The hind wing of the desert locust (Schistocerca gregaria Forskal). III. A finite element analysis of a deployable structure.
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Finite element analysis is used to model the automatic cambering of the locust hind wing during promotion: the umbrella effect. It was found that the model required a high degree of sophistication before replicating the deformations found in vivo. The model has been validated using experimental data and the deformations recorded both in vivo and ex vivo. It predicts that even slight modifications to the geometrical description used can lead to significant changes in the deformations observed in the anal fan. The model agrees with experimental data and produces deformations very close to those seen in free-flying locusts. The validated model may be used to investigate the varying geometries found in orthopteran anal fans and the stresses found throughout the wing when loaded. (+info)
The reduction of chromosome number in meiosis is determined by properties built into the chromosomes.
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In meiosis I, two chromatids move to each spindle pole. Then, in meiosis II, the two are distributed, one to each future gamete. This requires that meiosis I chromosomes attach to the spindle differently than meiosis II chromosomes and that they regulate chromosome cohesion differently. We investigated whether the information that dictates the division type of the chromosome comes from the whole cell, the spindle, or the chromosome itself. Also, we determined when chromosomes can switch from meiosis I behavior to meiosis II behavior. We used a micromanipulation needle to fuse grasshopper spermatocytes in meiosis I to spermatocytes in meiosis II, and to move chromosomes from one spindle to the other. Chromosomes placed on spindles of a different meiotic division always behaved as they would have on their native spindle; e.g., a meiosis I chromosome attached to a meiosis II spindle in its normal fashion and sister chromatids moved together to the same spindle pole. We also showed that meiosis I chromosomes become competent meiosis II chromosomes in anaphase of meiosis I, but not before. The patterns for attachment to the spindle and regulation of cohesion are built into the chromosome itself. These results suggest that regulation of chromosome cohesion may be linked to differences in the arrangement of kinetochores in the two meiotic divisions. (+info)
Correlation of diversity of leg morphology in Gryllus bimaculatus (cricket) with divergence in dpp expression pattern during leg development.
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Insects can be grouped into mainly two categories, holometabolous and hemimetabolous, according to the extent of their morphological change during metamorphosis. The three thoracic legs, for example, are known to develop through two overtly different pathways: holometabolous insects make legs through their imaginal discs, while hemimetabolous legs develop from their leg buds. Thus, how the molecular mechanisms of leg development differ from each other is an intriguing question. In the holometabolous long-germ insect, these mechanisms have been extensively studied using Drosophila melanogaster. However, little is known about the mechanism in the hemimetabolous insect. Thus, we studied leg development of the hemimetabolous short-germ insect, Gryllus bimaculatus (cricket), focusing on expression patterns of the three key signaling molecules, hedgehog (hh), wingless (wg) and decapentaplegic (dpp), which are essential during leg development in Drosophila. In Gryllus embryos, expression of hh is restricted in the posterior half of each leg bud, while dpp and wg are expressed in the dorsal and ventral sides of its anteroposterior (A/P) boundary, respectively. Their expression patterns are essentially comparable with those of the three genes in Drosophila leg imaginal discs, suggesting the existence of the common mechanism for leg pattern formation. However, we found that expression pattern of dpp was significantly divergent among Gryllus, Schistocerca (grasshopper) and Drosophila embryos, while expression patterns of hh and wg are conserved. Furthermore, the divergence was found between the pro/mesothoracic and metathoracic Gryllus leg buds. These observations imply that the divergence in the dpp expression pattern may correlate with diversity of leg morphology. (+info)
Auditory symmetry analysis.
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The study of biological symmetry continues to be an important and active area of research, yet in the hearing sciences there are no established quantitative methods for measuring auditory asymmetries and dissimilarities in threshold tuning curves (i.e. audiograms). Using a paired design and adopting methods from the analysis of fluctuating asymmetry, we describe methods for auditory researchers interested in delineating auditory asymmetries and comparing tuning curves, behavioral or neural. We illustrate the methods using audiograms of the prothoracic T-cell interneuron in a nocturnal katydid (Neoconocephalus ensiger). The results show that 87-92 % of T-cells had right-minus-left threshold asymmetries no larger than expected from measurement error alone. Thus, apart from small random fluctuating asymmetries, T-cell pairs in N. ensiger showed no sensory bias and were bilaterally symmetrical from 5 to 100 kHz. The sensitivity of the methods for detecting tuning curve dissimilarities was confirmed in a sound lateralization paradigm by comparing the 'symmetry' (i.e. similarity) of T-cell tuning curves measured at 0 degrees stimulation with tuning curves measured at 90 degrees stimulation for the same T-cell. The results show that T-cell thresholds measured frontally (0 degrees ) were significantly higher than those measured laterally (90 degrees ), particularly for ultrasonic frequencies. Statistically, the directional shift (increase) in auditory thresholds was detected as a directional asymmetry in T-cell tuning, whose origin and functional significance to an insect behaving normally are discussed. The paper discusses practical considerations for detecting auditory asymmetries and tuning curve dissimilarities in general, and closes by questioning the relevance of auditory symmetry for sound localization in both vertebrates and insects. (+info)
Neuroethology of the katydid T-cell. I. Tuning and responses to pure tones.
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The tuning and pure-tone physiology of the T-cell prothoracic auditory interneuron were investigated in the nocturnal katydid Neoconocephalus ensiger. The T-cell is extremely sensitive and broadly tuned, particularly to high-frequency ultrasound (>20 kHz). Adult thresholds were lowest and showed their least variability for frequencies ranging from 25 to 80 kHz. The average best threshold of the T-cell in N. ensiger ranged from 28 to 38 dB SPL and the best frequency from 20 to 27 kHz. In females, the T-cell is slightly more sensitive to the range of frequencies encompassing the spectrum of male song. Tuning of the T-cell in non-volant nymphs was comparable with that of adults, and this precocious ultrasound sensitivity supports the view that it has a role in the detection of terrestrial sources of predaceous ultrasound. In adults, T-cell tuning is narrower than that of the whole auditory (tympanic) organ, but only at audio frequencies. Superthreshold physiological experiments revealed that T-cell responses were ultrasound-biased, with intensity/response functions steeper and spike latencies shorter at 20, 30 and 40 kHz than at 5, 10 and 15 kHz. The same was also true for T-cell stimulation at 90 degrees compared with stimulation at 0 degrees within a frequency, which supports early T-cell research showing that excitation of the contralateral ear inhibits ipsilateral T-cell responses. In a temporal summation experiment, the integration time of the T-cell at 40 kHz (integration time constant tau =6.1 ms) was less than half that measured at 15 kHz ( tau =15.0 ms). Moreover, T-cell spiking in response to short-duration pure-tone trains mimicking calling conspecifics (15 kHz) and bat echolocation hunting sequences (40 kHz) revealed that temporal pattern-copying was superior for ultrasonic stimulation. Apparently, T-cell responses are reduced or inhibited by stimulation with audio frequencies, which leads to the prediction that the T-cell will encode conspecific song less well than bat-like frequency-modulated sweeps during acoustic playback. The fact that the T-cell is one of the most sensitive ultrasound neurons in tympanate insects is most consistent with it serving an alarm, warning or escape function in both volant and non-volant katydids (nymphs and adults). (+info)
Neuroethology of the katydid T-cell. II. Responses to acoustic playback of conspecific and predatory signals.
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Although early work on the tettigoniid T large fiber suggested that it might mediate early-warning and escape behavior in katydids, the majority of research thereafter has focused on the ability of the T-cell to detect, localize and/or discriminate mate-calling song. Interestingly, T-cell responses to conspecific song are rarely examined for more than a few seconds, despite the fact that many katydids sing for minutes or hours at a time. In this paper, the second of a pair examining the physiology of the T-cell in Neoconocephalus ensiger, we recorded T-cell responses using longer-duration playbacks (3 min) of conspecific song (Katydid signal 30 ms syllables, 9-25 kHz bandwidth, 12-15 kHz peak frequency) and two types of bat-like ultrasound, a 10 ms, 80->30 kHz frequency-modulated sweep (Bat 10 signal) and a 30 ms, 80->30 kHz frequency-modulated sweep (Bat 30 signal). Spiking responses were distinctly biased towards the short-duration ultrasonic signal, with more spikes per pulse, at a shorter spike latency and at a higher instantaneous firing frequency to the Bat 10 signal than to the Katydid signal or, surprisingly, to the Bat 30 signal. The ability of the T-cell to encode the temporal pattern of the stimulus was particularly striking. Only for the predatory bat signals did T-cell spiking faithfully copy the stimulus; playbacks of conspecific song resulted in significantly weaker spiking responses, particularly in male katydids. The results demonstrate that responses from the T-cell alone may be sufficient for katydids to discriminate biologically relevant signals pertinent to the phonotactic behavior patterns involved in mate attraction and predator avoidance. (+info)
Nitric oxide and cGMP influence axonogenesis of antennal pioneer neurons.
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The grasshopper embryo has been used as a convenient system with which to investigate mechanisms of axonal navigation and pathway formation at the level of individual nerve cells. Here, we focus on the developing antenna of the grasshopper embryo (Schistocerca gregaria) where two siblings of pioneer neurons establish the first two axonal pathways to the CNS. Using immunocytochemistry we detected nitric oxide (NO)-induced synthesis of cGMP in the pioneer neurons of the embryonic antenna. A potential source of NO are NADPH-diaphorase-stained epithelial cells close to the basal lamina. To investigate the role of the NO/cGMP signaling system during pathfinding, we examined the pattern of outgrowing pioneer neurons in embryo culture. Pharmacological inhibition of soluble guanylyl cyclase (sGC) and of NO synthase (NOS) resulted in an abnormal pattern of pathway formation in the antenna. Axonogenesis of both pairs of pioneers was inhibited when specific NOS or sGC inhibitors were added to the culture medium; the observed effects include the loss axon emergence as well as retardation of outgrowth, such that growth cones do not reach the CNS. The addition of membrane-permeant cGMP or a direct activator of the sGC enzyme to the culture medium completely rescued the phenotype resulting from the block of NO/cGMP signaling. These results indicate that NO/cGMP signaling is involved in axonal elongation of pioneer neurons in the antenna of the grasshopper. (+info)
Tension on chromosomes increases the number of kinetochore microtubules but only within limits.
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When chromosomes attach properly to a mitotic spindle, their kinetochores generate force in opposite directions, creating tension. Tension is presumed to increase kinetochore microtubule number, but there has been no direct evidence this is true. We micromanipulated grasshopper spermatocyte chromosomes to test this assumption and found that tension does indeed affect the number of kinetochore microtubules. Releasing tension at kinetochores causes a drop to less than half the original number of kinetochore microtubules. Restoring tension onto these depleted kinetochores restores the microtubules to their original number. However, the effects of tension are limited. Prometaphase kinetochores, when under normal tension from mitotic forces, have about half as many microtubules as they will in late metaphase. We imposed a tension force of 6 x 10(-5) dynes, three times the normal tension, on prometaphase kinetochores. The elevated tension did not drive kinetochore microtubule number above normal prometaphase values. Tension probably increases the number of kinetochore microtubules by slowing their turnover rate. The limited effect of tension at prometaphase kinetochores suggests that they have fewer microtubule binding sites than at late metaphase. The relatively few sites available in prometaphase may be the decisive sites whose binding of microtubules regulates the dynamics of transient kinetochore constituents, including checkpoint components. (+info)