Sensory projections of identified coxal hair sensilla of the scorpionHeterometrus fulvipes (Scorpionidae)

The topography of long hair sensilla on the coxae of walking legs and pedipalps of the scorpionHeterometrus fulvipes is described. Identified long hair sensilla are cobalt filled, and central projections of sensory fibres are reported for the first time in the suboesophageal ganglion of this scorpion. The afferent fibres arising from each long hair sensilla segregate into ventral, dorsomedial and dorsal tracts upon their entry into the suboesophageal ganglion. These transverse tracts bifurcate towards the middle of the leg neuromere and form three ipsilateral, plurisegrnental, longitudinal sensory pathways. Filling a pair of bilaterally distributed long hair sensilla shows bilaterally arranged longitudinal afferent tracts interconnected by distinct transverse commissures. Similar patterns of sensory projections are observed when filling homologous hairs on other legs and pedipalps. Numerous fine collaterals arise from the longitudinal sensory trancts that subdivide and end in small blebs presumed to be presynaptic endings. The dorsal and dorsomedial longitudinal tracts and their respective commissures are in close association with the dendritic arborisations of pedipalpal and leg motor neurons, suggesting direct contact between them. The probable functions of these multisegmental hair afferent pathways are discussed.     

Coxal hair plates in spiders: physiology,fine structure,and specific central projections

Coxal hair-plate sensilla in the spider Cupiennius salei are described with respect to their innervation, central projection pattern, electrical response to mechanical stimulation, and putative behavioral function.

Rhythmic changes in the activity of neurosecretory cells of the scorpion, Heterometrus fulvipes (C. Koch)

Changes in the activity of the neurosecretory cells in the cephalothoracic nerve mass (CTNM) of the scorpion, Heterometrus fulvipes (C. Koch) was studied over a 24-h period under natural light-dark cycles. Cytological and cytometric observations revealed that the first 3 groups of type 'A' cells in the sub-oesophageal ganglion of CTNM evidenced ultradian variations in their activity. Two peaks were seen in the activity of these neurosecretory cells, at 12(00) and 00(00). It is inferred that the cells show a biphasic rhythm in the synthesis of two different principles possibly having antagonistic effects on the central nervous system.     

Neural factors in the regulation of oxygen consumption rhythm in the tissues of the scorpion, Heterometrus fulvipes (C. Koch)

Circadian variation in the effect of cephalothoracic nerve mass (CTNM) extract and blood of the oxygen consumption of the isolated hepatopancreas of the scorpion, Heterometrus fulvipes, was studied. Both CTNM extract and blood from 2000 animal cause maximum increase in oxygen consumption. Another maximum increase was also noted at 0800. It is inferred that a chemical factor from the CTNM is responsible for the observed changes in oxygen consumption of hepatopancreas.     

Ultrastructure of the nerve cells and sensilla of Geocentrophora baltica (Platyhelminthes,Lecithoepitheliata) and the surface sensilla in the Geocentrophora group

Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata). Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups). Different kinds of vesicles and neuronal release sites are observed. Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities. In G. baltica and five endemic Geocentrophora spp. from Lake Baikal six types of surface sensillum were observed at the epidermal surface: 1. those with a long thin rootlet; 2. a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet; 3. a rootlet branching into many striated bundles; 4. a thick rootlet; 5. a reduced rootlet and numerous neurotubules;and 6. collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli. The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela. The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians.     

Proprioceptors involved in stinging response of the honeybee, Apis mellifera

Two types of mechanosensitive proprioceptor organ are present on the stinging apparatus of the honeybee: campaniform sensilla and mechanosensory hairplates. The campaniform sensilla are located on the surface of the tapering sting-shaft, which comprises an unpaired stylet and paired lancets. Each sensillum on the lancet differs from that on the stylet in terms of their topography and external morphology. The sensory afferents of the campaniform sensilla display slow-adapted firing responses to deformation of the cuticle that would be caused by the action of inserting the sting into a substrate, and their afferent signals induce and/or prolong the stinging response. By contrast, the mechanosensory hairplates are located at basal cuticular plates and on the posterior surface of the lancet valves. Two fields of hairplates on the second ramus at the ventral edge of the groove and on the antero-lateral edge of the oblong plate respond synchronously to protraction of the lancet. During the stinging response, these hairplates are likely to detect any sliding movement of the lancet and its position relative to the stylet. Afferent signals produced by them are likely to provide important information to the neuronal circuit for the generation and modulation of the stinging motor pattern.     

The mechanism of sensory transduction in the sensilla of the trochanteral hair plate of the cockroach,Periplaneta americana

Summary The trochanteral hair plate of the cockroach leg contains approximately 60 hair sensilla that are deflected by a joint membrane during flexion of the leg. Previous work has shown that the organ is a mechanoreceptor which limits leg flexion during walking by reflex connections to flexor and extensor motoneurons. Functional analysis of the largest sensilla has shown that their behaviour may be well approximated by a velocity detector followed by a unidirectional rectifier.We report here the results of an examination of the largest sensilla by scanning and transmission electron microscopy in an attempt to correlate the structure with the known functional elements. Each hair is innervated by a single sensory dendrite which is surrounded by an electron dense dendritic sheath. The dendrite terminates below the hair shaft in a tubular body containing a parallel array of microtubules embedded in an electron dense matrix, while the dendritic sheath extends beyond the tubular body to form the walls of the ecdysial canal. At the proximal end of the tubular body the dendritic sheath and sensory dendrite are anchored to the cuticular socket by a fibrous dome which seems to form a fulcrum around which the tubular body can be deflected by movements of the hair. We suggest that the basis for the detection of velocity may be mechanical differentiation by a fluid space between the dendritic sheath and the tubular body. The structure is also discussed with relation to the mechanism of sensory transduction and the possible causes of the unidirectional sensitivity.Supported by the Canadian Medical Research Council. The authors gratefully acknowledge the expert technical assistance of Sita Prasad     

Neuronal components of the circadian clock in the scorpion,Androctonus australis: Central origin of the efferent neurosecretory elements projecting to the median eyes

Summary The central origin of efferent neurosecretory fibers (ENSF) supplying the median eyes of the scorpion, Androctonus australis, with circadian information was investigated at the light- and electron-microscopic levels by the use of several tracer techniques. The results obtained convincingly show that the central neurons stained via the optic nerve are the source of the ENSF in the retina. Almost all of these neurons give rise to two different axon collaterals, one of which projects into the ipsilateral, the other into the contralateral median-eye nerve. Three protocerebral contact zones of the ENSF system are described, and their functional role within the circadian clock system is discussed.This investigation represents a portion of the Ph. D. thesis of one of the authors (S.H.) and was supported by grants to G.F. from the Deutsche Forschungsgemeinschaft (SFB 45).     

Electrophysiological characterisation of olfactory sensilla in the black bean aphid, Aphis fabae

Electrophysiological responses of three different olfactory sensilla (proximal primary rhinaria (PPR), distal primary rhinaria (DPR) and secondary rhinaria (SR)) to the sex pheromone components, nepetalactol, nepetalactone, and a plant volatile, (E)-2-hexenal, were investigated in four different morphs of the black bean aphid, Aphis fabae Scopoli. The DC responses recorded directly from the antennal sensilla and termed electrosensillograms (ESGs) were generally much larger (up to 14 mV) than electroantennogram (EAG) responses (up to 2.5 mV). Characteristic morph-specific response profiles to these compounds were observed in each type of rhinarium and response waveforms were different between (E)-2-hexenal and the sex pheromone components. (E)-2-Hexenal elicited the largest responses at PPR, while nepetalactol and nepetalactone elicited the largest responses at SR in gynoparae and males. Nepetalactol and nepetalactone also showed significant activities on DPR and PPR in all morphs. In contrast, (E)-2-hexenal had almost no activity on SR. However, almost all of the SR investigated in males and gynoparae were sensitive to both nepetalactol and nepetalactone. A small sub-set of male SR responded mainly to nepetalactol. SR of winged virginoparae did not respond to the sex pheromone compounds. Paraffin oil (the solvent control) also elicited significant responses at PPR in virginoparae but not in other morphs. In a further experiment, SR of winged virginoparae showed no response to 30 other plant volatile compounds or the alarm pheromone component, (E)-beta-farnesene. Nepetalactol and nepetalactone had similar dose-response profiles in the SR of gynoparae. The results indicate that SR in males and gynoparae are highly specialised to detect sex pheromone compounds, while the DPR and PPR are relatively broadly tuned to both plant volatiles and sex pheromone components. The presence of SR in winged virginoparae that are not responsive to sex pheromone components, alarm pheromone, or any of the plant volatile compounds tested may indicate a possible role of these sensilla to detect, as yet, unknown compound(s) with a high specificity. The present study also suggests that PPR may play a role in detecting paraffin-related compounds such as cuticular hydrocarbons of plants.     

Central nervous projections of mechanoreceptors in the spider Cupiennius salei Keys.

Summary The basic organization of sensory projections in the suboesophageal central nervous system of a spider (Cupiennius salei Keys.) was analyzed with anterograde cobalt fills and a modified Golgi rapid method. The projections of three lyriform slit sense organs and of tactile hairs located proximally on the legs are described and related to central nerve tracts. There are five main longitudinal sensory tracts in the central region of the suboesophageal nervous mass arranged one above the other. Whereas the three dorsal ones contain fibers from the lyriform organs, the two ventral ones contain axons from the hair receptors. Axons from all three lyriform organs have typical shapes and widely arborizing ipsilateral intersegmental branches and a few contralateral ones. The terminal branches of the afferent projections from identical lyriform organs on each leg form characteristic longitudinal pathways, typical of each organ: U-shaped, O-shaped, or two parallel bundles. The terminations of the hair sensilla are ipsilateral and intersegmental. Two large bilaterally arranged longitudinal sensory association tracts receive inputs from all legs including the dense arborizations from tactile hairs, lyriform organs, and other sense organs. These tracts may serve as important integrating neuropils of the suboesophageal central nervous system.     

Expression, regulation and function of the homeobox gene empty spiracles in brain and ventral nerve cord development of Drosophila

We analyse the role of the empty spiracles (ems) gene in embryonic brain and ventral nerve cord development. ems is differentially expressed in the neurectoderm of the anterior head versus the trunk region of early embryos. A distal enhancer region drives expression in the deutocerebral brain anlage and a proximal enhancer region drives expression in the VNC and tritocerebral brain anlage. Mutant analysis indicates that in the anterior brain ems is necessary for regionalized neurogenesis in the deutocerebral and tritocerebral anlagen. In the posterior brain and VNC ems is necessary for correct axonal pathfinding of specific interneurons. Rescue experiments indicate that the murine Emx2 gene can partially replace the fly ems gene in CNS development.     

Localization of [14C]DDT in the ventral nerve cord of the cockroach,Periplaneta americana (L.)

[¹⁴C]DDT was used as a probe to determine the subcellular localization of DDT in the ventral nerve cord (VNC) of the cockroach, Periplaneta americana (L.). Male cockroaches were injected intra-abdominally with [¹⁴C]DDT and their VNCs removed at 1 h post-injection. The VNCs were then subjected to homogenization and differential centrifugation to isolate plasma membrane, mitochondrial, and microsomal fractions. Results indicate that the plasma membrane fraction contained the greatest amount of [¹⁴C]DDT, with the mitochondrial and microsomal fractions containing significantly less. Calculations and a comparison with I₅₀ values for oligomycin-sensitive (OS)Mg-ATPase from the literature support the prediction that an insufficient amount of DDT reaches the ventral nerve cord mitochondria of a cockroach to effect an I₅₀ level of inhibition of the (OS)Mg-ATPase.     

Neurogenesis in larval stages of the spider crab Hyas araneus (Decapoda,Brachyura): proliferation of neuroblasts in the ventral nerve cord

Larvae of the spider crab Hyas araneus were reared in the laboratory from hatching through to metamorphosis. Neurogenesis was recorded in the ventral nerve cord during development of successive larval stages, zoea 1, zoea 2, megalopa and crab 1. Proliferating cells were detected immunocytochemically after in vivo labelling with 5-bromo-2-deoxyuridine (BrdU) which, as a thymidine analogue, is incorporated into the DNA of dividing cells. Segmental sets of mitotically highly active neuroblasts were found in newly hatched larvae. A dorsal neuroblast, a ventral-median neuroblast, 3–6 anterior-ventral neuroblasts and 1–3 lateral neuroblasts could be distinguished in each thoracic ganglion. Significantly fewer neuroblasts were labelled in the suboesophageal ganglion as compared to the thoracic ganglia. The number of active labelled neuroblasts was high throughout zoea 1 and about 30% of zoea 2 development and then dramatically decreased towards premetamorphosis. In the newly moulted megalopa, only a reduced set of neuroblasts was labelled which ceased dividing within the first few days of megalopa development. There is an indication that, although most ganglion mother cells born by unequal division of neuroblasts may go through their final division at an early stage, certain clusters of ganglion mother cells obviously delay their final mitosis. These results are discussed with regard to neuronal integration which necessarily changes during the course of metamorphosis in relation to the altered behavioural repertoire of the larvae.     

The effect of ventral nerve cord severance and male castration on female mating behavior, clutch size, and maternal care in the ring-legged earwig

Mating is critical for the expression of oviposition and maternal care in the earwig, Euborellia annulipes; additionally, mating diminishes receptivity to additional mating and promotes a decline in juvenile hormone synthesis at the end of the gonadotrophic cycle (in contrast to most insect species wherein mating stimulates juvenile hormone production). We report here that severance of the ventral nerve cord of virgin females similarly promoted egg deposition and maternal care of eggs, diminished mating receptivity, and elicited a timely decline in juvenile hormone biosynthesis. Mating of intact females to adult males that were castrated as larvae did not abolish oviposition; however, clutch size was reduced, and no eggs developed. Such castrated males had smaller seminal vesicles than did intact males, presumably attributable to lack of sperm in castrated males. In contrast, mating of intact females to males castrated on day 1 of adult life did not reduce clutch size compared with those of sham-operated animals and did not abolish fertilization; in fact, these castrated males produced viable offspring after six matings. These results are consistent with the notion that ventral nerve cord severance mimicked mating in intact animals. Following mating, the ventral nerve cord likely is a conduit to release the brain from inhibiting oviposition and maternal care. The presence of sperm in the spermatheca is not necessary for release of this inhibition but may modulate clutch size.