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575 1 591 53 595 •7 H Carausius •
:
•
:
Transference of induced food-habit from parent to offspring. I l l B y D. E. Sl a d d e n ,* D .I.C .
and
H . R . H e w e r , M.Sc.
Department of Zoology and Applied Entomology, Imperial College of Science and Technology (Communicated by E. W. MacBride,
— Received 10 March 1938)
I ntroduction
The two previous communications in this series dealt with experiments designed to test the preference for, and ability to eat, ivy (as opposed to privet) in the stick insect Carausius ( ix)morosus. I D under the conditions of the experiments, this insect increased its ability to eat ivy when reared on this food plant instead of on privet. In the present paper these experiments are continued for two more generations and several other types of experiment are described which help to clear up some of the rather puzzling features of the earlier work. The methods employed are identical with those previously used, not because they cannot be improved upon, but because these later experiments had to be strictly comparable with the earlier ones. The experiments fall under two heads: Presentation Tests and Preference Tests. In presentation tests the newly hatched nymph is offered the experimental food plant for 24 hours. If this is not eaten it is replaced by some of the parental food plant for 24 hours. Then the experimental plant is again presented; this time for 48 hours. A second refusal is again followed by a 24 hours presentation of parental food plant and a third presentation of the experimental food plant for 48 hours. When at last the insect feeds on the experimental food plant the number of the presentation is recorded as the result of th at experiment. Since the insect only feeds at night, each 24 hours period represents one opportunity for a meal; thus, except for the first presentation, there is an alternation of two experimental and one * T he tragic d eath of Miss Sladden occurred on 20 Ju n e 1937 when th e final figures included in this paper were on th e po in t of com pletion. The preparation of th e m anuscript is therefore solely th e responsibility of Mr Hewer. I t ought to be added th a t Mr H ew er deserves a considerable portion of th e credit for Miss Sladden’s previous papers as he devised th e ty p e of experim ent which she carried out. E . W. MacB. [ 30 ]
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Transference, of induced food-habit from parent to offspring
31
parental food plant opportunities. Newly hatched nymphs can only live for about 24 hours without food. The alternations of parental food plant (which is always eaten) provides for the continued existence of the nymph (see tables for the very small number of deaths within experimental times) but does not allow any great degree of growth. No nymph went through a period of ecdysis during the presentation tests even though 3 or 4 weeks old although the first instar is frequently over at the end of two weeks in normally fed insects. Preference tests consist in three presentations of both parental and experimental food plants at one and the same time so th at the insect can choose between them. If it accepts either parental or experimental food plant at all three presentations it is classed according to the food plant accepted. If it accepts both parental and experimental food plants at any one presentation it is classed as “ neutral” . This latter ruling probably accounts for the rather large numbers of “ neutral” insects, but it was considered th at no “ average” or “ dom inant” preference could be stated from three presentations only and th at a continuance of presentations would only result in the building up of a food preference, whether one existed previously or not. It is for this reason th at strict numerical com parison between experimental food-plant-preferring insects (in the prefer ence tests) and those accepting the experimental food plant at the first presentation (in the presentation tests) cannot be made. In order to make clear the relations of the various groups of insects employed in these experiments the genealogical table given in the second paper of the series (Sladden 1935, p. 34) has been extended in Table I.
P r esen ta tio n
and p r e f e r e n c e tests
These tests are a direct continuation of those carried out on insects reared on privet and ivy respectively, the former being known as the controls and the latter as the forced ivy-fed. The results of the 6th generation tests are shown in Tables II and III. One or two points call for comment. Only one parental group of forced ivy-fed insects was used for the production of offspring submitted to the accurate recording of the presentation tests because a number of other experiments, to be described later, were undertaken at the same time and it would have been impossible to carry out either efficiently had the numbers been larger. (Other parental groups of forced ivy-fed insects were used to produce offspring also forced ivy-fed but no detailed records were kept of these—see generation 7, Tables IV and V.)
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Transference of induced food-habit from parent to offspring
33
No emphasis can be laid on the 100 % acceptance at the first presentation because of the small number of insects used. Among the privet-fed stock four groups were tested. Fresh Forest Hill stock was introduced as a parallel strain to control stock of privet-fed T a ble I I P r esen ta tio n D uratio n of experim ent Aug.
P aren tal presentation group 1 .1 .1 .1
P resen ta tio n
test
D uration of experim ent Sept.-O ct. A ug.-Sept. Oct .-Dec. Feb.-M ar.
5. I v y -f e d
test
No. tested 100
pa r e n t s ,
P resen tation of ivy to progeny ^------ ------------- a-----4 5 2 3 1 100 — — — —
4 a . P r iv e t -fe d No. tested
F orest Hill Stock Control Group 1 Control Group 2 Control Group 3
100 100 100 100
Percentage
1935
pa r e n t s ,
D ead 0
1935
P resentation of ivy to progeny ------,------ ------------- A —N D ead 2 3 4 5 1 — — 0 27 — 73 0 8 92 — — 0 39 1 60 — — 0 5 1 65 19 10 0-2 1-2 5-0 58*8 34-8
T able III P r e fe r e n c e P aren tal presentation group
test
N 0. tested
1 .1 .1 .1
50
P r e fe r e n c e Forest Hill Stock Control Group 1 Control Group 2
test
4. I v y -fe d P riv et
pa r e n t s ,
N eutral 27
1
4 a . P r iv e t -fe d
N o. tested 50 50 50
P riv et 17 5 10
1935
parents
Iv y 22
D ead 0
1935
N eutral
Iv y
17 20 18
16 25 22
D ead 0 0 0
insects for comparison. All the insects used in the whole series of experi ments were originally derived from this source (Sladden 1934, p. 442). The three groups of control stock were used in order to test any possible variation due to emergence time. That there is the possibility of this effect is shown by comparison of the times of the duration of the experiments shown in the first column of Table II. This point will be dealt with fully later in the paper. Vol. CX X V I.
B.
3
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34
D. E. Sladden and H. R. Hewer
The preference tests give results according to expectations. In 1936 the seventh generation was similarly tested—the results are shown in Tables IV and V. T a ble IV P r esen ta tio n
test
6. I v y -fe d
pa ren ts,
1936
P resen tatio n of ivy to progeny D u ratio n of experim ent J u ly -S e p t. Ju n e-A u g .
P aren tal p resentation group 2 .3 .1 .1 1 0 .3 .2 .1
No. tested 100 100
Percentage
P r esen ta tio n
test
A
s -----------
1
2
3
4
D ead
86 78
14 21
— —
—
0 1
82-0
17-5
—
—
0-5
4
D ead
— —
0 0 0
—
0
5 a . P r iv e t -fe d
pa r e n t s ,
1936
P resentation of ivy to progeny D u ratio n of experim ent J u n e -J u ly Ju n e-A u g . Ju n e-A u g .
F o rest Hill Stock Control Group 1 Control Group 2
No. tested
1
2
100 100 100
85 86 87
15 13 13
86-0
13-6
P ercentage
3 — 1 — 0-3
T a ble V P reference P a re n ta l presen tatio n group
2.3.1.1 10.3.2.1 P reference F o rest Hill Stock Control Stock
test
5. I v y -fe d
pa ren ts,
1936
No. tested
P riv et
N eutral
Iv y
D ead
50 50
2 4
15 21
33 24
0 1
pa r e n t s ,
1936
test
5 a . P r iv e t -fe d
No. tested
P riv et
N eutral
Iv y
50 50
4
27 27
19 17
6
D ead
.
0 0
The comparatively small numbers do not allow any far-reaching con clusions. I t will be seen however th a t both experimental and control insects accept ivy with almost equal readiness and th at the period of the presentation tests extended over the months of June, July, August and September. As shown in the section of this paper dealing with periodicity it is during these months th at all the stick insects used show an equal ability to eat ivy. That this is not peculiar to this test (presentation tests 5 and
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Transference of induced food-habit from parent to offspring
35
4a) is seen from Table V II where insects (privet-fed) of three generations (1934, 1935 and 1936) show the same tendency. Reference to Table VI will show the consistency of this series of tests over six filial generations. T able VI S ummary
of pr ese n ta t io n tests
Filial D uratio n of generation experim ent
/ 1
2
Jan .-A p ril N ov.—Ja n . N o v .-Ju ly Sept .-N ov. Mar .-Aug. June-A ug.
8-0 30-7 54-2 61-0 58-8 86-0
32-0 46-0 40-9 39-0 34-8 13-6
1 2 3 4 5 6
P rivet-fed p arents. P resentation of ivy
O ct.-A pril A ug.-M ay Ju ly -F e b . Aug. Ju ly -S e p t.
2
1
78-25 18-99 6-2 93-7 9-5 90-3 100-0 82-0 17-5
S ummary Filial generation 2 3 4 5 6
—
5-0 0-3
3 1-5 0-1 0-3
—
—
1-2
0-2
—
8
9
10
2-4 0-0
0-8 0-0
0-0 0-7
0*8
—
—
—
— —
—
—
— —
—
— — —
35-0 44-0 46-0 36-0 54-0
4
5
0-12 0-12 — — — —
6
7
8
10
9
____
____
____
____
—
— —
— —
— —
— —
—
—
of p r e f e r e n c e tests
P rivet-fed p arents .---------------- -------------------, N eu tral Iv y P riv et 44-0 33-2 26-0 22-0 10-0
—
7
Ivy-fed parents. P resentation of ivy
t
11 2 3 4 5 6
4 3 5 6 21-6 12-0 11-2 11-2 14-7 2-0 3-3 0-7 — 4-8 0-0 0-2
21-0 22-8 28-0 42-0 36-0
Ivy-fed p arents ,----------------- -----------P riv et N eutral 28-2 37-0 16-5 39-9 14-0 39-0 2-0 54-0 36-0 6-0
Iv y 35-6 43-4 47-0 44-0 57-0
I t will be seen however th at the gradual concentration of privet-fed insects (controls) towards the first and second presentation groups which has been noted in a previous paper (Sladden 1935, pp. 42, 43), reached its maximum in the fourth filial generation and the last two tests in the fifth and sixth filial generations show a return to the earlier conditions. This would be more marked in the fifth filial generation if a number had not been tested in August which the analysis of periodicity shows to be a month when food preferences are not well marked. 3-2
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36
D. E. Sladden and H. R. Hewer T a ble V II P r esen ta tio n
g ro u ps .
P r iv e t -eed
pa ren ts
A. M onthly acceptances each year M onth an d y ear J a n . 1933 J a n . 1934 F eb. 1932 F eb. 1936 Mar. 1932 Mar. 1934 Mar. 1936 A pril 1934 M ay 1934 J u n e 1934 J u n e 1936 J u ly 1936 Aug. 1935 Aug. 1936 Sept. 1934 Sept. 1935 Oct. 1934 Oct. 1935 N ov. 1932 N ov. 1934 N ov. 1935 Dec. 1932 Dec. 1933 Dec. 1935
1
2
8 17 3 2 6 42 8 61 66 21 61 84 61 87 24 31 20 23 2 17 29 36 13 8
25 24 20 32 20 35 33 41 39 7 14 16 5 13 6 3 18 7 9 15 21 35 21
3 8 3 20 7 7 3 12 8 2 1
4
5
2 —
6
1 —
13 4 2
9 1 5
i —
7 2 —
1
10 —
3
8
9
10
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
1 —
1
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
11
2
—
—
i 2 '
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
1
—
2
1
1
2
—
Total 47 44 76 46 43 80 54 110 108 29 75 100 66 100 30 34 38 30 17 32 50 74 36 20
B. M onthly totals M onth
1
2
Jan. Feb. Mar. April M ay Ju n e J u ly Aug. Sept. Oct. Nov. Dec.
25 5 56 61 66 82 84 148 55 43 48 57
49 52 88 41 39 21 16 18 9 25 45 67
3 11 27 22 8 2 1
4
5
6
7
2 17 3
1 10 5
1 10 3
2 1
—
—
—
—
1 —
— — — — . — — — — 2 4 — —
8
9
10
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
— — — 2 2
— — — 1 —
— — — — —
— — — — — —
— — — — 1 —
— — — — — —
Total 91 122 177 110 108 104 100 166 64 68 99 130 1339
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Transference of induced food-habit from parent to offspring T able V II (
co
C. M ean p resentation tim es and sta n d ard deviations Jan. Feb. Mar. April May Ju n e Ju ly Aug. Sept. Oct. Nov. Dec.
M.P.T.
S.D.
2-08 3-07 1-99 1-52 1-43 1-22 M6 111 11 4 1-37 1*71 1-64
±0-78 ± 1-40 ± 103 ±0-63 ±0-58 ±0-44 ±0-37 ±0-32 ±0-35 ±0-48 ± 1-36 ±0-96
T able V III. P r esen ta tio n
g r o u ps .
I y y -eed
pa ren ts
A. M onthly acceptances each year M onth and year Ja n . 1933 Ja n . 1934 Ja n . 1935 Feb. 1933 Feb. 1934 Mar. 1933 Mar. 1934 Mar. 1937 April 1933 April 1934 May 1934 Ju n e 1936 Ju n e 1937 J u ly 1934 J u ly 1936 Aug. 1933 Aug. 1934 Aug. 1935 Aug. 1936 Sept. 1933 Sept. 1934 Sept. 1936 Oct. 1933 Oct. 1934 Nov. 1932 Nov. 1933 Nov. 1934 Dec. 1932 Dec. 1933 Dec. 1934
1 69 24 21 68 15 19 13 9 7 18 9 9 56 12 74 65 94 100 65 300 91 16 298 53 44 145 36 74 73 51
2 11 5 5 9 2 14 1 9 7 7 4 3 6 —
4
3
5
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
25 3 6
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
5 20 2 1 14 7 16 6 11 10 8 5
37
—
1
—
—
—
—
—
—
—
—
—
—
2 —
—
—
1
—
1
— —
—
—
—
—
—
—
—
—
—
—
'
Total 80 29 26 77 17 33 14 18 14 25 13 12 62 12 99 68 100 100 70 320 94 17 312 60 63 151 47 85 81 56
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38
D. E. Sladden and H. R. Hewer T able
V III (<
)
B. M onthly totals M onth Jan. Feb. Mar. April M ay Ju n e J u ly Aug. Sept. Oct. N ov. Dec.
1
2
114 83 41 25 9 65 86 324 407 351 225 198
21 11 24 14 4 9 25 14 23 21 33 23
3
4
5
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
I —
—
135 94 65 39 13 74 111 338 431 372 261 222
—
1
2 1
Total
—
—
—
2155 C. M ean p resentation tim es and stan d ard deviations M.P.T.
S.D.
M6
±0-36 ±0-32 ±0-49 + 0-48 ±0-48 ±0-33 ±0-41 ± 0-20 ±0-23 ±0-23 ±0-41 + 0-33
Jan. Feb. Mar. April M ay Ju n e J u ly Aug. Sept. Oct. Nov. Dec.
112
1-37 1-36 1-31 112
1-23 1 04 1-06 1-06 11 5 111
T a ble IX . P r e f e r e n c e
test
3 b . 1934
E xperim ental food: rose P rogenv of ivy-fed p aren ts [Per 50 Progeny of privet-fed p aren ts
No. tested 250
50
Iv y 71 14-2
N eutral 88 17-6
Rose 90 18-0
D ead 1 0-2]
P riv et 4
N eutral 26
Rose 20
0
Iv y 46 9-2
N eutral 77 15-4
Lilac 127 25-4
D ead 0 0]
P riv et 3
N eutral 19
Lilac 28
0
E xperim ental food: lilac P rogeny of ivy-fed p aren ts [Per 50 P rogeny of privet-fed paren ts
No. tested 250
50
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Transference of induced food-habit from parent to offspring
39
IX (i
con
T able
E xperim ental fo o d : fuchsia No. tested 250
Progeny of ivy-fed p arents [Per 50
50
Progeny of privet-fed p arents
Iv y 89 17-8
N eutral 81 16-2
Fuchsia 80 16-0
D ead 0 0]
P riv et 14
N eutral 24
Fuchsia 12
0
E xperim ental fo o d : blackberry Iv y 29 5-8
N eutral B lackberry 60 161 12-0 32-2
D ead 0 0]
P riv et 5
N eutral B lackberry 32 13
0
No. tested 250
Progeny of ivy-fed paren ts [Per 50 Progeny of privet-fed p arents
50
E xperim ental food: poplar No. tested 250
Progeny of ivy-fed p arents [Per 50 Progeny of privet-fed paren ts
P r esen ta tio n P aren tal p resen ta tion group 2 .3 .1 .1 1 0 .3 .2 .1
Iv y 249 49-8
N eutral 0 0
Poplar 0 0
D ead 1 0-2]
50
P riv e t 50
N eutral 0
Poplar 0
0
T able
X
4 b . I v y -fe d
test
P r iv e t -fe d 100 100
P r e fe r e n c e P aren tal presentation group 2 .3 .1 .1 1 0 .3 .2 .1 T otal
pa r e n ts ,
96 95 test
No. tested 50 50 100
50 50 100
— —
4 5
Iv y 10 8 18 pa r e n ts ,
2 8 10
Dead 0 0
1935
4 b . I v y -fe d
P r iv e t -fed Forest Hill Stock Control Stock T otal
1935
Presentation of lilac to progeny ______________ *_____________ _ 2 3 4 1 — — 92 8 4 96 — '—
NTn tested 100 100
Forest Hill Stock Control Stock
pa r e n t s ,
pa r e n ts ,
N eutral 8 16 24
— —
0 0
1935 Lilac 32 26 58
D ead 0 0 0
33 25 58
0 0
1935 15 17 32
0
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40
D. E. Sladden and H. R. Hewer Table XI P r esen ta tio n
P a re n ta l presen- No. ta tio n group tested 1
2 .3 .1 .1 10.3.2.1
100 100 200
T otal
4 c . I y y -eed
test
pa ren ts,
1935
P resentations of poplar to progeny ,---------- ■ -----------N
2
0 9 1 3 1 12
3 4 0 4
4 7 5 12
5 2 3 5
7 3 2 5
6 2 2 4
8 3 3 6
9 0 3 3
Re-
10 fused D ead 2 53 15 3 22 53 5 106 37
/
_____
T
143
57 P r iv e t -f e d F o rest H ill Stock Control Stock T otal
100 100 200
0 2 2
2 2 4
6 3
pa ren ts,
5 3 8
9
1935
1 3 4
3 1 4
5 1 6
1 0 1
2 1 3
1 1 2
V
J
43 P reference P a re n ta l presentation group
2.3.1.1 10.3.2.1
test
P r iv e t -f e d F o rest H ill Stock Control Stock
50 50
pa ren ts,
Iv y 50 50 pa ren ts,
6 16 22 —
157
4 c . I v y -fe d
No. tested 50 50
68 67 135
1935
N eutral
Poplar
0 0
0 0
N eutral
Poplar
0 0
0 0
1935
P riv et 50 50
P erio d ic it y
Previously it had been suggested th at the concentration was due to seasonal changes (Sladden 1935, pp. 43, 44). This has been the subject of further investigation. Fortunately, all records, with the exception of a few in 1932, contained data about the dates of emergence and consequent testing. I t was therefore possible to reorientate all the presentation tests figures in the monthly periods irrespective of parentage or year. These data are set out in Tables V II (ivy-fed parents) and V III (privet-fed parents). Each table contains the yearly composition of the monthly groupings, the monthly totals in presentation groups and lastly the mean presentation times expressed as decimals of group numbers. During 1936 and 1937 further presentation tests were carried out in order to increase the totals for certain months. The totals are now
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Transference of induced food-habit from parent to offspring
41
reasonably high, with the exception of th at for the month of May among the ivy-fed insect data. No emergences among these insects have occurred for several years in the month of May. Referring first to the privet-fed stock (controls) the periodicity postu lated is shown very obviously not only in the percentages accepting ivy at T a ble X II P resen ta tio n
test
5 c . P oplar -fe d
pa r e n ts ,
1936
Refused P resentation 10th presen groups 1-10 tatio n
P aren tal presentation group
No. tested
( 2 .3 .1 .1 .) 2 (2 .3 .1 .1 .) 4 (1 0 .3 .2 .1 ) 4 (1 0 .3 .2 .1 ) 7 (Forest Hill) 3 (Forest Hill) 7
20 50 30 50 95 100
0 0 0 0 0 0
4 39 26 40 74 87
16 11 4 10 21 13
345
0
270
75
T otal
P r e fe r e n c e P aren tal p resentation group ( 2 .3 .1 .1 ) ( 2 .3 .1 .1 ) (1 0 .3 .2 .1 ) (1 0 .3 .2 .1 )
test
pa r e n t s ,
1936
No. tested
Iv y
N eutral
P oplar
D ead
13 50 16 50
9 50 16 48
0 0 0 0
0 0 0 0
4 0 0 2
No. tested 50 50
P riv et
N eutral
Poplar
D ead 0 0
2 4 4 7
(Forest Hill) 3 (Forest Hill) 7
5 c . P oplar -f e d
D ead
50 50
0 0
0 0
Indices outside brackets refer to poplar presentation groups.
T able X III Stock (2 .3 .1 .1 ) 2 (2 .3 .1 .1 ) 4 (1 0 .3 .2 .1 ) 4 (1 0 .3 .2 .1 ) 7 (Forest Hill) 3 (Forest Hill) 7 Totals Totals
Eggs selected 300 315 225 225 330 255 1650
H atched 40 164 46 157 171 166 744
Used 33 100 46 100 145 150 574
% selected/ hatched 13-3 52-1 20-4 69-8 . 51-8 65-1 Means 45T
Cf. Table X , Sladden (1935) which gives: 4050 3660 2600 Means 90-4
% selected/ used 110 31-7 20-4 44-4 43-9 58-8 34-8 71-0
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the first presentation (as opposed to those accepting ivy at the second or third) but also in the number of presentations required to induce some insects to accept ivy during the winter and spring. The correspondence between the results for February and March in 1932 and 1936 is notable as indicating the probable validity of the figures, consistent as they are with the January figures of 1932 and 1933. Examination of these tables can leave no doubt th at a periodicity exists either in the palatability of the food plant or in the habits of the insect. There is no direct evidence to distinguish between these two possibilities, but this does not m atter since we are concerned solely with the elucidation of the anomalous results obtained with the controls from year to year. I t will be seen th at there is an approximation of observed data on both experimental and control insects during the months of June, July and August. In fact it is quite conceivable th at a repetition of these experi ments carried out over these three months would produce results, mathe matically significant, in direct contradiction to those obtained by us during other periods of the year. I t is to be regarded as fortunate th at our original experiments were carried out during the time of year when the differences in behaviour between the privet-fed and ivy-fed forms is at its maximum. This point must be borne in mind should these experiments be repeated. T ests
on o ther food plants
In the course of the experiments previously communicated it seemed possible to us th at the increased ability to eat ivy shown by the ivy-fed stock might not be specific but merely an expression of a more generalized ability to eat any other food plant. W ith a view to testing this view a number of preference tests were made with five different food plants, some of which had already been reported as possible foods for the insect. The results of these tests are set out in Table IX . I t will be noted that the ivy-fed stock were tested with ivy and the experimental food plant, and the privet-fed stock tested with privet and the same food plant. This appeared to be the only way to make com parable tests. The large number (250) progeny of ivy-fed parents tested is due to the use of five different parental groups each providing fifty experi mental progeny. There was no significant difference between any of these groups so th a t the average per fifty is quite justified. The following points emerge: (1) Poplar is consistently refused and is the only food plant of the five to be more distasteful to the insect than ivy or privet.
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Transference of induced food-habit from parent to offspring
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(2) Rose, lilac and blackberry appear to be more readily eaten than ivy or privet. (3) Fuchsia appears to be at about the same degree of “ preference” as ivy and privet. (4) Leaving “ poplar” on one side, in all the other experiments the progeny of ivy-fed parents appear to “ prefer” ivy to the experimental food plant more than the progeny of privet-fed parents “ prefer” privet. There is thus no evidence th at the induction of an ivy-feeding habit has “ broken” a privet-feeding habit since privet is certainly not one of the most palatable food plants to the insect. In fact to speak in such terms is obviously inconsistent with the delicacy of the process which has really taken place. The change in food habit is, in fact, extremely small and only detectable by a method of presentation and preference tests such as we have employed. As an example of this delicacy we may quote a further experiment carried out using lilac as the experimental food plant. In Table X are set out the results of presentation and preference tests on the progeny of both ivy-fed and privet-fed parents. The results of presentation test 4B are such th at any further generations for purposes of testing are little good since the percentage taking lilac at the first presentation is very high while those preferring lilac to ivy is over 50% (preference test 4B). Preference test 4B is practically a repetition of the preference test in lilac in Table IX and the figures will be seen to agree very well. Another example, in the opposite direction, is shown in the presentation and preference tests in poplar (Tables X I and X II). In these tables two generations are shown and the most noticeable feature of the experiment as a whole is the complete failure to induce the insects to increase their ability to eat poplar. After presentation test 4C it hardly seemed worth while to continue the experiment since so many individuals had either died or refused poplar for ten presentations. This limit had to be imposed as otherwise it became impossible to keep adequate records of the individuals tested. The experiment was however continued for a second generation to see whether selection was taking place. In Table X II it will be seen that none of the 345 progeny tested accepted poplar during the first ten presentations. The death rate was again high. I t is possible to account for this on the grounds that a modification of feeding habit towards ‘‘ability to eat poplar is outside the range of adaptability of the insect. Its repugnance for this food is very clearly shown in all the preference tests.
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D. E. Sladden and H. R. Hewer
Lastly it may be noted th at these experiments on poplar were accom panied by a great decrease in the fecundity of the parents and in the viability of the eggs. This is shown in Table X III with a comparison with totals from normal batches of eggs.
Conclusions
This series of experiments is now brought to an end, and it is very doubtful whether they could ever be repeated unless someone could be found willing to devote himself throughout the year to the tedious recording and experimentation carried out by Miss Dorothy Sladden, over a period of some six years. The conclusions appear to be fairly clear. Two sets of stick insects, one reared on privet in every generation but tested for their ability to accept ivy by presentation and preference tests on samples of each generation, the other reared on ivy in every generation after having been tested for their ability to accept it by identical tests, have demonstrated different relative tendencies to accept ivy as a food plant. Those forced to eat ivy from the first generation onwards rapidly developed an increased ability to accept this food plant, an ability which is comparatively little affected by the time of year when the test is made. Those reared on privet continuously displayed, during the first four filial generations, an increased ability to accept ivy but to a much less extent than did the ivy-fed stock. Further this increased ability to accept ivy appears to be a function of the time of year when the tests are made, since not only do the fifth and sixth filial generations show a decrease in the ability to accept ivy but analysis of the results on a monthly basis displays a distinct annual periodicity only slightly and hardly significantly shown by the ivy-fed stock. I t is therefore difficult to escape the conclusion th at the forced ivy feeding has induced the increased ability to accept ivy in succeeding genera tions in this parthenogenetic insect. I t is a m atter of the deepest regret th a t the work cannot be carried on and repeated with insects reproducing by sperm and egg. R e fer e n c e s Sladden, D. E. 1934 Proc. Roy. Soc. B, 114 , 441. — 1935 P™c. Roy. Soc. B, 119 , 31.
