Visualization of ensemble activity patterns of mechanosensory afferents in the cricket cercal sensory system with calcium imaging

The cercal sensory system of the cricket mediates the detection and analysis of low velocity air currents in the animal's immediate environment, and is implemented around an internal representation of air current direction that demonstrates the essential features of a continuous neural map. Previous neurophysiological and anatomical studies have yielded predictions of the global spatio-temporal patterns of activity that should be evoked in the sensory afferent map by air current stimuli of different directions. We tested those predictions by direct visualization of ensemble afferent activity patterns using Ca2+ -sensitive indicators. The AM ester of the fluorescent Ca2+ indicator (Oregon Green 488 BAPTA-1 AM) was injected under the sheath of a cercal sensory nerve containing all of the mechanosensory afferent axons from one cercus. Optical signals were recorded with a digital intensified CCD camera. Control experiments using direct electrical stimulation of stained and unstained nerves demonstrated that the observed Ca2+ signals within the terminal abdominal ganglion (TAG) were due to activation of the dye-loaded sensory afferent neurons. To visualize the spatial patterns of air-current-evoked ensemble activity, unidirectional air currents were applied repeatedly from eight different directions, and the optically recorded responses from each direction were averaged. The dispersion of the optical signals by the ganglion limited the spatial resolution with which these ensemble afferent activity patterns could be observed. However, resolution was adequate to demonstrate that different directional stimuli induced different spatial patterns of Ca2+ elevation in the terminal arbors of afferents within the TAG. These coarsely- resolved, optically-recorded patterns were consistent with the anatomy-based predictions.

Parasitology: parasite survives predation on its host

As prisoners in their living habitat, parasites should be vulnerable to destruction by the predators of their hosts. But we show here that the parasitic gordian worm Paragordius tricuspidatus is able to escape not only from its insect host after ingestion by a fish or frog but also from the digestive tract of the predator. This remarkable tactic enables the worm to continue its life cycle.

Complex Multivariate Sexual Selection on Male Acoustic Signaling in a Wild Population ofTeleogryllus commodus

Mate choice may impose both linear (i.e., directional) and nonlinear (i.e., quadratic and correlational) sexual selection on advertisement traits. Traditionally, mate recognition and sensory tuning have been thought to impose stabilizing (i.e., negative quadratic) sexual selection, whereas adaptive mate choice effects directional selection. It has been suggested that adaptive choice may exert positive quadratic and/or correlational sexual selection. Earlier, we showed that five structural components of the advertisement call of male field crickets (Teleogryllus commodus) were under multivariate stabilizing selection under laboratory conditions. Here we experimentally estimate selection on these five traits plus a measure of calling activity (the number of repeats in a looped bout of calling) in the field. There was general support for multivariate stabilizing selection on call structure, and calling activity was under strong positive directional selection, as predicted for a signal of genetic quality. There was, however, also appreciable correlational selection, suggesting an interaction between male call structure and calling effort. Interestingly, selection for short interbout durations of silence favored longer intercall durations in the field, in contrast to results from continuous looped call playback in the laboratory. We discuss the general importance of nonlinear selection in the honest signaling of genetic quality.

Evolutionary genetics of juvenile hormone and ecdysteroid regulation in Gryllus: a case study in the microevolution of endocrine regulation

During the past 15 years the first detailed synthesis of endocrinology and population genetics has begun, in which natural genetic variations for endocrine regulators have been characterized, almost exclusively in species of the cricket genus Gryllus. Artificial selection studies have documented that regulators of the juvenile hormone titer can rapidly evolve and exhibit levels of genetic variability similar to other physiological traits. Strong genetic correlations exist between some but not all regulators of the JH titer during the juvenile stage. No genetic correlation exists between regulators functioning in juvenile and adult stages, and thus, endocrine regulation can evolve independently in these stages. Genetic variation in the JH titer, the ecdysteroid titer, and JHE activity, in adult and juvenile stages, have been documented in genetic stocks of wing-polymorphic crickets; morph-specific differences in these endocrine traits are potentially responsible for genetically based differences in aspects of wing and flight muscle development, adult egg production, and adult dispersal. An unexpected morph-specific, genetic polymorphism for a circadian rhythm for the JH titer was observed in both the laboratory and field. Few comparable studies exist in non-Gryllus species, in which in vivo endocrine-genetic variation has been directly quantified using reliable analytical methods; many reported cases of endocrine variation in these species have been obtained using an inappropriate method and thus should be considered unsubstantiated. Obtaining basic information on the characteristics of natural genetic variation for endocrine regulators still remains one of the most important tasks of the fledgling subdiscipline of evolutionary endocrinology. Single gene endocrine mutants in Drosophila are promising candidates for investigating molecular-genetic variation in natural populations. Future studies should also focus on endocrine traits studied in the field and geographic variation in endocrine regulation.

Pulses, patterns and paths: neurobiology of acoustic behaviour in crickets

Crickets use acoustic communication for pair formation. Males sing with rhythmical movements of their wings and the mute females approach the singing males by phonotaxis. Females walking on a trackball rapidly steer towards single sound pulses when exposed to split-song paradigms. Their walking path emerges from consecutive reactive steering responses, which show no temporal selectivity. Temporal pattern recognition is tuned to the species-specific syllable rate and gradually changes the gain of auditory steering. If pattern recognition is based on instantaneous discharge rate coding, then the tuning to the species-specific song pattern may already be present at the level of thoracic interneurons. During the processing of song patterns, changes in cytosolic Ca(2+ )concentrations occur in phase with the chirp rhythm in the local auditory interneurone. Male singing behaviour is controlled by command neurons descending from the brain. The neuropil controlling singing behaviour is located in the anterior protocerebrum next to the mushroom bodies. Singing behaviour is released by injection of cholinergic agonists and inhibited by gamma-butyric acid (GABA). During singing, the sensitivity of the peripheral auditory system remains unchanged but a corollary discharge inhibits auditory processing in afferents and interneurons within the prothoracic auditory neuropil and prevents the auditory neurons from desensitisation.

Cannibal crickets on a forced march for protein and salt

Swarming and mass migration are spectacular and sometimes devastating features of the biology of various animal species. These phenomena are typically associated with actual or anticipated depletion of food resources after an increase in population density, but the mechanisms driving such collective movements are poorly understood. Here we reveal that insects in large, coordinated migratory bands consisting of millions of Mormon crickets in western North America were deprived of two essential nutritional resources: protein and salt. The insects themselves provided a major source of these nutrients, and cannibalism was rife. We show that protein and salt satiation reduced cannibalism and that protein satiation inhibited walking. Additionally, experimentally reducing the motility or mobility of crickets substantially increased their risk of being cannibalized by other band members. As a result, the availability of protein and salt in the habitat will influence the extent to which bands march, both through the direct effect of nutrient state on locomotion and indirectly through the threat of cannibalism by resource-deprived crickets approaching from the rear. The crickets are, in effect, on a forced march. Migratory band formation and subsequent mass movement, therefore, are manifestations of specific tradeoffs between the costs and benefits of group living. Bands afford antipredator benefits to individual group members. Group movement then mitigates the resulting costs of intraspecific competition, namely local depletion of nutritional resources and the associated increased risk of cannibalism.

Chirp rate is independent of male condition in a synchronising bushcricket

Males of the bushcricket Mecopoda elongata synchronise their chirps with neighbouring males, but because synchrony is imperfect, one male's chirp preceeds the other by some 50-200 ms. Since a male's intrinsic chirp rate is critical for the establishment of the leader role in a duet, and females prefer the leader in a choice situation, we investigated a possible condition dependence of this male trait. In a duet leader males are usually those calling at a higher intrinsic rate; therefore, we investigated whether calling at a higher rate indicates male condition. The calling metabolism was quantified in a respirometer; the factorial slope of males calling at a high rate was three times higher compared to males calling at lower rates. Males produce on average 3.4 singing bouts/per night, and there is a significant increase in chirp periods (CPs) with successive singing bouts. Call properties were investigated throughout a male's life; chirp period increases significantly with age. Two groups of males were reared on either a low- or a high-nutrition diet, and the influence of male condition on different song parameters was investigated. CPs in two feeding regimes did not differ significantly, although males of the low-nutrition diet group were significantly affected by nutrition with respect to mortality, a delayed last moult and reduced weight as adults. We therefore conclude that solo chirp rates do not reflect phenotypic male condition properly.

Divergence in the calling songs between sympatric and allopatric populations of the southern wood cricket Gryllus fultoni (Orthoptera: Gryllidae)

In the eastern United States the wood cricket Gryllus fultoni (Orthoptera: Gryllidae) occurs in sympatry with G. vernalis in an area between eastern Kansas and west of the Appalachian Mountains. Calling songs were recorded from 13 sympatric and allopatric localities. Both field and laboratory recordings showed that chirp rate (CR) and pulse rate (PR) overlapped extensively between allopatric populations of G. fultoni and sympatric populations of G. vernalis; by contrast, there was little or no overlap in these variables between sympatric populations of these two species. Divergence in PR and CR between the two species was thus greater in areas of sympatry than in areas of allopatry. Our field and laboratory studies of G. fultoni calling songs thus demonstrate the pattern expected of character displacement and support the genetic assumptions of this hypothesis. Other possible explanations for the sympatric divergence such as ecological character displacement and clinal variation are discussed.

Effect of the temporal pattern of contralateral inhibition on sound localization cues

We studied the temporal coding properties of identified interneurons in the auditory system of crickets, using information theory as an analytical tool. The ascending neuron 1 (AN1), which is tuned to the dominant carrier frequency (CF) of cricket songs, selectively codes the limited range of amplitude modulation (AM) frequencies that occur in these signals. AN2, which is most sensitive to the ultrasonic frequencies that occur in echolocation calls of insectivorous bats, codes a broader range of AM frequencies, as occur in bat calls. A third neuron, omega neuron 1 (ON1), which is dually tuned to both ranges of carrier frequency, was shown previously to have CF-specific coding properties, allowing it to represent accurately the differing temporal structures of both cricket songs and bat calls. ON1 is a source of contralateral inhibition to AN1 and AN2, enhancing binaural contrast and facilitating sound localization. We used dichotic stimulation to examine the importance of the temporal structure of contralateral inhibition for enhancing binaural contrast. Contralateral inhibition degrades the coding of temporal pattern by AN1 and AN2, but only if the temporal pattern of inhibitory input matches that of excitation. Firing rate is also decreased most strongly by temporally matched contralateral inhibition. This is apparent for AN1 in its mean firing rate; for AN2, high-frequency firing is selectively suppressed. Our results show that the CF-specific coding properties of ON1 allow this single neuron to enhance effectively localization cues for both cricket-like and bat-like acoustic signals.

Genitalic autogrooming: a self-filling trash collection system in crickets

Insects groom almost all parts of the body surface with their legs and mouth parts. However, some body regions are difficult to reach and keep clean. One is the genital chamber located in the last abdominal segment in males which houses the phallic complex for copulation and production of the spermatophore. In the male cricket, foreign substances can enter the genital chamber when it is opened during copulation and spermatophore formation. Moreover, the dorsal pouch and ventral lobes of the phallic complex, which mould the attachment plate, tube, and ampulla of the spermatophore, are inevitably soiled as a result of spermatophore production. We found a unique cleaning system in which foreign substances accumulated during copulation and spermatophore debris left in the dorsal pouch after copulation are quickly removed and collected in special pockets in the genital chamber. This trash is collected by undulation of the genital chamber's membranous floor which is entirely covered by small scales ( approximately 10 microm) similar to those in the ovipositor of female crickets. This self-filling trash collecting system may be used in some other insects which produce the spermatophore in a similar manner to that of crickets.

The cellular basis of a corollary discharge

How do animals discriminate self-generated from external stimuli during behavior and prevent desensitization of their sensory pathways? A fundamental concept in neuroscience states that neural signals, termed corollary discharges or efference copies, are forwarded from motor to sensory areas. Neurons mediating these signals have proved difficult to identify. We show that a single, multisegmental interneuron is responsible for the pre- and postsynaptic inhibition of auditory neurons in singing crickets (Gryllus bimaculatus). Therefore, this neuron represents a corollary discharge interneuron that provides a neuronal basis for the central control of sensory responses.

Hair canopy of cricket sensory system tuned to predator signals

Filiform hairs located on the cerci of crickets are among the most sensitive sensors in the animal world and enable crickets to sense the faintest air movements generated by approaching predators. While the neurophysiological and biomechanical aspects of this sensory system have been studied independently for several decades, their integration into a coherent framework was wanting. In order to evaluate the hair canopy tuning to predator signals, we built a model of cercal population coding of oscillating air flows by the hundreds of hairs on the cerci of the sand cricket Gryllus bimaculatus (Insecta: Orthoptera). A complete survey of all hairs covering the cerci was done on intact cerci using scanning electronic microscopy. An additive population coding of sinusoid signals of varying frequencies and velocities taking into account hair directionality delivered the cercal canopy tuning curve. We show that the range of frequencies and velocities at which the cricket sensory system is best tuned corresponds to the values of signals produced by approaching predators. The relative frequencies of short (< 0.5 x 10(-3) m) and long hairs and their differing responses to oscillating air flows therefore enable crickets to detect predators in a time-frequency-intensity space both as far as possible and at close range.

Auditory lateralization in bushcrickets: a new dichotic paradigm

Pair formation in the bushcricket Gampsocleis gratiosa is achieved through acoustic signalling by the male and phonotactic approaches of the female towards the calling song. On a walking belt in the free sound field, females tracked the position of the speaker broadcasting the male calling song with a remarkable precision, deviating by no more than 10 cm in either direction from the ideal course. Starting with stimulus angles of 6-10 degrees the females significantly turned to the correct side, and with stimulus angles greater than 25 degrees no incorrect turns were made. Using neurophysiological data on the directionality of the ear we calculated that with such stimulus angles the available binaural intensity difference is in the order of 1-2 dB. We developed a dichotic ear stimulation device for freely moving females with a cross-talk barrier of about 50 dB, which allowed to precisely apply small binaural intensity differences. In such a dichotic stimulation paradigm, females on average turned to the tronger stimulated side starting with a 1 dB difference between both ears. The significance of such a reliable lateralization behaviour with small interaural intensity differences for phonotactic behaviour under natural conditions is discussed.

Transepithelial transport of fluorescent p-glycoprotein and MRP2 substrates by insect Malpighian tubules: confocal microscopic analysis of secreted fluid droplets

Transport of fluorescent substrates of p-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (MRP2) by insect Malpighian tubules was examined using confocal laser scanning microscopy (CLSM). Isolated tubules of the cricket Teleogryllus commodus accumulated the MRP2 substrate Texas Red in the cells and lumen at concentrations up to 20 and 40 times,respectively, those in the bathing medium. Quantitative CLSM analysis of fluorochrome transport in some cricket tubules and all Drosophilatubules was not practical because of interfering effects of concretions in the cells and lumen. Samples of fluid secreted by tubules set up in Ramsay assays were therefore collected in hollow rectangle glass capillaries. Transepithelial dye flux was calculated as the product of fluid secretion rate(measured in the Ramsay assay) and dye concentration (measured by CLSM of the fluid samples). Dose–response curves for transport and the ratio of dye concentration in the secreted fluid to that in the bathing medium (S/M) were determined for Texas Red as well as for P-gp substrates (rhodamine 123,daunorubicin), the organic anion fluorescein and the organic cation quinacrine. Transepithelial transport of Texas Red was reduced by the MRP2 inhibitors MK571 and probenecid. Transport of daunorubicin was reduced by the P-gp inhibitors verapamil and quinacrine and also by the organic cation tetraethylammonium. The results indicate the presence of P-gp-like and MRP2-like transporters in the Malpighian tubules of both species.

Mate Choice: Been There, Done That

Females may benefit from mating with several males, but how can they avoid mating with the same male repeatedly? A new study of crickets has found that females mark their mates to avoid repeat copulations.

Cryptic sexual conflict in gift-giving insects: chasing the chase-away

The chase-away model of sexual selection posits that elaborate male sexual displays arise because they exploit preexisting biases in females' sensory systems and induce females to mate in a suboptimal manner. An essential element of this hypothesis is that such manipulation should quickly lead to female resistance to male displays. Nuptial food gifts may be a frequent conduit by which males attempt to influence the mating behavior of females against females' own reproductive interests. In decorated crickets Gryllodes sigillatus, such inducements come in the form of a spermatophylax, a gelatinous mass forming part of the male's spermatophore and consumed by the female after mating. We conducted experiments in which spermatophylaxes obtained from male G. sigillatus were offered as novel food gifts to females of a non-gift-giving species (Acheta domesticus) having no evolutionary history of spermatophylax consumption. Female A. domesticus that were allowed to consume the spermatophylax took significantly longer to remate than when given no such opportunity. In contrast, when female G. sigillatus were prevented from consuming their partners' nuptial gifts, there was no difference in their propensity to remate relative to females permitted to consume a food gift after mating. These results suggest that the spermatophylax synthesized by male G. sigillatus contains substances designed to inhibit the sexual receptivity of their mates but that female G. sigillatus have evolved reduced responsiveness to these substances.

Corollary discharge inhibition and audition in the stridulating cricket

The romantic notion of crickets singing on a warm summer's evening is quickly dispelled when one comes ear to ear with a stridulating male. Remarkably, stridulating male crickets are able to hear sounds from the environment despite generating a 100 db song (Heiligenberg 1969; Jones and Dambach 1973). This review summarises recent work examining how they achieve this feat of sensory processing. While the responsiveness of the crickets' peripheral auditory system (tympanic membrane, tympanic nerve, state of the acoustic spiracle) is maintained during sound production, central auditory neurons are inhibited by a feedforward corollary discharge signal precisely timed to coincide with the auditory neurons' maximum response to self-generated sound. In this way, the corollary discharge inhibition prevents desensitisation of the crickets' auditory pathway during sound production.

Social Learning in Noncolonial Insects?

Social-information use has generated great interest lately and has been shown to have important implications for the ecology and evolution of species. Learning about predators or predation risk from others may provide low-cost life-saving information and would be expected to have adaptive payoffs in any species where conspecifics are observable and behave differently under predation risk. Yet, social learning and social-information use in general have been largely restricted to vertebrates. Here, we show that crickets adapt their predator-avoidance behavior after having observed the behavior of knowledgeable others and maintain these behavioral changes lastingly after demonstrators are gone. These results point toward social learning, a contingency never shown before in noncolonial insects. We show that these long-lasting changes cannot instead be attributed to long re-emergence times, long-lasting effects of alarm pheromones, or residual odor cues. Our findings imply that social learning is likely much more phylogenetically widespread than currently acknowledged and that reliance on social information is determined by ecological rather than taxonomic constrains, and they question the generally held assumption that social learning is restricted to large-brained animals assumed to possess superior cognitive abilities.

Nuptial feeding of spermless spermatophores in the Hawaiian swordtail cricket, Laupala pacifica (Gryllidae: Triginodiinae)

Crickets in the genus Laupala (subfamily Trigonidiinae) have an elaborate courtship system, defined by a highly ritualized serial transfer of multiple spermatophores. Males produce multiple "micro" spermatophores followed by a final "macro" spermatophore during a single mating bout. Remarkably, the microspermatophores of L. cerasina, the first species whose mating system was studied in detail, were discovered to be spermless. However, in a study of another species, L. pacifica, sperm transfer was reported after every copulation suggesting that L. pacifica microspermatophores contain sperm. The presence or absence of sperm in the microspermatophore has important implications for the evolution of this exaggerated courtship system and the origin of nuptial gifts. In this study, we systematically examined L. pacifica spermatophore contents for sperm using a fluorescent nuclear stain. We detected sperm only in macrospermatophores. This finding suggests that spermless microspermatophores are typical for Laupala; thus, to determine the origin of this highly modified phenotype will require comparative analyses with closely related outgroups that exhibit less exaggerated courtship systems.

Auditory orientation in crickets: pattern recognition controls reactive steering

Many groups of insects are specialists in exploiting sensory cues to locate food resources or conspecifics. To achieve orientation, bees and ants analyze the polarization pattern of the sky, male moths orient along the females' odor plume, and cicadas, grasshoppers, and crickets use acoustic signals to locate singing conspecifics. In comparison with olfactory and visual orientation, where learning is involved, auditory processing underlying orientation in insects appears to be more hardwired and genetically determined. In each of these examples, however, orientation requires a recognition process identifying the crucial sensory pattern to interact with a localization process directing the animal's locomotor activity. Here, we characterize this interaction. Using a sensitive trackball system, we show that, during cricket auditory behavior, the recognition process that is tuned toward the species-specific song pattern controls the amplitude of auditory evoked steering responses. Females perform small reactive steering movements toward any sound patterns. Hearing the male's calling song increases the gain of auditory steering within 2-5 s, and the animals even steer toward nonattractive sound patterns inserted into the species-specific pattern. This gain control mechanism in the auditory-to-motor pathway allows crickets to pursue species-specific sound patterns temporarily corrupted by environmental factors and may reflect the organization of recognition and localization networks in insects.

Bacterial immunity traded for sperm viability in male crickets

The notion that a trade-off exists between immunity and reproduction is now a central concept in theories of sexual selection. However, whether such a trade-off exists between immunity and gamete viability has not been established. Here we show that genetic variance for high levels of an immune response required to fight bacterial infections is associated with genetic variance for low sperm viability. These data have implications for our understanding of sexual selection mechanisms and of reproductive costs in male longevity.

Discrimination of Conspecific Individuals via Cuticular Pheromones by Males of the Cricket Gryllus bimaculatus

Cuticular substances on the body surface of crickets serve as pheromones that elicit a variety of different behaviors in male crickets. Antennal contact between males and females resulted in courtship behavior, and that between two males resulted in aggressive displays. As a first step in elucidating how crickets recognize and discriminate individuals, behavioral responses of male individuals to cuticular substances of conspecific males or females were investigated. The behavioral responses of males to antennal or palpal stimulation with an isolated antenna from a male or a female were recorded. To both antennal and palpal stimulation with female antennae, the majority of males responded with courtship behavior; to stimulation with male antennae, males responded with aggressive displays. To gain insight into the chemical nature of the behaviorally relevant components, isolated antennae were washed in either n-hexane, acetone or ethanol before behavior assays. Washed antennae no longer elicited courtship or aggressive responses in males. Next, polypropylene fibers were smeared with substances from the body surface of females and used for antennal stimulation. This experiment showed that the quality and quantity of cuticular substances appear to be highly age-dependent. Significantly more males responded with courtship behavior to cuticular substances from younger females. Isolated males generally showed higher levels of aggression than males reared in groups. Grouped males also were more likely to display courtship behavior towards antennae from younger females, and aggressive behavior towards antennae from older females. These results suggest that male discrimination of mating partners depends on the nature of female cuticular substances.