The non-random behaviour of Aleochara bilineata gyll. (Coleoptera: Staphylinidae) in a Y-maze with neither reward nor punishment in either arm

THE NON-RANDOM BEHAVIOUR OF Aleochara bilineata Gyh . (Coleoptera : Staphylinidae) IN A Y-MAZE WITH NEITHER REWARD NOR PUNISHMENT IN EITHER ARM BY C. D . PUTNAM*

Zoology Department, Cambridge University

Introduction Aleochara bilineata is a parasite of the Cabbage Root Fly, Erioischia brassicae Bouche

(Diptera : Anthomyiidae) . It enters the puparium of the host as a first instar larva and leaves it as an adult . During preliminary observations of the behaviour of this beetle in a Y-maze, it was noticed that many beetles showed a significant preference for one or other arm of the maze although there was no indication of bias in the apparatus . This non-random behaviour was investigated further . Material and Methods Twelve of the seventy-three beetles used in these observations were reared from E . brassicae puparia collected in the field, and sixty-one were bred in the laboratory as described elsewhere (Putnam, 1957) . Thirty-one were males, and forty-one females, and one escaped before it could be sexed . The Y-maze used (Fig . 1) was made of Perspex sheet . The stem was 8 . 5 cm . long and the arms 5 . 0 cm. long, the angle between them being

90° . The beetle ran in a channel 1/16" high and 5 . 0 mm . wide lined with moist paper, and there was a small constriction at the entrance to the stem of the Y to ensure that the beetles were in contact with both sides to start with, thus eliminating any possible effect due to the experimenter placing them nearer one side than the other . The apparatus was washed in hot water, rinsed in alcohol and dried with compressed air before use, and a clean piece of moist paper was used for each series of choices by every beetle . The observations were made in a darkened room, the arms of the Y being equally illuminated by a cool light (D .C. "Point-o-lite"), and the stem shaded . The maze was placed in the centre of a piece of white card approximately two foot square . The beetles, which were kept in individual vials with ample food, were allowed to drink water from filter paper, before being placed at the entrance to the maze . They usually ran along one side of the stem, with the tip of the antenna on that side turned back and rubbing against the side of the channel, but they did not invariably choose the arm on the side along which they ran up the stem, nor did they always follow the same side all theawayyup the stem . When the beetles emerged from the end of the arm they had chosen, they were caught in a vial and put back at the entrance to the stem until they had completed a series of twenty consecutive choices.

Fig. 1 . The Y-maze . Two corners of the upper layer have been cut away to show the channel more clearly . The two upper layers were cemented together, and were separated from the lower layer by a sheet of moistened paper . *Present address : "Davenants," Sible Hedingham, Halstead, Essex . 118

One abnormal beetle, which emerged from its host with the left antenna limp and apparently without muscles, made three series of twenty runs through the maze. It invariably followed the right side of the stem and chose the right arm, and when the entrance to that arm was blocked, the beetle went along the right side of the "left" or only arm . The observations on this beetle are not included with those discussed below.

PUTNAM : BEHAVIOUR OF Aleochara bilineata

Results (1) Occurrence, of Combinations of Choices of the Two Arms of the Maze in a Series of Twenty Choices Table I shows how often the various possible combinations of choices of the two arms of the maze occurred in the first series of twenty choices by the seventy-two beetles . These are compared with the frequencies of those combinations expected if the beetles had chosen at random, the latter being calculated from the values of binomial-theorem coefficients given by Glaisher (1917) . Table I . Frequencies of Different Combinations of Left (L) and Right (R) in the First Series of Twenty Choices in the Y-maze made by Seventy-three Individuals of A . bilineata . Combinations (L/R)

Observed frequency

Frequency expected if choices at random

20/0, 19/1, 18/2

12

0 . 015

17/3, 16/4, 15/5

10

1 . 496

14/6, 13/7 12/8

5

16 . 865

11/9,10/10, 9/11

13

36 . 248

8/12, 7/13, 6/14

12

16 .865

5/15, 4/16, 3/17

9

1 .496

2/18,1/19, 0/20

12

0 .015

Forty-three of the beetles (58 . 9 per cent .) deviated significantly from choosing the two arms equally often, i .e . they chose one side fifteen or more times out of twenty . If they had been choosing betweem them at random, only 3 . 02 (4 . 14 per cent .) should have done this . This difference is highly significant (P << • 1 percent . N'=310-4), establishing definitely that the beetles were not behaving at random . In particular they showed a tendency to prefer one of the arms of the maze, as the twelve choice combinations with fifteen or more choices of one side all occurred more often than expected, while seven of the other nine combinations occurred less frequently than expected. Of the forty-three beetles deviating significantly from choosing the two arms equally often, twenty-two preferred left and twenty-one preferred right . The population as a whole showed no preference for either side, and left was chosen 717 times and right 743 times .

GYLL .

1 19

The beetles showing a significant preference for one alley did not differ significantly in their sex ratio (dd :9Y : : 44 . 1 :55 .9) from the population as a whole (aa :9? : :43 .0 :57 .0). (2) The Relationship Between Pairs of Consecutive Choices These deviations from equal choice of the two arms of the maze could be accounted for if, on any occasion, the beetles tended to choose the same arm that they had chosen on the previous run . Analysis of the sequence of choices made by individual beetles showed that on their second choice fifty-two of them (71 . 2 per cent .) chose the same arm that they had chosen on their first run . If they had chosen at random ., only half of them would have chosen the same alternative both times, and the difference between the observed proportion where the first and second choices were the same and the expected value is significant (P<-1 per cent . X'=13-16) . Thus there was a positive association between the first and second choices made in the maze . This was also true for all other pairs of consecutive choices, significantly more than half the beetles choosing the same arm both times . The mean number was 52 . 05 ± 3 . 65 (71 . 3 per cent .), but the value declined over the series of choices . The mean number of beetles choosing the same side both times for the first five pairs of choices was 54 . 6, and for the last five pairs of choices was 48 . 6 (see also Fig . 2) . (3) The Relationship of the Third Choice to the First Two Choices Further analysis showed that choices were not only related to the one which immediately preceded them . The third choice was the same as the second exactly as often as the second was the same as the first (i .e . fifty-two times out of seventy-three or 71 . 2 per cent.). However, when the first two choices were the same, the third was the same as the second more often (thirtyone times out of fifty-two or 78 . 8 per cent.), and when the first two choices were different, the third was the same as the second less often (eleven times out of twenty-one or 52 .4 per cent .), there being no association between them . Thus after having chosen one arm of the maze twice, a beetle was more likely to choose it again than after having chosen it once, but after one choice of each arm, the beetles showed no preference for either on the third choice . These two effects were studied further .

120

ANIMAL BEHAVIOUR, X, 1-2

(4) The Effect on a Choice of Several Previous Choices of One Arm of the Maze The effect of a series of several previous choices of one arm, starting with the first, on the beetles' next choice is shown in Table 11 . Table II. The Proportion of Beetles Choosing an Arm of the Maze Again after a Varying Number of Previous Consecutive Similar Choices, Starting with the First . No . of previous choices i 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

No . (and percentage of beetles choosing the same arm again

i i

52 out 41 „ 34 30 „ 27 „ 26 23 „ 20 „ 18 „ 18 16 14 „ 12 12 10 9

of 73 (71-24%) ,, 52 (78 . 84 %) ,, 41 (82-92%) ,, 34 (88-22%) ,, 30 (90-00%) ,, 27 (96-29%) ,, 26 (88 .45 %) ,, 23 (86-96%) „ 20 (90-00%) "18 (100'00%) „ 18 (88 - 88 --) „ 16 (87 . 50%) „ 14 (85-72%) „ 12 (100 . 00%) „ 12 (83 . 33 %) „ 10 (90 - 00%)

As the number of previous choices of one arm made by the beetles increased from one to six, the proportion of them choosing the same side again increased steadily . Then, as the number of previous choices increased further, it fluctuated around 90 per cent., and as the later values are based on fewer beetles, it is not clear whether an asymptotic level was reached or not . (5) The Effect on a Choice of Several Previous Choices of One Arm Followed by One Choice of the Other Arm Some of the beetles chose the same arm of the maze on their first one to four runs, and then chose the other arm once . Their behaviour on the next run is summarised in Table III . Table III. Behaviour of Beetles after Several Previous Choices of One Arm of the Maze Followed by One of the Other. 1 Previous sequence No. (and percentage) of choices of left Freof beetles choosing the (L.) and right (R) quency arm chosen first on the next choice LR LLR LLLR LLLLR Total

and and and and

RL RRL 1 RRRL RRRRL

21 11 7 4

10 (47 .6 %) 6(54-5%) 3 (42-9%) 1(25-0%)

43

20 (46 .5 %)

These beetles showed no preference for either side on their next choice . This was not unexpected in the beetles which had previously chosen each arm once, but in the others, although the average ratio of one arm to the other was 2 . 68 :1, there was no preference for the one previously chosen most often. After two to three choices of one side approximately 80 per cent . of the beetles chose the same one again (see Table II) but when the most recent choice was of the other arm it apparently cancelled out the effect of from two to four choices of the one chosen first . 6) The Relationship of Choice Repetition to all Previous Choices The probability of any choice being the same as the previous one was related to the proportion of all preceding choices which were of the same side, i.e. the chance of choice (n+ 1) and (n---2) both being, say, right increased as the proportion of choices of right in the first n choices increased . These data are given below in the form of a 2 x 2 table (Table IV) which shows a positive association between these two factors (P<2 per cent . : X',=6-46 (calculated allowing for the small values in two cells)) . Table IV . Relation Between the Percentage of the First n Choices Which were of the Arm Chosen on the (n+1)st Choice, and the Proportion of Beetles Choosing the Same Arm Again on the n+2)nd Choice . Percentage of the first n choices when the arm chosen on the (n+l)st choice was chosen . <68% No . of beetles, out of 73, choosing the same>51 as (n+1) on their (n+2)nd choice <51

0

>68% 8

j

7

3

10

7

11

18

It was mentioned above (Section 2) that the percentage of consecutive pairs of choices which were the same fell during the series of twenty choices. This can now be explained in terms of a similar decrease throughout the series of choices in the proportion of the first n choices which were of the same arm as that chosen in the (n+1)th choice (Fig . 2), and this decrease was probably due to the relatively small number of beetles making a long sequence, from their first

PUTNAM : BEHAVIOUR OF

Aleochara bilineata

GYLL .

121

-59 E d

74--

y

-55

k 4.4-

0

o C 0

y 0

-50

v 0

h d

1

62-

I

5 No .

of

10 15 previous choices (n) .

18

Fig. 2 . Graph showing the decrease both in the percentage of the first n choices which were of the same arm as that chosen on the (n+1)st (left hand scale x- - -x) and in the number of beetles choosing the same arm on' both the (n+1)st and (n+2)nd choices (right hand scale + - +), as the number of previous choices increased (horizontal axis) . run through the maze, of choices which were all the same (see Table II) . (7) The Relationship Between Two Series of Twenty Choices by the Same Beetles Twenty-nine of the beetles made a second series of twenty choices in the Y-maze from a few hours to thirteen days after the first series, the mean interval being 4 .9 days . The relationship between the presence or absence of a significant preference for one arm of the maze in the two series of choices is summarised in Table V . In all cases where there was a significant preference for one arm in both series, the same side was preferred both times . There is a significant association between either the presence or the absence of a significant preference for one side in both the series (P<5 per cent . : X =4 . 18 (calculated allowing for the

Table V. The Occurrence of a Significant Preference for One Side (i .e . fifteen or more choices out of twenty) in Two Series of Twenty Choices in the Y-maze by 29 Individuals of A . bilineata. Second series

First series

Significant preference

No preference

Significant preference

9

5

14

No preference

3

12

15

12

17

29 i

small value in one cell)) . Thus individual beetles tended to show either a significant preference, for the same side, in both series, or no preference for either. There was no indication that as

122

ANIMAL BEHAVIOUR, X, 1-2

the time between the two series of choices increased, the association between them decreased . Discussion The large proportion of beetles showing a significant preference for one of the two alternatives was thus a result of a definite relationship between a beetle's choice on any occasion and its previous behaviour . Throughout the series of twenty choices there was an association between any choice and the previous one, but less recent behaviour could also exert an effect, as several previous choices of one arm of the maze resulted in a stronger association, while when there had been several previous choices of one arm followed by one of the other, there was no association with either. The occurrence of two consecutive choices of the same arm was related to the proportion of the two sides in the previous choices . Any preferences shown in the first series of twenty choices were still present in a second series up to thirteen days later . Possible explanations of these phenomena will now be considered . The association between consecutive choices could have been due either to some factor such as an intrinsic bias to one side influencing both choices, or to the first choice influencing the second in some way . The former possibility can be checked, as the strength of a bias which would cause the observed degree of association between the first and second choices can be calculated, and further consequences of a bias of this magnitude, such as the relative frequencies of the combinations of twenty choices which would have occurred if the beetles had been affected by a bias of that strength throughout the series of choices, can be determined . This has been done in full elsewhere (Putnam, 1957), and there is no resemblance between the observed frequencies of the choice combinations and those expected . Thus the beetles could not have been affected throughout the series by a constant internal bias towards one side which was present before the series began . The making of the second choice must therefore have been influenced in some way by the first choice, and so on through the series, and this could have happened in one of two ways . Firstly, the beetles could have left some trail in the maze on their first run through which they could follow on their second, such as, for example, an odour trail like that found in ants and some other insects (Vowles, 1955 ; Williams,

1954), or, secondly, if the beetles remembered their first choice, for some reason, this memory could have influenced the making of the second choice . In the former case the two arms of the maze would have been different when the beetles made their second choice, and in the latter the two sides would still not have differed, but the beetles would have changed . The crucial experiment to decide between these two explanations would be to have two clean Y-mazes available, and to allow the beetles to make one choice in one followed by two choices in the other . If the first ("odour trail") explanation was correct, association would be expected between the second and third choices but not between the first and second, while if the second ("memory") explanation was correct, association would be expected between the first and second choices, and also between the second and third, if the first two were of the same arm of the maze . Unfortunately it was not possible to make these observations, and get a definite decision, but there is some evidence available which favours the "memory" explanation rather than the other . In particular, the association between the two series of choices on the average over four days (Table V) can only have been the result of some "memory" of the first series influencing the beetles' behaviour during the second, as the maze was clean at the start of the second series, so on the basis of the "odour trail" explanation, no association between the two series of choices would have been expected . Some psychological change must thus have occurred in some of the beetles, at least by the end of their first series of choices, and it seems likely that such a change would have occurred during the first series, and have affected the behaviour of the beetles then as well . Furthermore, the fact that the effect of several choices of one side could be cancelled out by one choice of the other (Table II) would be difficult to explain in terms of the "odour trail" hypothesis without some elaboration, while it can be simply accounted for in terms of "memory", as will be shown below. The evidence thus favours the conclusion that after the first run through the maze, the beetles changed, and, in particular, became more likely to choose the same alternative again, although the two arms remained the same. This resembles the situation in many animals which have developed a preference for one of two alternative arms in a T- or Y-maze by

PUTNAM : BEHAVIOUR OF Aleochara bilineata GYLL .

trial-and-error learning (for many examples see Thorpe, 1956, passim), but in such cases there has been a definite and, usually, deliberately placed reason for the animals to choose one arm, Le, a reward, or to avoid the other, i .e. a punishment, and as a result of this the two alternative pathways have clearly been different, and all the animals have developed a preference for the same one . A . bilineata, however, had a choice between two similar maze-arms with no intentional reward or punishment in either, yet, firstly, there was a positive association between consecutive choices, and, secondly, a preference for either side occurred equally often . The experimental situation must thus have unintentionally included some reward in both arms of the maze, despite which, individual beetles developed a preference for only one of them . Although this situation does not seem to have been explicitly considered before, except, mathematically, by Bush & Mosteller (1955, pp . 154 and 163), it can be shown quite simply that animals choosing between two alternatives which are both associated with reward, could develop a preference for either one of them, provided that they were unable to detect the presence of a reward in either alternative when at the choice point, and had no intrinsic bias to turn towards one side . The first choice would then be made at random, the two arms would be chosen equally often, and all animals would gain a reward . On the second choice the memory of the reward associated with the first choice would increase the probability of the same arm being chosen again, and more than half the animals would do this . On the third choice, the animals which had chosen the same arm twice would be at least as likely, and probably more likely still, to choose it again, but those which had by then chosen each side once would associate reward with both sides, and thus have no reason to prefer either . The more often an animal had chosen one side, the less likely it would be either to choose the other, or, if it did so, to lose its preference for the one chosen most often . However, an animal choosing one arm a few times and then discovering the reward in the other arm as well would probably show no preference for either in subsequent choices . If the first choice was made at random, and both the arms were chosen equally often on the first choice, then a preference for either would occur equally often, but if there was an initial bias towards one side, a preference for this side would be proportionally more fre-

123

quent than one for the other. Thus, in this situation, apparent "trial-and-error" learning could occur, though there would in fact be no possibility of error. Some observations which confirm these deductions have been made on rats, where, although the situation is complicated by their tendency to alternate between two alternatives, even if choice of one or both are rewarded (reviewed by Dember & Fowler, 1958), it has been found that with a large enough reward a preference for one of two rewarded alternatives will develop. Most rats in a group rewarded with food in both arms of a T-maze developed a preference for one arm, and after thirty trials the proportion of rats choosing the preferred side had risen to 85 per cent . (Cotton et al., 1959), and over half the rats showed a significant preference (21 or more choices out of 30) for one side (Cotton, 1960) . Similarly, rats which were rewarded in both arms of a T-maze with escape from electric shock in the stem showed a definite tendency to repeat their response . (Fowler, et al., 1959), and in their first four choices 50 per cent . of them chose the same arm every time, compared with the 12 . 5 per cent . expected if they had been choosing between them at random, this difference being significant (Pu . 1 per cent . ; x2=51 . 43) (Fowler, 1959) . Finally, rats in a Y-maze with no difference between the arms, and no intended reward in either, showed about 67 per cent . response repetition (Jagoda & D'amato, 1960) . As A . bilineata behaved in the same way, it is likely that there was some reward in both arms of the maze which could not be detected at the choice point. This reward cannot, however, be identified with certainty, although certain possibilities, such as food and drink, can be ruled out as neither was provided, nor were the beetles deprived of them. One possibility which cannot be eliminated is that the "opportunity to explore", which the beetles had between leaving the end of the arm they had chosen and being recaptured in their vial, acted as a reward. Such "opportunity to explore" an interesting space, to manipulate a puzzle, or even to "explore" visually or aurally can act as an effective reward for discrimination in rats, monkeys and some other animals (reviews in Thorpe, 1956 ; Glanzer, 1958 ; Barnett, 1958) . While it has not yet been shown to act as a reward in insects, a general exploratory tendency has been found not only in higher insects such as ants (Caldwell & Phaup, 1959), honey-bees (Lindauer, 1952), and the

.124

ANIMAL

BEHAVIOUR,

hunting-wasps Amophila (Baerends, 1941) and (Tinbergen & Kruyt, 1938 ; van Beusekom, 1948), but also in Blatella (Dictyoptera) (Darchen, 1952 ; 1955) where it was found that stimulus changes increased exploratory activity . Thus this possibility cannot be ruled out, especially as an exploratory tendency would be useful to A. bilineata which under natural conditions lives semi-socially in subterranean tunnels and chambers (Burks, 1952) . However, as no reward can definitely be shown to have acted as a reinforcement for this apparent learning by the beetles, one other possible explanation must be considered, namely that the behaviour was not due to any reward, but was mediated at a purely motor level, the mere fact of having chosen an alternative once making a beetle more likely to choose it again ("motor learning", Wigglesworth, 1956) . However, in the first place, though this could explain the positive association between pairs of consecutive choices, and between two series of choices, it would not account for the behaviour of the beetles after three or four choices of one arm and then one of the other, and in the second place, in one of the few recorded cases of this type of behaviour considerable "training" of larvae of Ephestia (Lepidoptera) by a forced circling movement in one direction did not result in any of them choosing the appropriate side in a T-maze more than sixteen times out of twenty (Brandt, 1934). There are several records of significant deviations from equal choice of the two sides of a Y- or T-maze with neither reward nor punishment in either in other invertebrates. Fifteen of twenty individuals of Helodrilus (Annelida) given from 70 to 1,500 trials in a Y-maze did so (Swartz, 1929) and in both Calcinus herbsteii (Crustacea ; Decapoda) (Mackay, 1945) and Clibnarius zebra (Decapoda) (Mackay, 1946) over 80 per cent . of the individuals showed a significant preference for one arm of a Y-maze, approximately 60 per cent . of these preferring the left arm . There is not, however, enough data to explain these results, and they may have been partly due to an intrinsic bias towards one side in some or all individuals . Such bias has been observed in Blatella (Dictyoptera) (Hullo, 1948), Pediculus humanus (Anoplura) (Wigglesworth, 194 1) and Coccinellid larvae (Coleoptera) (Banks, 1957), It may also have occurred in the insects used by workers on olfactory responses who have made control observations in which several insects have passed Philanthus

X,

1-2

through a Y-olfactometer at once (e .g . Mclndoo, 1926, Leptinotarsa (Coleoptera) ; Thorpe & Jones, 1937, Nemeritis (Hymenoptera) ; Crombie 1944, Calliphora and Lucilia (Diptera)), and in the control observations of Grosslight & Harrison (1961) on Tenebrio molitor larvae (Coleoptera). The equal choice of the two arms found in these cases shows that there was no bias in the apparatus, but does not rule out the possibility of bias towards one side in some or all the individuals, as, provided that bias to either side occurred equally often, equal choice of the two arms would be expected . The present study shows that a preference for one side could develop during Y-olfactometer control observations, and, as such preferences might affect subsequent responses to olfactory stimuli, this gives further support to recent criticisms of the Y-olfactometer method of investigating olfactory responses, and to suggestions of other ways, in which animals can move in a less restricted way (Varley & Edwards 1953 ; Bartel & Davenport, 1956) . Conclusions The non-random behaviour of A. bilineata in the Y-maze could have been the result of the association of both arms with a reward not deliberately included in the experimental set-up which was not detectable by the beetles at the choice point . An "opportunity to explore" might have acted as a reward, but this cannot be definitely established . Some other possible explanations, such as "motor learning" and intrinsic bias in the beetles can be ruled out, but no final conclusion can be reached at present . Summary Adults of the beetle A . bilineata deviated significantly from random choice between the two arms of a Y-maze with neither reward nor punishment in either. Over half of them showed a significant preference for one side in a series of twenty choices, preference for right and left occurring equally often . As the number of consecutive choices of one arm made by a beetle increased from one to six, the probability of it choosing the same side again increased linearly . If the first two choices were of different arms, no preference for either was shown on the third . Most beetles which had shown a significant preference for one arm in their first series of twenty choices also showed a significant preference for the same side in a second series made from a few hours to several days later .

PUTNAM : BEHAVIOUR OF Aleochara hilineata GYLL .

Possible explanations of this non-random behaviour are considered, and examples of similar behaviour in other animals are discussed . Acknowledgements While making these observations in the Department of Zoology, Cambridge University, during 1954 and 1955, I had a D .S .I .R . maintenance grant. I am most grateful to my supervisor, Dr . G. Salt, F.R .S ., for his advice and criticism, and to many friends for helpful discussions of various points while the work was in progress . This work formed part of a dissertation approved for the degree of Ph .D . by the University of Cambridge . REFERENCES Baerends, G. P . (1941) . Fortpflanzungsverhalten and Orientierung der Grabewespe Ammophila campestris. Tijd. voor Entom ., 84, 71-275. Banks, C . J. (1957). The behaviour of individual Coccinellid larvae on plants . Brit. J. anim . Behav ., 5, 12-24 . Barnett, S. A . (1958) . Exploratory behaviour, Brit . J. Psychol ., 49, 289-310 . Bartel, A . H. & Davenport, D . (1956) . A technique for the investigation of chemical responses in aquatic animals . Brit . J. anim . Behav., 4,117-119 . Beusekom, G. van (1948) . Some experiments on the optical orientation in Philanthus triangulum Fabr. Behaviour, 1, 195-226. Brandt, H . (1934) . Eine Gewohnheitsbildung in der Bewegungsrichtung der Mehlmottenraupe Ephestia kuehniella Zeller. Z. vergl. Physiol., 21, 545-551 . Burks, B . D . (1952) . Insects, enemies of insects . In Insects. The yearbook of Agriculture, 1 952 . pp . 373-380 . U .S . Dept . o f Agric ., Washington . Bush, R . R. & Mosteller, F . (1955) . Stochastic models for learning. New York : Wiley . Caldwell, W. E . and Phaup, M. R . (1959) . Some aspects of the behaviour of the ant in the maze under various conditions of stimulation. J. genet . Psycho!., 95, 27-39 . Cotton, J . W. (1960) . Personal communication, October 28th, 1960 . Cotton, J. W ., Lewis, D . J. & Jensen, G. D . (1959) . Partial reinforcement effects in a T-maze . J . comp . physiol. Psycho!., 52, 730-733 . Crombie, A . C. (1944) . On the measurement and modification of the olfactory responses of blowflies . J. exp . Bio!., 20, 159-166. Darchen, R . (1952) . Sur l'activit6 exploratrice de Blatella germanica . Z. Tierpsychol., 9, 362-372 . Darchen, R . (1955) . Stimuli nouveaux et tendance exploratrice chez Blatella germanica . Z. Tierpsychol., 12, 1-11 . Dember, W. N. & Fowler, H . (1958). Spontaneous alternation behaviour . Psychol . Bull., 55, 412-428,

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