An analysis of building behaviour of the termite Macrotermes subhyalinus (Rambur)

Although the work presented in this study is little more than a first survey of the causal factors involved in construction behaviour of Macrotermessubhyalinus , it clearly indicates a fertile field of research which has scarcely been explored. Past studies have concentrated on the release of building behaviour and have done little (with the exception of Grassé, 1959) to account for the formation of the various types of structural elements present in termite nests. The present study gives a general account of building behaviour and of the formation of several structural elements: volatile chemical cues in concerted action with tactile stimuli provided by physical objects, cause workers to grasp soil pellets, transport them, and subsequently deposit them at a building site. The ability of termite workers to employ pheromones in a versatile way in their orientation is the underlying condition which gives rise to the formation of a variety of structural elements.Building behaviour in this species is predominantly released by chemical stimuli like the building pheromone of the queen, the 'cement' pheromone and the trail pheromone (Bruinsma, unpublished). In view of the fact that the active space of the latter two pheromones directly depends on the number abd distribution of workers, it would appear, then, that the building latency time is inversely related to the workers group size. Grassé, (1959) has described such a density dependent effect on building activity in Cubitermes.The orientation required by building workers is mediated by trail- and 'cement' pheromone (directional orientation) and in the appropriate situation also by the building pheromone emanating from the queen (providing distance orientation). Other authors have documented examples of chemical orientation among termites. Leuthold (1975) has reviewed a body of evidence which shows that termites to a great extent rely on odour trail orientation. Stuart (1967) discovered that Zootermopsis is recruited to building sites by chemical trails. Grassé, (1959), Wilson (1971) and Deneubourg (1977) indicated the possibility of odours emanating from construction sites, orienting nearby motivated workers to such sites. This study furnishes convincing authentication of a pheromone emanating from a building site and confirms its rôle in worker orientation. Soil transporting workers arriving at a building site, e.g. located along a trail or in a deposition zone around the queen, attain a stable orientation with respect to that site prior to and during cementing their load. This response is probably to be due to the perception of a local stimulus, or stimulus complex. Such a mechanism of orientation is named telotaxis (Schöne, 1973), and implies as already emphasized by Fraenkel and Gunn (1961), simultaneous perception and evaluation of several stimuli.The results obtained with physical objects (small spheres) are in accordance with those of Stuart (1967) who demonstrated that surface irregularities release building behaviour in Nasutitermes. In respect of the ability of M . subhyalinus to perceive spatial relations between the discussed stimuli, it is of interest to mention a mechanism of orientation which has been described by Forel as: "By topochemical I mean a sense of smell which informs the ant as to the topography of the places surrounding it by means of chemical emanations which give an odour to objects" (Forel, 1928; cit. by Wilson, 1971, p252). To date such a mechanism integrating olfactory and mechanical information has been only demonstrated in honey bees (Martin, 1965) and the termites referred to in this study.Grassé, (1959, 1967) described in detail building behaviour in M . bellicosus , M . mülleri and Cubitermes sp. According to this author, it is the product of work previously accomplished, which constitutes the social stimulus for the workers to perform additional work (the concept of 'stigmergy'). In summary, two stages are distinguished: 1) A phase of 'unco-ordination' during which the workers first explore the container they are placed in, and after a certain time lapse start depositing soil pellets anywhere in the arena. When at some place the deposited material reaches a 'critical density' that is when several soil pellets are stuck together, this incipient structure proves to be very attractive to the workers compared to single pellets. 2) Subsequently the phase of 'co-ordination' is started during which workers transform the incipient structures into pillars, which, when a neighbouring one is within a critical distance, are combined to form arches. The concept of 'stigmergy' can be very well viewed in terms of the mechanism treated in the present study, except the observation that, initially, building is random. This observation is at variance with that of Stuart (1969), and the finding in the current study that workers employ the edge of a pheromone trail as a zone to co-ordinate soil depositions.Stuart (1967, 1969, and 1972) concludes that the behavioural basis of building is a response to a 'low level excitatory' stimulus, like air movement, odour, light, temperature etc., deviating from the normal nest environment. The subsequent building activity gradually eliminates the causal stimulus, and when no more environmental stimuli are forthcoming, building will stop. In conclusion, the immediate function of building is a homeostatic one. Since termites like most other social insects control the environment within the nest by actively maintaining several steady states, based on behavioural and physiological regulation (this phenomenon has been named 'social homeostasis' by Emerson, 1956), it is important to know whether the building mechanism proposed in this study is compatible with homeostatic regulation. At present it is only possible to indicate that there are several negative feedback mechanisms operating during building activity. At first, the ephemeral activity of both the trail- and the 'cement' pheromone is likely to modify the number of workers recruited to building sites. Secondly, it could be shown that major and minor workers during subsequent building runs loose between 20 - 40% of their fresh weight. In case workers are not allowed to compensate this loss by drinking at the water supply, they invariably stop building (Bruinsma, unpublished).The fact that pheromones, together with structural signals, are involved in the initiation and co-ordination of building behaviour in termites constitutes a plausible mechanism which accounts for most aspects of nest building behaviour in M . subhyalinus . The findings presented in this study may serve a useful purpose, increasing the understanding of the way in which termites perform such remarkable engineering feats as the construction of their impressive nest mounds.