Visual Orientation Performances of Desert Ants Toward As Tromeno

Visual Orientation Performances of Desert Ants

(cataglyp h is ~~COZQY)Toward As tromenotactic Directions and Horizon Landmarks

RUDIGER WEHNER Zoologisches Imtitut der Universitat Zurich

N MANY ORGANISMS different systems of (2) These hunting ants can be success- Idirection finding and position measuring fully trained to feeding places and will return are simultaneously involved in maintaining a regularly for many days when rewarded aftcr particular orientation course. Using a whole each predatory run. set of direction-indicatingstimuli of the phys- (3) The optically uniform natural envi- ical environment, these organisms may first ronment of the desert ant enables us to pre- increase the accuracy of their orientation per- sent visual patterns which can be exactly de formances and may also be able to measure fined in their stimulus properties and may their position in space even when one part of not be thought to interfere with other ter- their navigation system no longer receives in- restrial cues left uncontrolled. formation about the external directional stimuli. This presentation gives some experimental data on the visual orientation of desert ants toward astromenotactic courses and horizon landmarks involving the co-operation of different direction-findingsystems. Cataglyphis bicolor, an ant widely distributed in desert areas of northern Africa (refs. 1 and 2) and southwest Asia, is most suitable for investigations on optical orientation mechanisms, in comparison with most of the central European species : (1) Cataglyphis bicolor is a predatory and solitary hunter (fig. 1) ,never performing mass foraging along scent trails but mainly FIGURE 1. CatagZyphis bicolor foraging with orienting by means of visual cues. gaster placed upright in its typical position.

42 1 422 ANIMAL ORIENTATION AND NAVIGATION

Therefore, the two main demands for tended over the whole experimental area studying navigation systems in animals are (fig. 2A). To receive directional data from fulfilled by training Cataglyphis bicolor these graphs, a set of concentrically arranged under natural conditions : a precise presenta- circles was drawn around the point marking tion and measurement of the physical cues the start of the ant’s orientation course. and a well established method for recording Using this procedure, one obtains a fre- the animal’s response to the orienting stimuli. quency distribution in angular intervals of Here, I would like to deal with three as- 5 O. According to statistical methods of circu- pects of the ant’s visual orientation. First, it larly distributed data (ref. 7), a mean vector must be determined whether the ants choose can be calculated for every distance from the a compromise direction between an astro- start (figs. 2B and C). This vector represents menotactic angle and the direction toward a the mean direction of orientation as well as, horizon landmark when both angles compete by its length, the dispersion about the mean with each other or whether they decide alter- direction. natively between the two courses. Second this Before we can test orientation towards ce- presentation refers to adaptations of the vis- lestial and terrestrial cues by means of com- ual system to the special demands of direc- petition experiments, we first must prove the tion finding by astromenotactic orientation or accuracy and time compensation of the Sun- pattern recognition. Finally some data on the compass courses. In contrast to the circum- parameters of visual learning behavior will be stantial work with bees (refs. 8 to 10) and presented since all the ’results dealt with here some other insects (Orthoptera, Hemiptera, were obtained by using training experiments Coleoptera) , there exists only little evidence with well-defined learning parameters. for a time-compensated Sun-compass orientation in ants (Formica rufa, Lasius niger: ref. ASTROMENOTACTIC VERSUS 4, see also Brun (ref. 11) who failed to prate LANDMARK ANGLES time compensation in these two species). Therefore, Cataglyphis bicolor was trained Until recently the most extensive work on for 20 m to a special azimuth and then dis- competition between an astromenotactic placed in a testing grid far away from the angle and the azimuth of a horizon landmark training grid and completely unknown to the has been done with bees (ref. 3), followed by ants. There they were released and their runs some preliminary experiments with ants (refs. recorded (fig. 3). The mean directions of the 4 to 6). However, in all these studies per- returning ants, graphed for a distance of 5 m, formed with natural landmarks, neither the respectively, 10 m from the releasing point, physical stimulus properties of the terrestrial did not show any statistically significant dif- cues nor the different stages of learning ference from the home direction, irrespective processes were taken into account. Up to of whether the ants were released at once or now, the temporal aspects of spatial orienta- captured in the dark for 3.5 hr before being tion have been ignored, as no regard has displaced. If there was no compensation of been given to the succceeding stages of one the Sun’s course during the time of capture, orientation course. To consider these temporal one would expect a deviation of 52.5’. By a parameters, the foraging runs of the indi- special experimental procedure (which can- vidually trained ants were exactly recorded not be described here in detail) it was ex- by means of a grid of thin threads which ex- cluded that these results were influenced by SESSION V: SENSORY MECHANISMS-OPTICAL 423

FIGURE 2. Method of recording directional data. (A) Recording of a single course by means of a rectangular grid. Concentrically arranged circles give directional information for varying distances from start of run (nest entrance).

NEST ENTRliNeE NEST WiRMICE

FIGURE 2 (Concluded). (B) Frequency distribution in angular intervals of 5" (n = 20 ants, training angle a = go", three rewards). (C) Mean vectors of frequency distributions graphed in (B).r = length of mean vector; r > 0.47: Preferred direction is statistically signiscant with p < 0.01. the anemomenotactic orientation of Catagly- the ants show a bimodal distribution of direc- phis bicolor. tions according to the symmetrical pattern of It must be added that the term "Sun- polarized light at that time.l That ants are compass" orientation also includes the orientation toward the pattern of polarized light WEHNERj R.: Die ~onkurrenzvon kompass- und Horizontmarken-Orientierung bei der in the sky. This orientation Performance Can Wiistenameise CatagZyphis bicolor (Hymenoptera, be proved before sunrise or after sunset when Formicidae). Zool. Anz. Suppl. (in press). 424 ANIMAL ORIENTATION AND NAVIGATION

lOm Wend: 3-W0-l - dlredlon d a tlme- A contpenratfd return wrse lwnmnl almulhl W.5°t”--I-dlredl~ d a nm-llme- cwnpensatsd return cwrse konrlant angle to Sun’s azlmulhl Aa - devlatlpn from a.M0 P * mean Wrd return dlfecllon d ants releared at o 3.5 hrdler arrival at .c - mean valor d the return dlredlonr d ants lrnmfdlatelydlrplacfd fmm o to lwntrdrl A. nest entrance *feeding place

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FIGURE 3. Proof of a thecompensated Sun-compass orientation. (A) Directions of foraging runs (all shifted to lSOo). (B) Directions of return runs recorded in a test grid. able to use polarized light for their orienta- angle. However, the ants were not rewarded tion performances is already known for Las- until they had reached the black screen. In ius niger (ref. 12), Myrmica ruginodis (refs. the following runs called “competition train- 13 and 14) and Formica rufa (ref. 4). ing” runs, the position of the black screen In the basic competition experiment, the varied alternatively between the 0”- and ants were trained to a particular Sun-compass 60°-direction so that the ants were always direction, which was marked by an artificial confronted with the competition situation be- horizon landmark consisting of a black screen tween the direction to the horizon landmark having an angle size of 10” x 10”. This pro- and the Sun-compass direction of the preced- cedure is called “simultaneous training.” ing foraging run. As figures 4A and B demon- After a varying number of simultaneous strate, the fifth run (indicated by the open training runs, the position of the black screen squares and dotted lines) is mainly oriented was relocated from the trained direction by a towards the black screen. Furthermore, figure specific amount of degrees (60” in fig. 4). The 4B qualitatively shows that changing from directions of the following foraging runs, in- one orientation course to another does not dicated by the filled circles in fig. 4A, prove a consist of a continuous shift of a compromise clear orientation toward the astromenotactic direction but consists of an alternative SESSION V: SENSORY MECHANISMS-OPTICAL 425

FIGURE 4. Competition between an astromenotactic course and the direction to a horizon landmark. The ants have been trained to both (a = 0", simultaneous training, 3 reinforcements), before the landmark is offered at 60°. See text for explanation. (A) Frequency &- tribution of recorded angles.

FIGURE 4 (Continued). (B) Courses of 10 individuals graphed in a generalized way.

switching over from the Sun-compass course cated until it reaches the nest entrance where to the terrestrial cue. The theoretically deter- the foraging runs begin. This statement is mined compromise directions, graphed in proved in greater detail by figure 5. Hence, a figure 4C, do not coincide with the experi- mean vector cannot be calculated because of mental curves in figure 4B. With an increas- the bimodality of the angle distributions; an- ing number of competition experiments, the gles a! and p and their relations are deter- point of switching over is successively relo- mined for the succeedingstages of the courses : 426 ANIMAL ORIENTATION AND NAVIGATION

Astromendadk course Oiredlon d horhontal

__ Comprwnlse directions kept constant from bginnlng d runs -Compromisedlredians correctedfor eazh slale d runs xcordlngto wrying angles bffnaen dlrectlon to hWlm landmark and ant's langiiudinalaxis

FIGURE 4 (Cancluded). (C) Theoretically determined compromise directions, which do not correspond to the experimental curves in figure 4(B).

(Y means the deviation from the direction to recorded exactly; (2) with kinaesthetic the horizon landmark and the angle in- angles in spiders (Agelena labyrinthica, cluded by the astromenotactic direction and ref. 15) ; (3) with anemomenotactic angles in the direction marked by the black screen. The beetles (Geotrupes syhaticus, ref. 16) and relations a//? fall into two distinct groups: desert scorpions (Androctonus spec., ref. 17). one about the quotient = 1, character- Agelena can also start in a compromise direc- izing pure Sun-compass orientation; and the tion (refs. 15 and 18). Jander (ref. 4) also other about the quotient a//?= 0, meaning describes intermediate angles in the competi- orientation toward the terrestrial cue. As the tion situation of terrestrial and celestial cues lower graph (fig. 5B) shows, the proportion in Formica rufa. It must be noted, however, of the latter group relative to the first group that compromise directions can be proved is the larger as the distance from the nest only when more than one of the animal's entrance increases and as more competitive positions are recorded during one course. training runs are performed by the ants. Otherwise an uncritically determined inter- Therefore, we can summarize that in the com- mediate angle can mask succeeding alterna- petition experiments the ants first follow the tive decisions. The hypothesis that compro- Sun-compass direction, but they can switch mise directions are built up by means of pre- over to a horizon landmark by alternative ceding alternative adjustments needs further decisions and not by shifting a compromise analysis that may give more detailed informa- direction. tion on the cooperation of the two mecha- Alternative decisions are also found when nisms competing with each other. the astromenotactic orientation competes : At this points I wish to insert an addi- ( 1) with horizon landmarks in bees (Apis tional remark on another cooperation of two mellifica, ref. 3), where, however, the orienta- direction-indicating systems in Cataglyphis tion courses of the flying bees could not be bicolor-the cooperation of astromentactic SESSION V: SENSORY MECHANISMS-OPTICAL 427

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4 MD I 1.0 1.5 2.0 2.5 3D d (m) FIGURE 5. Orientation of Catuglyphis bicolor in competition situation between astromenotactic direction (MD) and direction to horizon landmark (HM). For explanation see figure 4 and text. Mean values are only graphed when based on at least five single values. (A) Ratio a/p in relation to different numbers of reinforcements (ST and CT) and different stages (d) of the orientation courses. a = deviation of the course from the direction to the horizon landmark (HM); p = angle between astromenotactic direction (MD) and direction to horizon landmark; ST = simultaneous training preceding competition experiments; CT = competition training to HM during competition experiments; d = distance from nest entrance. (B) Percentage of the runs directed toward horizon landmark n(HM, a/p u 0) in relation to the total number N of runs. and anemomenotactic orientation. Since our the testing grid in order to exclude all known experiments on this subject are not yet fin- terrestrial cues. The bimodal distribution, ished, I am able to present only the main however, only holds for the situation when qualitative results instead of the whole quan- the wind is absolutely absent. When it is titative data. As mentioned above, before present and its direction coincides or-this is sunrise and after sunset Cataglyphis bicolor now important-only nearly coincides with orients toward the pattern of polarized light the one during the training situation, a uni- in a bimodal way by preferring the home and modal distribution in the right direction is ob- its counter direction. The ants were trained tained. However, when the wind has shifted the previous day to an area far away from to the counter direction, a mean vector not 428 ANIMAL ORIENTATION AND NAVIGATION statistically different from the direction 180" tion only decides which of the two courses (home direction 0") is due for that situation, according to the pattern of polarized light is again irrespective of the precise direction of to be taken. the wind. But as soon as the Sun rises, in every Now the question may be answered: Are case Cataglyphis bicolor will perform the astromenotactic angles still recorded in an right home direction. ant orienting by means of horizon landmarks, From these results three main conclusions or is the mechanism of Sun-compass orienta- may be drawn : tion switched off in that situation? The ex- (1) There exists a time-compensated periment described in figure 6 proved the for- Sun-compass orientation toward the pattern mer to be true. After having been trained to of polarized light because the home direction the two places, PI and Pz which is marked is always the counter direction of the pre- by a black screen, the ants are not rewarded vious training direction, notwithstanding the at the actual place of the screen (for exam- varying training times during the day. ple, PI) but must search for the other place (2) There does not exist a time-compen- (for example, P,) by means of Sun-compass sated anemomenotactic orientation although orientation. If the astromenotactic mecha- the shifts in wind direction correspond with nism had been switched off during the orien- certain hours of the day. tation toward the horizon landmark, the ants (3) Between the different orientation would follow the direction leading to PZ as mechanisms a hierarchy can be stated as fol- seen from the nest entrance. But, they start lows: The Sun-compass orientation domi- from PI in the correct direction to P,. nates the orientation toward the pattern of Therefore, an ant orienting to horizon land- polarized light, and the latter dominates the marks still records the astromenotactic angle anemomenotactic orientation because varying and applies that information to further navi- wind directions do not result in proportion- gation tasks that must be solved without the ally varying home directions. The wind direc- aid of landmarks. That an astromenotactic

p d * 1.0m LaJeM. A -nest entrance. dart d G.9.4" forqinq runs r) .-93 6landP2 -pOinislowhichanlsare tralnad by means d black screen - pmnlon d black screen . _- In proson1 experlmenl b a * -&=-and-+- mean dhdiani d antr 4.1° starllngtrnm Plto P2

aslramtndacticawls are rgor4ad during run to P1 o*Wo-exp~addiradiom.Wkn mechanismd aslrornendadic orientationIs swnchad df

FIGURE 8: Mechanism of astromenotactic orientation is not switched off during orientation toward horizon landmarks. SESSION V: SENSORY MECHANISMS-OPTICAL 429

3qGURE7. Stereoscan electron micrograph of compound eye of Cataglyphis bicolor. View from inside-ventral Eye was coated by a 300 .%gold layer. Prepared and photographed by R. Wessicken, Institute of Electron Miemsmpy, ETH, Zurich.

course can be determined by means of other electron microscopy (figs. 7 and 8) one can physical cues than horizon landmarks (geo- study receptor properties of the ant’s com- menotactic angles, ref. 19; anemomenotactic pound eye, for example, radius of curvature angIes, ref. 17) is known from Arctosa and and diameter of the corneal facets. When Androctonus (Arachnida) . these and other parameters are measured for different size classes of the ants and dif- VISUAL CUE ADAPTATIONS ferent regions of the compound eye, highly significant differences can be found (fig. 9). Adaptations of the visual system on astro- Proceeding from the ventral to the dorsal menotactic orientation and pattern recogni- parts of the eye, the diameter of the corneal tion are now discussed. Since we have now facets decreases whereas the radius of curva- proven two mechanisms of visual orientation ture increases. Both parameters are shown for which are not linked together by building up three size classes of ants with proportionally a compromise direction, one may ask whether varying numbers of ommatidk2 They prove the visua1 system has developed special adaptations supporting one m the other mecha- * MENZEL,R.; AND WEHNER, R. : Augenstruk- turen bei verschieden grossen Arbeiterinnen von nism. Let us first look at some topological Cataglyphis bizolor (Fcrmicidae, Hymenoptera). relations within the visual field. By scanning Z. vergl. Physiol. (in press). 430 ANIMAL ORIENTATION .AND NAVIGATION

frequencies of the bees to the four test patterns are related to the reaction frequency 1 of the actual training pattern. The smaller the reaction frequencies are, the more effective is the insertion of the contrasting area into the training pattern. These results lead to the conclusion that distributions of black and white areas are most precisely analyzed in the lower part of the frontal visual field upon which horizon landmarks are normally projected. As known from electrophysiologi- cal recordings, single units in the optic lobes of crustaceans (ref. 22) and flies (ref. 23) have their receptive fields in special parts of the whole visual field of the eye. That special parts within the visual fields of arthropods are due to certain orientation performances is for optomotor FIGURE 8. Corneal hcets from ventral-extemal also known region in compound eye of CatagZyPhis bicolm. responses ( Uca pugnax, ref. 24) , telotactic Stereoacan electron micrograph by R. Wessicken, orientation toward single light sources (sev- Institute of Electron Microscopy, ETH, Zurich. eral insect species, ref. 25) ,and prey catching (Stagmatoptera bioceUata, refs. 26 to 28). that the interommatidial indinations become Color receptors are also disproportionally dis- the smaller, the more dorsal the ommatidia tributed over the wmpound eye (Periplaneta are situated (for bees see ref. 20). The small- americana, ref. 29; Libellula quadrimaculata, est values are obtained in the internal quarter ref. 30; Notonecta glauca, refs. 31 and 32; of the dorsal half of the eye-where, even Ascalafihus macaronius, ref. 33; Apis melli- morphologically, a small depression can be fica, ref. 34). Therefore, the visual system of seen looking straight upward to the sky. insects may be topologically subdivided ac- Until now we have not proved whether, in cording to the demands of the different opti- desert ants, there are also adaptations of the cal orientation mechanisms. visual system to the orientation by terrestrial cues. But our work dealing with the mech- LEARNING PARAMETERS anism of pattern recognition in bees gives clear evidence that this is the case. By means Finally we must consider some central of an apparatus (ref. 21) we are able to train processes involved in the two mechanisms of bees to screens with black and white areas in visual orientation. However, only a few re-

different positions of the visual field. When ’ marks can be made in this presentation con- the bees are trained to disbthe upper and cerning the visual learning capacities of Ca- lower halves of which are black and white, taglyphis bicobr. First the question arises, respectively (fig. 10)-the disks are chosen to whether the Sun-wmpass direction to a spe- a lesser degree when contrasting areas are in- cial feeding place is learned during the pre- serted into the lower rather than into the ceding return or foraging runs. Figure 11 upper part of the visual field. The reaction shows the latter is true. In this experimental SESSION V: SENSORY MECHANISMS-OPTICAL 43 1

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VENTRAL 4xNll7MDoRspL DEPRESSION FIGURE 9. Radius of curvature (upper graph) and diameter of corneal facets (lower graph) of compound eye of CatagEyfihis bicolor as taken from stereoscan electronic micrographs. Values are determined for small, medium and large individuum and for four regions of the eye (see inset figure). Body size (lateral length of head 1.0, 1.5 and 2.0 mm) and number of ommatidia (577, 825 and 1004) are positively correlated. In lower part, mean errors of mean values are graphed. Number beside each set of data represents total number of ommatidia in the eye.

setup the ants were trained to a feeding mean direction of the preceding return runs. place and displaced to a nearby releasing Therefore, no learning of a new compass point. The mean vectors of the first stages of direction occurs when the ants return to the the return runs are indicated by the black nest after being rewarded (refs. 10 and 35). arrows. ‘Afterward, the ants returned to the This conclusion is confirmed by another nest entrance. When measuring the directions more direct experiment in which an astro- of the following foraging runs (marked by menotactic course competes with the direc- the white arrows) , one finds these directions tion toward a horizon landmark during the to coincide with the Sun-compass course of return run. In order to obtain that test situa- the preceding foraging runs but not with the tion, Cataglykhis bicolor was trained in a 432 ANIMAL ORIENTATION AND NAVIGATION

1.0 0 0 TRAlNlNGPAllERNS

I I TRAlNlNG BLACKSECTOR WHITE SECTOR MlERNS INSERTED INSERTED Legend: 0 Calculated mean value Standardizedtests

FIGURE 10. Dorsoventral asymmetry in visual field of honey bee (A$& meZZifica). Reaction frequencies of trained bees to four test patterns (diameter 130") are related to reaction frequency 1 to actual training pattern. Smaller reaction frequencies are more effective in insertion of contrasting area in training pattern. Each mean value is calculated from reaction frequencies of three standardized tests. particular Sun-compass direction by means of tion to a special point after reaching it in a a set of black screens. After rewarding the roundabout way, but this does not hold for ants, the terrestrial cues were simultaneously the return runs. When the horizon landmarks displaced in such a way that the ants, when are removed, even after more than 20 compe- released, had to choose between the Sun- tition experiments, the ants never follow the compass direction and the direction toward direct astromenotactic return course but the black screens including an angle of 90'. choose the counter-direction of the immedi- They first followed the former direction but ately preceding foraging course. In contrast switched to the horizon landmarks with pro- to the foraging situation, no integration and ceeding competition situations, as is true for learning of astromenotactic angles seen at the the foraging runs. Even during the return return runs can be proved. runs no compromise directions were per- As the astromenotactic angle of the re- formed, furnishing further proof for our pre- turn run is determined by the counter direc- vious statement. While foraging, however, the tion of the foraging run, one may now ask ants learn the direct astromenotactic direc- whether a special constellation of horizon SESSION V: SENSORY MECHANISMS-OPTICAL 433

(1) FR p 4 HR 0 - (3) FR e- 02 (7) * 75s d. 1.0 ac, .78.3O d-1.0 ac,.n.s" 1.5 77.7O 1.5 75.4" Legend: 20 m .oo 20 78d FR. Forzqing run HR- Homing lreturni run 4 -Thswdbaily axpeded -C- Mean dlrectlons d return dlrectlm. whenangled runs IU d .Distance artromndactic orientdim -0. Mean direction d loraging om* Mean direction I31 Is ddermlnedby the . runs 01. B .Nert entrance preceding foraging run I11 4 ulpected directim. when *Feeding . I Plxe mean direction d rdurn 0. RelwIngpMnt run 121 IS essential for Idlowing foraging run 131 .--b- Direction and distance' d displacement

FIGURE 11. Competition between the astromenotactic angles of the precediig foraging run and the precediig homing run.

landmarks seen during the foraging run is return runs. Similar results can be taken from also reversed for direction finding on the re- experiments on maze learning in bees, wasps, turn run-more generally, whether terrestrial and ants (Apis mellifica, Vespa germanica, cues learned while foraging are also used ref. 36; Formica schaufussi, ref. 37). Other during the return runs. Our experiments learning parameters (e.g., acquisition, reten- show that ants returning to the nest can only tion, and reversal learning) cannot be dis- orient toward those horizon landmarks which cussed here in detail.3 they have seen during the preceding return What I wanted to show is that separate runs, irrespective of whether or not these orientation mechanisms are responsible for landmarks were presented during the forag- the orientation of the desert ant (Cataglyphis ing runs. From these and other experiments bicolor) toward astromenotactic angles and we may conclude that the direction of the horizon landmarks. If both sets of visual sti- return run is determined as the astromeno- muli compete with each other, the ants tactic counter direction of the foraging run switch over from one to the other orientation but not by the reversed constellation of horizon landmarks seen while foraging. The ter- * WEHNER, R.: Die Konkurrenz von Sonnen- kompass- und Horizontmarken-Orientierungbei der restrial cues used in the return situation must Wiistenameise Cataglyphis bicolor (Hymenoptera, be separately learned during the preceding Formicidae) . Zool. Am. Suppl. (in press). 434 ANIMAL ORIENTATION AND NAVIGATION mechanism and do not perform a compro- WEHNER: The solitary wasps (Ammophila, mise direction. Depending on the physiologi- Bembix, Philanthus) studied by the Dutch school (ref. 38) react more precisely to natural landmarks cal demands of the two mechanisms, the vis- as do Cataglyphis bicolor in the desert regions of ual system may be topologically subdivided. southern Tunisia when confronted with artificial Finally the storage of information presented terrestrial cues. If Cataglyphis bicolor is displaced by both sets of visual stimuli is restricted to to a strange place, the ant will accurately run in special stages of the orientation runs. the correct home direction and will search at random looking for the nest entrance after having performed a definite percentage of the real homeward course distance. Therefore, Sun-compass orientation mechanisms can be more precisely studied in these DISCUSSION desert ants. Furthermore, that advantagt is supported by the possibility of training the ants to spe- QUESTION:What is the natural food of the cial places and recording the orientation courses of ants and how do they forage? the running ants (in contrast to the flying wasps). WEHNER: In the regions where our experiments Additionally horizon landmarks can be presented in were performed (southern Tunesia) , the natural a definite way. Therefore, in Cataglyphis many ori- food of Cataglyphis bicolor consists of desert beetles entation problems may be analyzed that are diffi- (e.g., Tenebrionidae) and other desert insects as cult to study with other insects such as wasps and well as amphipod crustaceans in areas near the bees. coast. The ants reach their feeding places, which may be 100 to 200 m from the nest entrance, first in a straight course and then by searching around REFERENCES at random. The development of the well estab- iished learning capacities in these ants may have 1. SANTSCHI,F.: Etude sur les Cataglyphis. Rev. been supported by the necessity of foraging a freely Suisse Zool., vol. 36, 1929, pp. 25-70. moving prey, for a special feeding place can be 2. DELYE,G.: Physiologie et comportement de kept constant only for a few foraging runs. quelques fourmis (Hymenoptera, Formici- KLEERKOPER:Has the locomotor pattern of for- dae) du Sahara en rapport avec les princi- aging ants been studied to any extent? Is there any paux facteurs du climat. Insectes sociaux, indication that the moves are either random or pro- vol. 14, 1967, pp. 323-338. grammed ? 3. FRISCH,K. v.; AND LINDAUER,M.: Himmel WEHNER:We have studied random movements und Erde in Konkurrenz bei der Orientier- by releasing ants from a point not known to them. ung der Bienen. Naturwiss., vol. 41, 1954, They search at random in irregular, increasing cir- pp. 245-253. cles. After searching around, they return to the 4. JANDER, R.: Die optische Richtungso- point of original release. 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