Patrick A. Guerra

Evaluating the life-history trade-off between dispersal capability and reproduction in wing dimorphic insects: a meta-analysis.

A life‐history trade‐off exists between flight capability and reproduction in many wing dimorphic insects: a long‐winged morph is flight‐capable at the expense of reproduction, while a short‐winged morph cannot fly, is less mobile, but has greater reproductive output. Using meta‐analyses, I investigated specific questions regarding this trade‐off. The trade‐off in females was expressed primarily as a later onset of egg production and lower fecundity in long‐winged females relative to short‐winged females. Although considerably less work has been done with males, the trade‐off exists for males among traits primarily related to mate acquisition. The trade‐off can potentially be mitigated in males, as long‐winged individuals possess an advantage in traits that can offset the costs of flight capability such as a shorter development time. The strength and direction of trends differed significantly among insect orders, and there was a relationship between the strength and direction of trends with the relative flight capabilities between the morphs. I discuss how the trade‐off might be both under‐ and overestimated in the literature, especially in light of work that has examined two relevant aspects of wing dimorphic species: (1) the effect of flight‐muscle histolysis on reproductive investment; and (2) the performance of actual flight by flight‐capable individuals.

A magnetic compass aids monarch butterfly migration

Convincing evidence that migrant monarch butterflies (Danaus plexippus) use a magnetic compass to aid their fall migration has been lacking from the spectacular navigational capabilities of this species. Here we use flight simulator studies to show that migrants indeed possess an inclination magnetic compass to help direct their flight equatorward in the fall. The use of this inclination compass is light-dependent utilizing ultraviolet-A/blue light between 380 and 420 nm. Notably, the significance of light <420 nm for inclination compass function was not considered in previous monarch studies. The antennae are important for the inclination compass because they appear to contain light-sensitive magnetosensors. For migratory monarchs, the inclination compass may serve as an important orientation mechanism when directional daylight cues are unavailable and may also augment time-compensated sun compass orientation for appropriate directionality throughout the migration.

Flight behaviour attenuates the trade-off between flight capability and reproduction in a wing polymorphic cricket

Flight-dimorphic insects have been used extensively to study trade-offs between energetically costly traits. Individuals may develop and maintain structures required for flight, or alternatively they may invest in reproduction. Previous experiments have not examined whether flight itself might affect investment into reproduction. As in other Gryllus species, flight-capable individuals of the wing polymorphic cricket, Gryllus texensis, incur an apparent reproductive penalty for being able to fly, expressed as smaller ovaries in females and lower courtship propensity in males, than their flight-incapable counterparts. We find that a short bout of flight eliminates the trade-off. Two days after the flight, the ovaries of flight-capable females were comparable with those of short-winged females. Similarly, flight markedly increased the probability of courtship behaviour. Our results suggest that the impact of the flight–reproduction trade-off described in earlier studies may have been overestimated.

A Life History Trade-Off Between Flight Ability and Reproductive Behavior in Male Field Crickets (Gryllus texensis)

Male field crickets (Gryllus texensis) that differ in flight ability incur a life history trade-off between flight ability and reproduction, where flight ability comes with a male fitness cost. In courtship trials, flight-capable males produced courtship song, a necessary signal for mating success, with a significantly lower probability than flight-incapable males. The trade-off was evident in young males, and a similar trend occurred in older males. Males that lost the ability to fly through histolysis of flight muscles produced courtship song with a similar probability as males incapable of flight for their entire lives. Time of day did not affect the expression of the trade-off. Neither male morph nor time of day influenced female mating behavior.

Coldness Triggers Northward Flight in Remigrant Monarch Butterflies

Each fall, eastern North American monarch butterflies (Danaus plexippus) migrate from their northern range to their overwintering grounds in central Mexico. Fall migrants are in reproductive diapause, and they use a time-compensated sun compass to navigate during the long journey south. Eye-sensed directional cues from the daylight sky (e.g., the horizontal or azimuthal position of the sun) are integrated in the sun compass in the midbrain central complex region. Sun compass output is time compensated by circadian clocks in the antennae so that fall migrants can maintain a fixed flight direction south. In the spring, the same migrants remigrate northward to the southern United States to initiate the northern leg of the migration cycle. Here we show that spring remigrants also use an antenna-dependent time-compensated sun compass to direct their northward flight. Remarkably, fall migrants prematurely exposed to overwintering-like coldness reverse their flight orientation to the north. The temperature microenvironment at the overwintering site is essential for successful completion of the migration cycle, because without cold exposure, aged migrants continue to orient south. Our discovery that coldness triggers the northward flight direction in spring remigrants solves one of the long-standing mysteries of the monarch migration.

Neurobiology of Monarch Butterfly Migration

Studies of the migration of the eastern North American monarch butterfly (Danaus plexippus) have revealed mechanisms behind its navigation. The main orientation mechanism uses a time-compensated sun compass during both the migration south and the remigration north. Daylight cues, such as the sun itself and polarized light, are processed through both eyes and integrated through intricate circuitry in the brains central complex, the presumed site of the sun compass. Monarch circadian clocks have a distinct molecular mechanism, and those that reside in the antennae provide time compensation. Recent evidence shows that migrants can also use a light-dependent inclination magnetic compass for orientation in the absence of directional daylight cues. The monarch genome has been sequenced, and genetic strategies using nuclease-based technologies have been developed to edit specific genes. The monarch butterfly has emerged as a model system to study the neural, molecular, and genetic basis of long-distance animal migration.

Discordant timing between antennae disrupts sun compass orientation in migratory monarch butterflies

To navigate during their long-distance migration, monarch butterflies (Danaus plexippus) use a time-compensated sun compass. The sun compass timing elements reside in light-entrained circadian clocks in the antennae. Here we show that either antenna is sufficient for proper time compensation. However, migrants with either antenna painted black (to block light entrainment) and the other painted clear (to permit light entrainment) display disoriented group flight. Remarkably, when the black-painted antenna is removed, re-flown migrants with a single, clear-painted antenna exhibit proper orientation behaviour. Molecular correlates of clock function reveal that period and timeless expression is highly rhythmic in brains and clear-painted antennae, while rhythmic clock gene expression is disrupted in black-painted antennae. Our work shows that clock outputs from each antenna are processed and integrated together in the monarch time-compensated sun compass circuit. This dual timing system is a novel example of the regulation of a brain-driven behaviour by paired organs.

Colonists and desperadoes: different fighting strategies in wing-dimorphic male Texas field crickets

In many species, males fight one another for mates or resources. Fighting can vary among males, however, such that some males are highly aggressive whereas others are not. We observed that fighting was negatively associated with flight capability in the wing-dimorphic field cricket, Gryllus texensis. Long-winged males capable of flight had a significantly lower fighting propensity and displayed lower levels of aggression than short-winged, flight-incapable males in staged contests. Long-winged males no longer able to fly as a result of the histolysis of their flight muscles were more aggressive than long-winged males that had functional flight muscles, but were still less aggressive than short-winged males. Long-winged, flight-capable males were more aggressive after a brief flight just prior to an interaction with a rival, but only in contests with other flight-capable males. Flight capability and flight itself had no effect on the outcome of fights. Our results support the existence of an aggression flight syndrome in flight-dimorphic insects.

CALLING COMMUNICATION IN MEADOW KATYDIDS (ORTHOPTERA, TETTIGONIIDAE): FEMALE PREFERENCES FOR SPECIES-SPECIFIC WINGSTROKE RATES

Summary During their search for a mate, female Conocephalus brevipennis (Orthoptera: Tettigoniidae) exhibit different phonotactic preferences. We presented females with calling song stimuli in no-choice playback experiments to determine the physical form of the sound signals they prefer to approach. Females were attracted by sustained noise, a signal lacking any speciesspecific amplitude modulation (AM) but possessing the ultrasonic carrier band typical of Conocephalus spp. This stimulus approximates the collective output of distant aggregations of calling males in which the species-specific AM features of the song are obscured by the grouped singing. When presented with close-range broadcasts, where detailed song features were not obscured (i.e. the call of a single male), female preference was always for conspecific calling song. We also studied female preference for various modifications of the major physical components of the buzz-tick-gap structure of C. brevipennis calling song to uncover parameters

Panoploscelis specularis (Orthoptera: Tettigoniidae: Pseudophyllinae): extraordinary female sound generator, male description, male protest and calling signals

Abstract Females of Panoploscelis specularis present a dramatic modification of their forewings for stridulation. The female generator is illustrated and its distinct form contrasted with that of males. The physical form of the signals that females might produce is inferred; male calling and protest signals are characterized. The male of P. specularis is described for the first time.