On Death Feigning in Aphodius sp. By S.C. SAXENA and Y. SAXENA. Department of Zoology, University of Rajasthan, Jaipur, India

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1 On Death Feigning in Aphodius sp. By S.C. SAXENA and Y. SAXENA Department of Zoology, University of Rajasthan, Jaipur, India The response of insects to various mechanical stimuli, by passing into a motionless characteristic state has been reported by several authors. Various terms have been used by different workers to express this state of tonic immobility. HOLMES (1903, 1906), SEVERIN and SEVERIN (1911), LOHNER (1914) and WEISS (1940, 1947,) used the term death feigning, REISINGER (1928) and TEYROVSKY (1949) preferred katalepsy, and AUDOVA (1929), KOZANSHIKOV (1931) and SAXENA (1957, 1958, 1959, 1961, 1962) called it thanatosis state. ROEDER (1953) attempted to define akinesis or tonic immobility and suggested the term catalepsy for those who assume a stick insect like or protective posture while retaining their foothold and thanatosis or death feigning when the insect falls to the ground after the foothold is released. Without going into the controversy of the terms used, the authors, in the present paper, intend to describe some of the observations on thanatosis or death feigning in Aphodius. METERIAL AND METHOD The experimental insects collected from light were kept in the laboratory in muslin topped jars at the room temperature. Since the handling of the insects results in the increased activity leading to the stage of excitement when they become unresponsive to the stimulus, they were allowed reasonable time to remain undisturbed before the duration of thanatosis was recorded. For this purpose each individual was kept under a muslin topped glass funnel standing on filter paper for 4 hours at room temperature, after which the duration of thanatosis was recorded by inducing it by applying a needle to the thorax on the dorsal side. The state of death feigning was taken as terminated and the time was noted no sooner the legs started moving. In order to study the effect of light, it was essential to check the heat of the bulb from reaching the experimental insects as the temperature influences death feigning. This was achieved by using a special box as described in an earlier paper (SAXENA, 1957) in which the heat was checked by keeping a trough with water between the experimental insects and the light. The experiments were performed in the dark room and the insects were conditioned for 24 hours in the dark before subjecting them to light. All the experiments were performed at room temperature 28 }2 Ž and the insects for any experiment were taken at random from the population. The data were applied with square root transformation in order to avoid any uneven distribution of the data and thus to make the variance in any group of observations more nearly independent of mean. RESULTS AND DISCUSSION The observations that the posture assumed by the insects during death feigning was quite different from those assumed by dead specimens, was in accordance with DARWIN quoted by HOLMES (1908) and SAXENA (1961). Induction of death feigning The beetles were thrown into the state of death feigning for maximum period by (Received for publication, July 10, 1964) 305

2 306 Japanese Journal of Applied Entomology and Zoology Vol.8, No.4 applying a needle to the thorax on the dorsal side. It was also noted that the ventral surface is also equally sensitive, for evoking thanatosis. The sensitivity may even be considered of higher degree on the ventral side rather than on dorsal side. This beetle is very susceptible to any of the mechanical stimuli. Even the picking of the insect by finger or forcep or dropping it from a height of few inches brings on this state. Sometimes the insect went into the motionless state simply on tapping the medium on which it was resting. The method of evoking thanatosis varies in different arthropods due to their differential sensitivity of different parts of the body to mechanical and other stimuli (SAXENA, 1961). To induce thanatosis in coccinellids a light pressure on the ventral side of the insects by means of the index finger was used (SAXENA, 1958), whereas in Carausius morosus the stimulus used was a pressure on the sides of the thorax by the thumb and forefinger (SAXENA, 1962). Posture during death feigning In general the arthropods assume a compact form by ceasing all their movements and tightly pressing their legs and antennae to the body; the antennae and the forelegs are usually held parallel to the head and the mid and hind legs tightly close to the body (SAXENA, 1961). But this generalised posture exhibits vide variation not only among different species but even in the same species, although but rarely. In coccinellids head usually moves down and assumes a curve like structure touching the fore legs with the antennae pressed to the head and the legs contracted to the body with femora, tibia and tarsi folded on each other (SAXENA, 1958), in Carausius morosus in most cases the antennae and the forelegs are tightly pressed to the body with the antennae stretched forward but the posture of the legs may vary in different individuals (SAXENA, 1962), and unlike all these postures Armadillidium vulgare rolls up into a ball like structure (SAXENA, 1957). The posture assumed by this beetle resembles to that of coccinellids to the extent that the legs contract to the body and femora, tibia and tarsi fold on each other. The antennae takes up a position unlike other beetles; it does not tightly press to the head but remains in the normal position. Sometimes the forelimbs do not take up their usual position and remain hanging. Termination of thanatosis Just as different arthropods assume different postures their termination of thanatosis also varies. SAXENA (1961) pointed out two categories, one of instantaneous termination as in some coccinellids and the other of gradual termination as in granary weevils. In general the indication of termination of thanatosis is a slow movement of antennae followed by the slight right and left movement of the head, which is then followed by the movement of the legs. But again there are variations from this generalised method of termination; in Carausius morosus after the movement of the antennae no head movement takes place (SAXENA, 1962), in coccinellids it is quicker begining with the unbending of antennae followed by a sudden jerky movement of forelegs and then of mid and hind legs (SAXENA, 1958), in Armadillidium vulgare it is gradual begining with unrolling of the body during which the antennae and legs get into motion and peep out (SAXENA, 1957). In this beetle the termination is instantaneous like coccinellids, with the difference that the antennae do not move first but the legs straightway come into motion and the insect stands up ready to run. Apparent insensibility to pain while in the death feint The beetles during the state of thanatosis fail to respond to the minor disturbances. When they were made to stand on their legs, they simply fell down and behaved like dead insects. In some cases even when they were pricked by the needle, no symptom of pain was shown by

3 December, 1964 SAXENA & SAXENA: Death feigning in Aphodius sp. 307 them. DEGUR and SAXENA (1957) have also reported similar observations. Response to different stimuli The experiment was performed with 40 insects divided into 4 batches and thanatosis response was recorded on applying the mechanical stimuli of five grades. Different duration of death feigning were obtained on subjecting the insects to different stimuli, such as air, cotton, brush, glued thread and needle (Table 1). It is Table 1. Response to different stimuli. mechanical also clear from the observations that harder the stimulus the longer is the duration of death feigning. The observations are in accordance with SAXENA (1957, 1958, 1961) and the authors incline to support an earlier suggestion (SAXENA, 1961) that the cause of different responses on applying the stimuli of different intensities, may be due to the variations in the number and frequency of nerve impulses transmitted from the receptors. WEISS (1947) also thought of some correlation between the strength of the stimulus and the period of thanatosis. Response to continuous application of stimulus Ten specimens were subjected to continuous application of the stimulus (needle). Each individual was given a second and successive applications of stimulus soon after the termination of death feints induced by the previous application. The results (Table 2) show a decline in the duration of thanatosis with the increase Table 2. Death feigning on repeated applications of stimulus. in the number of applications of the stimulus. Similar observations are reported by earlier workers (KOZANSHIKOV, 1931; SAXENA, 1957, 1958, 1961, 1962). SAXENA (1961) has already explained the decline in magnitude of response by suggesting that it is due to synaptic fatigue and does not appear to be as a result of the adaptation of the sense organs as the various stimuli were applied successively but relatively slowly. Effect of physical factors (I) Effect of illumination 30 insect divided into 3 batches were subjected to ordinary room light and 100W bulb light, for 1 hour and 3 hours. The results (Table 3) show that the duration of thanatosis increases with the increase in the exposure period of the illumination. HOLMES (1906, 1907) and SAXENA (1957, 1958, 1961) reported the decrease in the period of thanatosis with the increase in the light intensity or the increase in the exposure period at the same level of illumination. But SAXENA (1962) while working on C. morosus observed the persistance of thanatosis under illumination. The present is the case similar to C. morosus. These beetles were found attracted to light and like stick insect exhibited their sensitivity to illumination by running in a state of thanatosis for a longer period under the artificial light. Keeping in mind both the views that of HOLMES (1906, 1907) and of SAXENA (1961, 1962) and the results of the present experiment, it appears that those insects which move away from the light and get excited show a fall in the duration of thanatosis under different light intensities whereas the other group of insects which is attracted to light shows its sensitivity to the illumination by ex- Table 3. Death feigning under different light exposures.

4 hibiting a prolonged period of death feigning under light. But the authors feel that more observation with different insects would be able to throw more light on this aspect. (II) Effect of temperature To study the effect of temperature two batches of beetles of 10 each were conditioned at 22 Ž and 32 Ž and the duration of death feigning was recorded. The observations (Table 4) reveal that at low temperature the insects show an increase in the duration of Table 4. Death feigning at different temperatures. thanatosis. HOLMES (1906) and SAXENA (1957, 1958, 1961) obtained the similar results. Insects were also found more excited at high temperature and inactive at low temperature. The authors support an earlier interpretation (SAXENA, 1961) that if thanatosis is induced when the nervous system of insects is in an excited conditions, the reversion occurs quicker than if it is in a less excited condition. (III) Effect of heat radiation The effect of heat radiation on death feigning was studied by transmitting the heat radiations to the insects by bringing a needle heated in a flame for 30 minutes, about less than a centimeter away from both the antennae of the insects which were in the state of thanatosis. A control was also run. The results obtained (Table 5) Table 5. Thanatosis response to heat radiation. Table 6. Thanatosis response to starvation. starved, and the other was run as control. The ovservations were recorded after 18, 36 and 96 hours of starvation. As the figures (Table 6) indicate the duration of thanatosis goes up with an increase in the period of starvation. SAXENA (1958, 1961) recorded similar increase in duration after keeping the weevils and coccinellids starved for different periods. The insects also appeared to be very inactive after starvation. The authors feel that just as at low temperature when the insects are inactive and not in a normal or excited condition they show a longer duration of thanatosis, probably in the same way as when they are starved they are inactive and exhibit death feigning for longer than the normal period. ACKNOWLEDGEMENTS The authors wish to express their deep gratitude to Mrs. S. BHARTIYA, the Director, University Maharani's College for Women, Jaipur, for her prompt and liberal, moral, as well as financial help, keen interest and encouragement which enabled us to complete this investigation. Thanks are also due to Prof. L.S. RAMASWAMI, Head of Zoology Department for his encouragement throughout the work. We also thank the Forest Research Institute, Dehradun, India for kindly identifying the beetle. SUMMARY show the termination of thanatosis earlier than the normal period, which is in accordance to SAXENA (1961). (IV) Effect of starvation Out of two batches each of 10 insects one was kept Thanatosis are death feigning may be induced in Aphodius by applying the mechanical stimulus such as needle to the thorax on the dorsal side or the ventral side. During thanatosis state the insects

5 December, 1964 SAXENA & SAXENA: Death feigning in Aphodius sp. 309 assume a posture which somewhat resembles to that of coccinellids; the femora, tibia and tarsi of the legs fold on each other and the antennae do not press to the head but take up the normal position. The termination of thanatosis is instantaneous like coccinellids; the antennae do not show any movement but the legs straight way start moving with jerk. The insect does not show any symptom of pain during thanatosis state. There appears to be a relation between the intensity of the stimulus and the duration of thanatosis, as the different periods of death feigning are recorded on subjecting the beetles to mechanical stimuli of different grades. A decline in the duration of thanatosis takes place on subjecting the insects to repeated applications of the same stimulus, probably due to the approach of the fatigue stage. Like Carausius morosus the beetles, under long exposures of light at fixed level of illumination run into a prolonged state of thanatosis thereby exhibit their sensitivity to the light. At low temperature since the insects become inactive they show an increase in the duration of death feigning which decreases when the temperature is raised as they get excited. The insects terminate thanatosis earlier than the normal period on subjecting them to heat radiation while they are in thanatosis state. The starved beetles as they become inactive show an unusual rise in the duration of death feigning. REFERENCES AUDOVA, A. (1929) Z. Morph. u. Oekol. Tiere 13: 722 `744. HOLMES, S.J. (1903) Biol. Bull. 4: 191 `196. HOLMES, S.J. (1906) Jour. Comp. Neurol. Physiol. 16: 200 `216. HOLMES, S.J. (1907) Biol. Bull. 12: 158 `164. HOLMES, (1908) Pop. Sci. Mon. 72: 179 `185. KOZANSHIKOV, I. (1931) Bull. Plant Protection, Leningrad 3: 233 `240. LOHNER, L. (1914) Z. allg. Physiol. 16: REISINGER, L. (1928) Biol. Zentrabl. 48: 162 `167. ROEDER, K.D. (1953) Insect physiology. SAXENA, S.C. (1957) Jour. Zool. Soc. India 9: 192 `199. SAXENA, S.C. (1957) Jour. Univ. Sagar 6: 90 `98. SAXENA, S.C. (1958) Proc. Natl. Acad. Sci. India 28: 41 `48. SAXENA, S.C. (1959) Bull. Zool. Soc. Coll. Sci. Nagpur 2: 29 `31. SAXENA, S.C. (1961) Beitr. Entomol. 11: 269 ` 280. SAXENA, S.C. (1962) Proc. Natl. Acad. Sci. India 32: 189 `196. SEVERIN, H.H.P. and H.C. SEVERIN (1911) Behav. Monograph. 1, 44 pp. TEYROVSKY, V. (1949) Acta Soc. Zool. Csl. (Prague) 13: 334 `347. WEISS, H.B. (1940) Jour. N.Y. Entomol. Soc. 48: 37 `46, 303 `304. WEISS, H.B. (1947) Jour. N.Y. Entomol. Soc. 55: 275 `279.