Appl. Entomol. Zool. 42 (3): 473–478 (2007) http://odokon.org/

Light compass in the provisioning navigation of the subsocial shield bug, japonensis (: )

Mantaro HIRONAKA,1,* Sumio TOJO2 and Takahiko HARIYAMA1 1 Department of Biology, Faculty of Medicine, Hamamatsu University School of Medicine; Hamamatsu, Shizuoka 431–3192, Japan 2 Department of Applied Biological Science, Faculty of Agriculture, Saga University; Saga 840–8502, Japan (Received 10 March 2007; Accepted 25 April 2007)

Abstract The female of the subsocial shield bug, , provisions food to her nymphs using path integra- tion, a type of navigational strategy that acquires information indicating the traveler’s direction and distance. In this study we investigated whether and how P. japonensis females use a light compass involved in orientation at a constant angle with respect to a light source. First, we let the female run toward its burrow in an experimental room when a lamp set on the floor was turned on. Then, we changed the direction of the light source 180° horizontally by switching off the lamp and turning on another lamp on the opposite side. In response to this change, the homing bug turned back immediately. Next, we examined the effects of the light source from two different elevations. It accomplished its path integration task when the light was placed at an azimuth of 45°. When the light was at the zenith (90°), however, the bug lost the correct home direction. These results suggest that in their path integration system P. japonensis use light sources as compass references and, further, that they can estimate their direction from a point located at mid-sky but not the zenith.

Key words: Homing; navigation; path integration; sun compass; subsocial shield bug

(Wehner, 1992). Path integration, which refers to a INTRODUCTION method for computing one’s present location, re- Many use a light compass for long-dis- quires external or internal information indicating tance navigation, which involves orientation at a the navigator’s instantaneous direction and distance constant angle with respect to a celestial light of trip (Mittelstaedt and Mittelstaedt, 1982). Cen- source, such as the sun, the moon, or the stars (von tral-place foragers such as ants also use the sun as Buddenbrock, 1917). For navigators with a means for defining direction in the path integra- sensory access to the sky, celestial cues undoubt- tion (Santschi, 1911, 1923). edly offers angular information because they are In addition to the well-known navigators of Hy- effectively at infinity and thus not subject to the menoptera, the female of the subsocial shield bug, phenomenon of motion parallax. Therefore, the in- Parastrachia japonensis, which belongs to Het- sect navigators can readily use the celestial light eroptera, is also a remarkable navigator. The fe- sources and/or celestial cues derived from the light male forages on the forest floor and provides food sources as a means for defining direction (Rossel for its nymphs by walking. In mid-June, after the and Wehner, 1984; Wehner, 1984; Dacke et al., nymphs hatch, the female leaves its burrow to find 2003; Homberg, 2004). Central-place foragers like drupes, fallen fruit with a hard stone-like en- social hymenopterans are semi long-distance navi- dosperm, from the host tree, Schoepfia jasmin- gators, have been observed to make the round trip odora (Olacaceae: Rosidae: Santalales) (Tachikawa between their nest and foraging sites regularly in and Schaefer, 1985; Tsukamoto and Tojo, 1992). their environment using the path integration The female searches for drupes far away from its

*To whom correspondence should be addressed at: E-mail: [email protected] DOI: 10.1303/aez.2007.473

473 474 M. HIRONAKA et al. burrow, which is located usually more than 5 m fed them on the ripe drupes of a single host tree, S. from the foraging area (Filippi-Tsukamoto et al., jasminodora. A plastic lid was used to cover the 1995; Filippi et al., 2001). After the bug leaves its cups to keep the bugs from escaping. We turned burrow, it searches arduously until it encounters a the light source in the incubator on at 5:00 and off drupe. When a drupe is discovered, the bug always at 19:00 in order to imitate the appropriate seasonal takes the shortest route back to its burrow using outdoors conditions. path integration (Hironaka et al., 2007a, b). We conducted the experiments in a temperature- It is possible that P. japonensis uses the visual controlled room (85m and 2.5 m high). To acti- information in its path integration because it has vate the provisioning behavior, the temperature in been observed that direct homing females become the experimental room was kept at 252°C. The disoriented to their burrow direction when visual floor was covered with soil and fallen leaves to information is disturbed by the painting of the simulate the natural forest floor. In addition, the compound eyes and ocelli (Hironaka et al., 2003). whole area of the floor was transacted with yellow The question of what kind of visual information lines into a grid of 50 cm mesh to allow us to the bug acquires, however, still remains to be an- record the bugs’ paths on a reduced scale. All walls swered. and the ceiling were covered with a blackout cur- In this study, we investigated whether P. japo- tain. A compact fluorescent lamp (EFA25ED/22, nensis females use a direct light source as a com- Matsushita Electric Industrial Corp. Ltd., Tokyo, pass reference in their provisioning navigation. Japan) equivalent to a 100-watt incandescent bulb was used as the light source. After the females’ eggs hatched, we continued to MATERIALS AND METHODS rear the females with their nymphs on drupes until We carried out laboratory experiments from the nymphs reached the third stadium. Then we mid-June to mid-July 2000 and 2004 at Saga Uni- conducted the following two experiments during versity in Japan. We collected P. japonensis fe- the daytime from 14:00 to 17:00. In all experimen- males guarding their egg masses from the field site, tal trials, we tested those bugs that had never left Hinokuma-yama, a small forested hill in Saga Pre- the burrow before the start of the trial and used fecture, Japan (33°16 N, 130°16 E), during June. each individual bug only once. We reared egg-guarding females individually in Experiment 1: Horizontal shift of light. We clear plastic cups (diameter 8 cm; height 4 cm) carried out a horizontal shift experiment to investi- lined with substrate (soil and fallen leaves) for 1 gate the possibility that the homing bug uses a light week at 22°C on a 14:10 h light-dark regime and source for directional reference. We set an experi- mental plastic burrow and a square feeding site (one side 10 cm) containing several ripe drupes on the floor at a distance of 1 m. Two light bulbs (shown as ‘a’ and ‘b’ in Fig. 1) were placed on the floor (elevation of 0°) at a distance of 4 m. The axis between the two bulbs was perpendicular to the axis between the burrow and the feeding site. The light switching was controlled by the experimenter when the bug crossed the line between the two bulbs. First, only lamp ‘a’ was switched on. When the lid of the cup was opened, the female sponta- neously started the provisioning trip for its young, that is, the female left the burrow to search on the Fig. 1. Three-dimensional schematic representation of the floor in a wandering path. When it reached the experimental set-up, in which light sources were set in the room so that a bug located at the intersection would see the feeding site and found a drupe, the female started light from horizontal directions (a or b) as in Experiment 1, an to return to its burrow showing a direct trajectory, elevation of 45° (c) or 90° (d) in Experiment 2. so that it walked roughly toward the intersection Light Compass in Shield Bug 475 point of the axis between the two bulbs and the males in the 45° and 90°, respectively. axis between the burrow and feeding site. When Statistics. Statistical analysis of the distributions the female crossed the line between the two bulbs, of homing directions was performed according to we immediately switched light ‘a’ off and turned the methods reported by Batschelet (1981). For the light ‘b’ on. At the same time, we placed a metal distribution, the mean resultant vector was calcu- marker where the bug was located precisely at the lated and a V test was applied to determine whether switching point. For the control experiment, we did the distribution differed from randomness. not change the position of the light when the fe- male reached the intersection point, that is, we kept RESULTS lamp ‘a’ turned on for the duration of the experi- ment. We did, however, as with the experimental Experiment 1: Horizontal shift of light group, place a metal marker on the point where the Each bug, once its burrow had been placed in bug crossed the line between the two bulbs. To the experimental room, foraged around on the ex- measure the homing direction, we scratched a cir- perimental floor until it encountered a ripe drupe, cle 50 cm in diameter with a compass centered on which had been placed at the feeding site. In the the metal marker. A small flag was placed at the homing trip, it walked back toward its burrow di- point where the bug crossed the circle, and we then rectly while dragging the drupe in its proboscis fol- drew twine between the metal marker and the flag, lowing the line between the feeding site and the and between the metal marker and the burrow, re- burrow. The bugs under the control conditions spectively. Next, using a protractor, we measured homed with great precision. The homing directions the interior angle between the two pieces of twine of the control bugs were significantly clustered to- as the homing direction. wards their burrows (Fig. 2A; V test: u4.219, If the bug did not reach the feeding site within N10, p0.0001) when measured at the crossing 15 min or take a wide detour around the direct out- point of the line between the two bulbs. In the ex- bound course, we ceased the trial and omitted it perimental condition, the light position was moved from the data. This criterion was also used in Ex- to the opposite side when the homing bug carrying periment 2. In the Experiment 1, we analyzed 10 a drupe crossed the line between the two bulbs. As data of 28 and 22 females in the control and the ex- soon as the position of the lamp was shifted, the periment conditions, respectively. bug stopped and turned or rotated several times. A Experiment 2: Effect of the zenithal position few seconds later, the bug started to walk in the op- of light. Only one light was used in Experiment 2, posite direction, moving away from its own burrow. placed at an elevation of either 45° or 90° (zenith) After walking to a place near the feeding site, the from the intersection point (Fig. 1). The positional bug showed typical burrow-searching behavior arrangement between the burrow and the feeding (Fig. 2B). The homing directions of the experimen- site was same as that of Experiment 1 except for tal bugs were significantly clustered towards the the elevation of the light source. In these light set- feeding site (Fig. 2A; V test: u3.566, N10, tings, the angle at which the bug looked up at the p0.0001). lamp from the burrow or feeding site was ca. 76°. When the bug reached the feeding site and found a Experiment 2: Effect of the zenithal position of drupe, we placed a metal marker to designate the light point of drupe discovery. To determine the homing In this experiment, each bug foraged around and direction, we scratched a circle of 50 cm in diame- returned to its burrow under a lamp set at two dif- ter with a compass centered on the point of drupe ferent elevations, but not horizontally on the floor discovery in the feeding site. In addition, we placed as in Experiment 1 (Fig. 1). A comparison of the a small flag at the point where the bug crossed the two conditions, i.e., 45° or 90°, with respect to the circle. We measured the angle between the two elevation from the intersection to the lamp, the be- lines: the line from the burrow to the drupe discov- haviors as reflected in the outbound trajectories of ery point and the line from the drupe discovery the homing bugs were indistinguishable (not point to the crossing point of the circle. In this Ex- shown), both being similar to the case of the con- periment 2, we analyzed 14 data of 26 and 21 fe- trol bugs described in Experiment 1. However, 476 M. HIRONAKA et al.

Fig. 2. Effects of horizontal light-shift on homing behav- ior. (A) Distributions of homing directions of control () and experimental () bugs. The continuous arrow and the dotted arrow indicate mean vectors in the control and experimental conditions, respectively. ‘a,’ mean angle of orientation; ‘r,’ length of the mean resultant vector; ‘N,’ number of tested. (B) A typical example of the homing path of the experi- Fig. 3. Effects of light-source elevations on homing be- mental bug. The continuous line indicates the homing trajec- havior. When the light was at the 45° elevation, a typical ex- tory, and the following dashed line indicates the trajectory ample of the homing path (A) and the distribution of homing after the light shifted from ‘a’ to ‘b,’ when the bug crossed the directions (B) showed their direct homing. At the 90° eleva- intersection. The bug changed direction by nearly 180° and tion, the bugs lost their way to home (C, D). Arrows in the cir- took a direct route towards the opposite direction. cular graphs indicate mean vectors, respectively. ‘a,’ mean angle of orientation; ‘r,’ length of the mean resultant vector; ‘N,’ number of insects tested. there was an apparent difference in the inbound tra- jectories. When the light bulb was placed at the 45° elevation, the bugs left the feeding site and rather eral other species have shown that insects largely directly returned to their burrows (Fig. 3A). The navigate by a vector-based mechanism of orienta- homing directions in this case were significantly tion called path integration (Wehner, 1992; Collett clustered towards their burrows (Fig. 3B; V test: and Collett, 2000). In the path integration system, u4.820, N14, p0.0001), similar to those ob- an insect updates an accumulator that keeps a run- served in the control bugs in Experiment 1. On the ning tally of its current direction and distance upon other hand, when the bulb was placed at the 90° leaving a starting point, such as a nest, so that it (zenith) elevation, the homing bugs lost their way can always take a direct path back to the starting and showed wandering behaviour mainly around point. Hence, the insect in effect utilizes a compass the feeding site; moreover, the homing directions by which it can measure the angular components of were randomly distributed (Fig. 3C and D; V test: its movement (Collett and Collett, 2000). Gener- u1.495, N14, p0.05). ally, insects continually monitor their direction of travel using celestial light sources (Wehner, 1984). Santschi (1911) first demonstrated in mirror exper- DISCUSSION iments that ants use the angle subtended by the sun Behavioral studies in honeybees, ants, and sev- and the nest to guide their return trip to the nest Light Compass in Shield Bug 477 using path integration. For nocturnal foraging ants, mately coincides with the angle (ca. 76°) at which a moon compass response is equally feasible, and they looked up at the lamp from their burrow or has, in fact, been demonstrated by Santschi (1923) feeding site in Experiment 2. Given this observa- and Jander (1957). It has been reported that honey- tion, we can suspect that P. japonensis females are bees also use the sun compass in their path integra- either able to perceive the sun’s position relative to tion (von Frisch, 1965). Our results in Experiment the zenith accurately to within only a few degrees, 1 confirmed that P. japonensis use a similar light similar to honeybees (New and New, 1962), or that compass to monitor their angular movement as a they use different cues to recognize direction, as part of their path integration system. This is the has been observed in equatorial sandhoppers first report in the heteropterans to demonstrate the (Ugolini, 2002). Further investigation is needed to utilization of the light compass for path integration. clarify this point. Some insects are known to use stars as a light Numerous ecological and agricultural studies compass reference for their navigation in addition have demonstrated that attraction to artificial lights to the sun and moon (Sotthibandhu and Baker, generally occurs in heteropteran (e.g. Ito et al., 1979; Baker and Mather, 1982), but such utiliza- 1993; Kato et al., 1995; Schuh and Slater, 1995). tion of stars as a reference was doubted by Wehner Therefore, it is reasonable to suppose that the light (1984). In addition, environmental structures such compass is a basic and widely shared trait in Het- as canopy gaps may also function as references. In eroptera. However, among heteropteran, thus far fact, P. japonensis females have been observed to only one study has demonstrated the existence of provision for entire days using visual cues in the the light compass (Birukow, 1956). The study forest during the busiest provisioning period (Hi- showed that a water bug, Velia currens, uses the di- ronaka et al., 2003, 2007b). Thus, they seem to be rection of the sun when determining its escape re- able to use different cues as light compass refer- sponse. When an artificial light source in a room ences depending on the current celestial conditions was moved to another position, V. currens adjusted and structural surroundings in the field. its escape direction in accordance with the new az- For angle measurement in the sun compass, in- imuth. Further, when the light source was changed sects refer only to the azimuth of the sun and not to to be exposed to the bug from an angle between its elevation (Wehner, 1984). Therefore, if the sun 85° and 90°, the bug became completely disori- is directly in the zenith at noon, or its zenithal ented and tried to escape to an indefinite direction. angle is indistinguishable from 0°, then the sun Thus, as the present results show that a similar compass is useless around midday (Wehner, 1984). light compass mechanism also functions in P. japo- For example, the homing orientation in nensis, we can conclude that the mechanism is Cataglyphis ants has been found to not be influ- more general, or even universal, in heteropteran. enced even when the altitude of the sun was in- With respect to the foraging behavior of P. japo- creased by 30° using a mirror (Duelli and Wehner, nensis, the effects of several cues on the light com- 1973). Moreover, Lindauer (1957) confirmed, from pass have been confidently proven, and further observations in Ceylon (Sri Lanka), which is lo- studies of the light compass and its biological cated near the equator, that honeybee dances be- background in this species will provide new in- come disorientated when the sun passes within 2.5° sights into ecological and agricultural management of the zenith. Coinciding with the results of these in heteropteran. studies, our present study demonstrated that the ACKNOWLEDGEMENTS homing P. japonensis bugs became disoriented to their burrows by several degrees when the light We thank Dr. S. Nomakuchi of Saga University and Dr. L. was set to the zenith (Fig. 3C and D). This result Filippi of Hofstra University for their many useful comments. suggests that P. japonensis also measures the angle This research was partially supported by a Grant-in-Aid for JSPS Fellows, no. 16003679, 2004 from the Ministry of Edu- from the azimuth in their light compass system. We cation, Culture, Sports, Science and Technology of Japan. estimate that the P. japonensis females in our ob- servatory, located at 33° N, are typically exposed to REFERENCES the sun at its highest altitude of 80° at noon in their Baker, R. R. and J. G. Mather (1982) Magnetic compass provisioning season of late June, which approxi- sense in the large yellow underwing moth, Noctua 478 M. HIRONAKA et al.

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