The research, conducted in Tsukuba, Ibaraki Prefecture, documents how overwintering adults in reproductive diapause coordinate their activity with the diel thermal cycle. Unlike most diapausing insects, which are expected to stay put and conserve their fat reserves, these grasshoppers move up and down through as much as 12 centimeters of leaf litter every day, emerging around midday and hiding again in the afternoon. The behavior is striking because the animals are sluggish, vulnerable, and gain no obvious foraging benefit from their excursions, since most plants in their winter habitat are wilted.

Field observations in a semi-natural outdoor enclosure revealed how tightly the movements track temperature. The daily maximum number of adults appearing on the litter surface correlated strongly with air temperature, and in January and February typically fewer than 60 percent of the insects emerged on any given day. By March and April, more than 80 percent appeared regularly. Adults were immobile below about minus 1 degree Celsius, could stand up once their body temperatures reached roughly 3.7 degrees Celsius on average, yet under semi-natural conditions they generally did not leave the litter until temperatures approached 14 degrees Celsius. Intriguingly, the body temperature at which they emerged in the morning, about 14.1 degrees Celsius, was virtually identical to the temperature at which they began hiding in the afternoon, about 14.0 degrees Celsius.

That symmetry posed a puzzle: if the same body temperature triggers both behaviors, what tells the grasshopper to climb up in the morning but burrow down in the afternoon? The answer lay in the trend of the temperature rather than its absolute value. During the hour before the first adult emerged from the litter, the litter temperature rose in every single case observed, with a mean increase of 8.9 degrees Celsius. During the hour before the first adult hid, the temperature fell in 92.5 percent of cases, with a mean change of minus 5.0 degrees Celsius. Rising warmth apparently signals the insects to move toward light, while falling warmth signals them to move away from it.

To disentangle the roles of light and heat, the researcher ran a series of elegant tube experiments. Transparent horizontal tubes were divided into a dark half covered with black paper and a light half open to illumination. Adults placed in the dark half migrated into the light around midday, and adults placed in the light returned to the dark in the afternoon, reproducing the natural cycle in a simplified setting. Crucially, the light half was often slightly cooler than the dark half during the morning transition, meaning the grasshoppers were moving against the thermal gradient. They were not simply seeking warmth.

Further tests ruled out thermotaxis as the driver of the afternoon retreat. When a cardboard screen blocked sunlight from the dark half, making it cooler than the light half, adults still moved into the dark. When an incandescent bulb artificially heated the light half in the afternoon, the insects still abandoned it for the dark. In both cases the animals moved into a colder microhabitat, demonstrating that once the daily switch in phototaxis is triggered, the grasshoppers follow light cues independently of any temperature gradient.

The most decisive experiment probed whether temperature change alone could override time of day, circadian rhythm, and ambient light. On winter evenings, after all adults had naturally retreated into the dark halves of their tubes, treatment groups were suddenly transferred from cold outdoor conditions into a warm room at 20 degrees Celsius under artificial lighting. Within an hour, 50 to 70 percent of these insects appeared in the light halves of their tubes, even though it was dark outside and they would normally be hidden. Control insects that had already experienced warm indoor temperatures, and therefore encountered only a small temperature increase on transfer, remained in the dark. A sufficiently large, rapid warming, it turns out, can commandeer the animals’ phototactic response at any hour.

From these results, the study concludes that phototaxis in overwintering P. japonica is controlled by the direction, magnitude, and absolute range of temperature change. Rising temperatures elicit positive phototaxis, drawing the insects out of the litter and into the light; falling temperatures elicit negative phototaxis, sending them back down. The final decision of whether to stay above ground for the night is made late in the day by monitoring litter temperature, since the number of adults remaining on the litter at 18:00 correlated best with temperatures recorded at that hour rather than earlier in the afternoon. The behavior is therefore flexible rather than rigidly programmed, integrating current thermal information before committing to a course of action.

Why would a diapausing grasshopper take such risks? The author suggests that periodic sun-bathing may serve as hygiene. Grasshoppers are notoriously susceptible to fungal pathogens during cold, wet weather, and an earlier attempt to overwinter the insects in poorly ventilated containers ended with moldy corpses. Regularly drying and warming their bodies in sunlight may suppress fungal infection, while the intermittent warming may also temporarily boost metabolism in a way that benefits the dormant animals. The cryptic body coloration of the adults, which blends with the litter and soil, offers some protection, but the insects cannot jump or fly effectively when cold, leaving them exposed to winter-active birds and small mammals.

The findings add a curious chapter to the overwintering biology of insects, a field dominated by studies of diapause physiology and cold hardiness rather than behavior. Comparable daily shuttling occurs in other arthropods, such as pill bugs that are attracted to light by day and repelled by it in the evening, and in locusts that emerge from grass clumps for a few hours around midday in winter. What sets P. japonica apart is that it performs these movements in the depths of winter, in a species that is not a migratory locust, and that the switch between attraction and aversion to light is governed by a quantifiable thermal rule. The author notes that the underlying mechanism remains unknown, and points to biogenic amines such as octopamine and tyramine, known modulators of insect phototaxis, as promising candidates for future investigation into how a grasshopper’s nervous system translates a warming trend into a turn toward the light.

Subject of Research: Temperature-dependent phototaxis controlling daily vertical movements in overwintering grasshoppers

Article Title: Temperature-dependent phototaxis in overwintering adults of the grasshopper Patanga japonica ( Orthoptera , Acrididae )

Article References: Tanaka, S. (2024). Temperature-dependent phototaxis in overwintering adults of the grasshopper Patanga japonica (Orthoptera, Acrididae). Journal of Orthoptera Research, 33(1), 71-86. https://doi.org/10.3897/jor.33.102749

Image Credits: AI Generated

DOI: 10.3897/jor.33.102749

Keywords: Patanga japonica, grasshopper, phototaxis, overwintering, diapause, thermoregulation, insect behavior, leaf litter, temperature change, entomology, Japan, winter ecology