Louse Oniscus Asellus to Environmental Stimuli

295 STUDIES IN DIURNAL RHYTHMS II. CHANGES IN THE PHYSIOLOGICAL RESPONSES OF THE WOOD- LOUSE ONISCUS ASELLUS TO ENVIRONMENTAL STIMULI BY J. L. CLOUDSLEY-THOMPSON From the Department of Zoology, King's College, University of London {Received 19 July 1951) (With Three Text-figures) INTRODUCTION In an earlier paper in this series, it was pointed out that diurnal rhythms are basically of two kinds: exogenous, a direct response to environmental changes, and endogenous rhythms which persist under constant conditions. For example, the West African millipede Ophistreptus sp. exhibits a 24 hr. periodicity under constant conditions for 19 days (Cloudsley-Thompson, 1951a). Many species of animals show a combination of both types and such rhythms are termed 'composite' by Park (1949). Endogenous rhythms are frequently correlated with environmental changes such as light, temperature and relative humidity. As they are not a direct response to these changes, however, Dr D. L. Gunn has suggested that it is preferable to refer to them as 'clues' rather than 'stimuli'. Another factor to be considered is that although many species are active during a certain period of the day or night and are quiescent for the remainder of the 24 hours, some exhibit different kinds of activity at different times. Thus the water-skater Gerris spends the daytime on the surface of ponds and streams, but flies from one locality to another at night (Riley, 1925); many other aquatic insects such as beetles, fly mostly at night, but swim actively throughout the day. Most of the work on rhythmic behaviour in animals has, in the past, been con- cerned with establishing the existence of 24 hr. periodicities, and determining the 'clues' with which they are correlated (Calhoun, 1944). In the case of locomotory rhythms, little attention has been directed to the changes in physiological response which may be correlated with outbursts of activity. An example of this kind of change is afforded by the pill-woodlouse Armadillidium vulgare. This species is more resistant to desiccation than other species of Isopoda (Edney, 1951), and shows locomotory activity principally in the morning when it is often to be seen walking about in the sunlight (Cloudsley-Thompson, 1951c). This is probably correlated with the fact that in this species some individuals exhibit positive photo- taxis when the temperature rises (Henke, 1931). The majority of woodlice such as Oniscus and Porcellio spp. are nocturnal, however, and come out at night from their hiding places under stones, logs and bark, J. L. CLOUDSLEY-THOMPSON and wander in dry places where they are not found during the day. For example, they are often to be seen climbing up walls after dark. Atmospheric humidity is greater during the night, the temperature drops and light is absent. In this paper an attempt has been made to indicate how nocturnal changes in environmental conditions may alter the daytime reactions of Oniscus asellus, and thus engender the observed locomotory activity. MATERIAL The species used in this investigation was Oniscus asellus L. collected from my garden in Esher, Surrey. Adult animals of both sexes were used, but these were not separated. ESTABLISHING THE RHYTHM Diurnal periodic locomotory activity was investigated using the Aktograph apparatus described by Gunn & Kennedy (1936). Instead of the original long writing lever, however, a short rod was connected with a gymbal lever, and a clockwork baro- graph drum was employed as a kymograph in place of a 12 inch motor-driven drum. 20 30 15 J J 20 10 10 5 III 1 I 201 151 30 10 20 10 10 Days Fig. 1. Activity of Oniscui, in damp surroundings under natural lighting and in darkness at room temperature; and later in darkness at room temperature followed by artificially fluctuating temperature reaching a minimum during the day. The object of these modifications was to reduce the overall size of the apparatus so that it could be placed in a large incubator, and light and temperature controlled artificially. The floor of the arena was lined with damp filter paper so that the humidity was maintained at a high level, and did not fluctuate with temperature. The results obtained are plotted as block histograms (Fig. 1), and show that locomotory activity at room temperature is mostly confined to the hours of darkness. Studies in diurnal rhythms 297 Th preliminary experiments the arena was exposed to normal daylight and darkness, and fluctuating temperatures, the latter being registered on a recording thermometer. When, however, light was excluded, the periodicity disappeared after a few days, suggesting that the rhythm is 'composite', the 'clues' fluctuation in light rather than in temperature. Fluctuations in temperature alone did not maintain the rhythm, but it was immediately resumed in alternating light and darkness. Throughout the work it has been assumed therefore, that if woodlice are kept for several days in darkness, their normal periodicity is eliminated and the behaviour of such animals resembles that of control animals at night: check tests have con- firmed this. DIURNAL FLUCTUATIONS IN THE HUMIDITY RESPONSE Experiments were carried out on Onucus asellus by a modification of the method described by Gunn (1937), using two choice-chambers identical with those employed in the investigation of the humidity responses of millipedes (Cloudsley- Thompson, 1951 b). One choice-chamber was kept in light, the other was darkened with a wooden cover. The air in one hah0 of each arena was kept dry, using a mixture of sulphuric acid and distilled water calculated to give a relative humidity of 50 % (Buxton & Mellanby, 1934). On the other side, distilled water was placed beneath the perforated zinc gauze. A paper hygrometer recorded 60 and 90 % relative humidity respectively on the two sides of the arena. All experiments were carried out at room temperature 18 ±2° C. in subdued daylight during March and April The apparatus was set up overnight, and on the following morning five woodlice were placed in each arena. Their positions were noted at intervals of 15 min. Animals moving were counted separately, as were any within 1*5 cm. of the boun- dary. After each reading, the animals were stirred up with a glass rod so that after each stirring there were either two animals on each side and one in the middle, or three on the side which had previously had two. After ten readings giving fifty position records, the animals were returned to the cultures. The apparatus was rotated between experiments so that external factors were cancelled out, and the positions of the perforated zinc platforms, and of the sides and lids were interchanged. The intensity of the reaction was so marked that on one occasion a slight leak of acid into the distilled water was immediately detected through the abnormal behaviour of the woodlice in the arena. Two series of experiments were carried out. In the first, the responses in light of animals from a control culture exposed to daylight were compared with those of similar animals in the darkened choice-chamber. In the second series, the responses in light of control animals were compared with those of animals in the darkened choice-chamber which had previously been kept in darkness at room temperature for several weeks (Table 1). The intensity of the reaction was calculated in each case by dividing the number of stationary woodlice on the moist side of the arena plus half the number in the 298 J. L. CLOUDSLEY-THOMPSON middle, by the number on the dry side plus half the number in the middle. It was also calculated using Gunn's (1937) method in which those in the middle are omitted; but since they were considerably more numerous in the experiments carried out in darkness, they cannot in this case justifiably be ignored. Table 1. Number of looodtice moving or stationary on the dry (50 % R.H.), middle or moist (100 % R.H.) sides of humidity choice-chamber apparatus. Totals Percentages Totals Percentages of first of first of second of second Grand Percentagi halves of halves of halves of halves of totals of grand experiments experiments experiments experiments totals A. Moving "3 226 87 17-4 200 2O-O On dry side 21 4-2 13 26 34 3-4 In middle 49 9-8 24 48 73 7-3 On moist side 317 63-4 376 75-2 693 693 Total 5°o 100 500 100 1000 100 Intensity of reaction — i5-I — 28-9 — 204 (Gunn'8 method) Intensity of reaction — 75 — — 104 (alternative method) B. Moving 23 92 17 6-8 40 80 On dry side 32 128 3 1-2 35 70 In middle 31 124 16 6-4 47 9-4 On moist side 164 656 214 85-6 378 75-6 Total 250 100 250 100 500 100 Intensity of reaction — S-I — 7i-3 — io-8 (Gunn's method) Intensity of reaction — 3-8 — 2O'3 — 69 (alternative method) C. Moving 7 28 9 36 i7 3-4 On dry side 46 18-3 7 28 53 io-6 In middle 54 216 33 132 86 17-2 On moist side 143 57-3 201 80-4 344 68-8 Total 250 100 250 100 5°o 100 Intensity of reaction — 3-i — 287 — 6-5 (Gunn's method) Intensity of reaction — 3-3 — 93 — 4a (alternative method) A, controls in light; B, controls in darkness; C, animals in darkness from a culture kept in darkness for seve days previously. Room temperatures 18 ±2° C. From the results (Table 1) it can be seen that not only was the intensity of the humidity reaction of woodlice in darkness (B) considerably less than in controls (A), but it was still further reduced in animals (C) which had been kept in darkness for some days prior to the experiments.

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