Neural Coding with Graded Membrane Potential Changes and Spikes

The neural encoding of sensory stimuli is usually investigated for spike responses, although many neurons are known to convey information by graded membrane potential changes. We compare by model simulations how well different dynamical stimuli can be discriminated on the basis of spiking or graded responses. Although a continuously varying membrane potential contains more information than binary spike trains, we find situations where different stimuli can be better discriminated on the basis of spike responses than on the basis of graded responses. Spikes can be superior to graded membrane potential fluctuations if spikes sharpen the temporal structure of neuronal responses by amplifying fast transients of the membrane potential. Such fast membrane potential changes can be induced deterministically by the stimulus or can be due to membrane potential noise that is influenced in its statistical properties by the stimulus. The graded response mode is superior for discrimination between stimuli on a fine time scale.     

Hierarchical Perception of Stimuli During Case Construction in the Bagworm Moth Eumeta crameri (Lepidoptera: Psychidae)

We studied case construction behavior of bagworm moth, Eumeta crameri (Lepidoptera: Psychidac) in the host plant, Acacia nilotica, at two sites. At the time of the study, Site A had 1-year-old A. nilotica saplings only, whereas Site B had full grown trees. The larva of bagworm moth used either thorns or cut-twigs or both as the materials for building its case. It renovated its case three times, and during each instance it increased the volume of its case by replacing older thorns or cut-twigs by newer and longer ones. However, it exhibited a spectacular predilection for thorns, irrespective of the sites, during the first-instar stage of its development. Thereafter, at Site A it used exclusively cut-twigs, whereas at Site B it preferably used thorns provided they were available in the range of the required length and within the threshold distance. It appears that the bagworm has an ability to process thorn length and distance signals hierarchically to fulfill its priority, that is, case building. Thus it optimizes time and energy expenditures during the period of its larval growth leading to pupation by toggling its preference between thorns and cut-twigs.     

DasFestuca vivipara-Problem in den Alpen

The differences betweenFestuca vivipara (tetraploid) andF. ovina subsp.supina (diploid), two often confused taxa, are demonstrated in regard to morphology, leaf anatomy, ecology and distribution. New maps illustrate the distribution ofF. vivipara in the Alps and the Northern Hemisphere. The development of different polyploidy levels withinF. vivipara as a consequence of occasional sexual processes is discussed. (English Summary on p. 39.)

     

Acoustic and Morphological Differentiation in the Frog Allobates femoralis: Relationships with the Upper Madeira River and Other Potential Geological Barriers

We studied patterns of call acoustics and external morphological differentiation in populations of the dart-poison frog Allobates femoralis occurring in forested areas along a 250-km stretch of the upper Madeira River, Brazil. Multivariate analyses of variance using principal components representing shared acoustic and morphological parameters distinguished three groups in relation to call structure and external morphology: (1) populations belonging to a two-note call morphotype; (2) populations with four-note calls inhabiting the left riverbank; and (3) populations with four-note calls inhabiting the right riverbank. Our results report a case of Amazonian anuran diversity hidden by current taxonomy and provide evidence for the upper Madeira River being a boundary between distinct populations of A. femoralis , and suggest a new taxonomic interpretation for these groups. Samples that did not fit into the general differentiation pattern and the existence of a well-defined contact zone between two morphotypes on the left riverbank indicate that mechanisms complementary to river-barrier hypotheses are necessary to explain the phenotypic differentiation between populations. Our study shows that at least one anuran species shows congruence between population differentiation and separation by a large Amazonian river, as documented for birds and mammals. Conservation efforts should not consider the taxon now known as A. femoralis as a homogeneous entity. There is much within-taxon variability, which can be probably explained partly by the existence of cryptic species, partly by geological barriers and part of which currently has no obvious explanation.     

Flight behaviour of Encarsia formosa in response to plant and host stimuli

The flight behaviour of Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a parasitoid of Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) has been studied in response to the following natural components: bean plant (Phaseolus vulgaris L.) infested with T. vaporariorum (PHC, plant host complex), host damaged bean plant from which whiteflies were removed (HDP, host damaged plant), T. vaporariorum third instar larvae (H, host), uninfested bean plant (P, plant) and T. vaporariorum empty pupal cases (EPC, empty pupal cases). A combination of chemical and visual stimuli is necessary in host location from a distance by E. formosa and seems to deeply affect the dispersal of this parasitoid. The presence of host (PHC, H) or of its parts (EPC) led to a substantial increase of oriented flights suggesting the occurrence of host derived chemicals. At a closer range, a combination of visual and chemical stimuli deriving from all the components of the plant host complex elicited the transition flying-landing and the following landing on the source.     

Mechanisms of host discrimination and intraspecific competition in the aphid parasitoid Ephedrus cerasicola

The aphid parasitoid Ephedrus cerasicola Starý (Hymenoptera: Aphidiidae) discriminates between parasitized and unparasitized aphid hosts (Myzus persicae (Sulzer), Homoptera: Aphididae) in two different ways. By antennal contact with the parasitized host, it can detect an external marking pheromone which is effective only for a few hours. The behaviour of the parasitoid also indicated that it responded to an internal marker, probably by means of chemoreceptors on the ovipositor. The parasitoid behaviour was studied in encounters with parasitized hosts 0–10 min, 2–3 h, 6–7 h and 23–24 h after the first parasitization. Aphids were superparasitized within 1 h or after 24 h, and aphids in each experiment were later dissected at two different times. It is suggested that the supernumerary offspring were eliminated by chemical means, whether the time between the two ovipositions was 0–1 h or 24 h.

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Résumé Le parasitoïde de puceron, E. cerasicola Starý (Hym.: Aphidiidae) sélectionne les pucerons, Myzus persicae (Sulzer), parasités ou non, de deux façons différentes. Il peut percevoir une phéromone externe de marquage perceptible seulement pendant quelques heures par contact antennaire. Son comportement montre qu'il réagit aussi à un marquage interne, probablement par des chémorécepteurs de sa tarière. Le comportement du parasitoïde a été étudié en le mettant en contact avec des hôtes parasités depuis 0–10 min, 2–3 h, 6–7 h et 23–24 h. Les pucerons ont été surparasités pendant la première heure ou après 24 heures. Les pucerons de toutes les expériences ont été disséqués à deux moments différents. Ceci a conduit à estimer que les descendants en surnombre ont été éliminés chimiquement quand la période entre deux pontes successives était inférieure à 1 h ou supérieure à 24 h.

     

Mercury concentrations in hair exposed in vitro to mercury vapor

Hair is often used as an index of environmental and industrial exposure to different metals. The interpretation of metal levels in hair is difficult because of the risk of external contamination. The aim of this study was to define the degree of external contamination of hair exposed in vitro to mercury vapor. Specimens of hair were exposed to concentration: 0.026, 0.21, and 2.7 mg Hg/m³ for 2–28 d. Mercury levels in hair increased during 28 d of exposure 2, 3 and 13, times, respectively, when compared to initial values. Mercury levels in hair exposed to the first and second (but not third) concentration of mercury vapor attained a steady state on the 21st d of exposure. The contamination of hair with mercury could not be removed by washing with water, solvent, and detergent. Hair may be used as an index of internal uptake of mercury provided that it was not externally exposed to mercury vapor. In cases of occupational exposure to mercury vapor, hair could become a useful tool for monitoring exposures.