before the beginning of ecdysis no reaction to sound could be detected. Once the old cuticle was shed maximum sensitivity returned as soon as the replacement hairs were erect. The sensilla are therefore physiologically functional at all developmental stages except for 30–60 min during actual ecdysis.

K+ and Ca++ in the receptor lymph of arthropod cuticular mechanoreceptors

SummaryTwo types of cuticular strain detectors, the campaniform sensilla on the haltere of the blowfly,Calliphora vicina, and the slit sensilla on the tibia of the spider,Cupiennius salei, were investigated. In campaniform sensilla a transepithelial voltage (43.6±10.7 mV), which depends on an intact metabolism, occurs. In spider slit sensilla no transepithelial voltage exists. The occurrence and the lack of a transepithelial voltage is paralleled with differences in the ionic composition of the receptor lymph in the two arthropod sensilla. We used double-barrelled ion-selective microelectrodes to measure potassium and calcium content in the receptor lymph with respect to the hemolymph. The potassium concentration in campaniform sensilla (121±15 mM) is five times larger than that of the wing hemolymph (25±7 mM) and nine times larger than that of the haltere hemolymph (13±3 mM). These differences are statistically significant. The calcium concentration in campaniform sensilla (0.8±0.5 mM) does not differ significantly from that of the hemolymph (1.2±0.7 mM). In spider slit sensilla no significant difference occurs between the potassium concentration of the receptor lymph (9.5 mM±5.5 mM) and that of the hemolymph (8±3 mM). The calcium concentration of the hemolymph (1.6±0.9mM) is 3 times higher than that of the receptor lymph (0.6±0.3 mM). This difference is significant.

Morphogenesis of mechanoreceptor and epidermal cells of crickets during the last instar, and its relation to molting-hormone level

Summary(1) The fine structure of the cercal campaniform sensilla and epidermal cells of Gryllus bimaculatus Deg. (Saltatoria, Gryllidae) was examined, and the ecdysteroid level was monitored throughout the last larval instar. (2) The epidermal cells show changes in shape, cytoplasmic inclusions and differentiation of the apical cell membrane, coupled to the phases of buildup and breakdown of the (cercus) cuticle. (3) The imaginal epicuticle of the epidermal cells begins to form later (by about approximately 6 h) than that of the campaniform sensilla. (4) The campaniform sensilla were studied with respect to (a) the morphogenesis of the cuticular apparatus, (b) the inclusion of phenol oxidases in the cuticular apparatus, and (c) changes in the sensory apparatus preparatory to molting. (5) After apolysis the folding of the tormogen-cell wall into microvilli transiently disappears. Microvilli re-form shortly before imaginai ecdysis, and at the same time an outer receptor-lymph space develops. The role of the tormogen-cell “plaques” is discussed. (6) The levels of α- and β-ecdysone were determined separately by radioimmunoassay. (7) At the beginning of the instar the hormone level, especially that of β-ecdysone, falls. Prior to apolysis, the concentration of α-ecdysone rises, reaching an intermediate peak after apolysis is complete. The maximum hormone concentration (approximately 2,000 ng/g) is reached after the cuticulin layer is deposited, primarily due to the increase in β-ecdysone. While the proecdysial cuticle is forming, the hormone titer is reduced; at this time β-ecdysone is its chief component. (8) The identification of the ecdysteroids monitored by radioimmunoassay was confirmed by gas chromatography.