Aspects of the biology of aestivation in Bulinus (Physopsis) africanus (Krauss) (Gastropoda, Pulmonata)
- Authors: Heeg, J. (Jan)
- Date: 1973
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:21054 , http://hdl.handle.net/10962/6178
- Description: Aestivation, a period of dormancy during hot, dry conditions, is known to occur in both the Prosobranchia and the Pulmonata among freshwater Gastropoda. While there have been numerous accounts, mainly by field ecologists concerned with bilharziasis control, of the survival value of aestivation in the Gastropoda, little is known of its underlying physiological mechanisms. Results of investigations into the physiological basis of aestivation in the freshwater Prosobranchia, confined to the family Ampulariidae, have been conflicting, fundamental differences having been shown to exist between different species of the genus Pila . Only a single comprehensive study, that by von Brand and his coworkers on the planorbid snail Australorbis glabratus*, forms the basis of our knowledge of the aestivation process in the aquatic Pulmonata. In view of the conflicting results obtained in investigations on freshwater Prosobranchia, the general applicability of the findings for Australorbis glabratus to other freshwater Pulmonata was open to question. The present investigation on the planorbid snail Bulinus (Physopsis) afriaanus, a species known to be a successful aestivator, was prompted by the obvious need for a further comprehensive study on an aquatic pulmonate species . The results presented here show that aestivation in this species is a definite physiological state, characterised by a depression of the metabolic rate, which not only aids in husbanding the snails I metabolic resources. during the enforced starvation which must necessarily accompany aestivation, but also constitutes a form of resistance adaptation to the transient high temperatures which aestivating snails are bound to encounter. Loss of body water is shown to be important in initiating aestivation, but it is also the factor most likely to prove lethal during the course of aestivation. The results confirm and extend the earlier findings for Australorbis glabratus. The results are discussed in the context of our present knowledge of aestivation in other Gastropoda, both aquatic and terrestrial, and also in relation to other forms of resistance adaptation. As expected, the investigation raises more questions than it answers, thus directions for further research, arising out of the present findings, are suggested.
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- Date Issued: 1973
Contributions to our knowledge of the biology of Machiloides Delanyi Wygodzinsky and Ctenolepisma Longicaudata Escherich : (Hexapoda Thysanura)
- Authors: Heeg, J. (Jan)
- Date: 1963
- Subjects: Insects -- Adaptation , Insects -- Physiology , Thysanura , Apterygota
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5856 , http://hdl.handle.net/10962/d1012261 , Insects -- Adaptation , Insects -- Physiology , Thysanura , Apterygota
- Description: Among the South African Thysanura, the order Lepismatoidea has spread over the whole sub-continent, while all except one species of the order Machiloidea are confined to the discontinuous forest belt which lies below the escarpment of the inland plateau. The Machiloidea are not , however, strictly confined to the actual forests within their geographical region, some species invading regions of considerable aridity. Investigations on the ecology, water relations and orientation behaviour of a representative species of each order have been carried out. These have revealed that: (i) the physical conditions in the typical niche of the Machiloidea are extremely stable, whereas those in the habitat of the Lepismatoidea are subject to some considerable fluctuation. (ii) the Lepismatoidea are more resistant to desiccation than the Machiloidea; in both cases this resistance is due in part to physical barriers in the cuticle and partly to an active metabolic process. (iii) the Machiloidea rely on their eversible vesicles, situated on the abdominal coxosternites, for the uptake of water which cannot be drunk, such as a thin film of water or soil capillary water. (iv) the Lepismatoidea are able to absorb water from a subsaturated atmosphere. (v) the behavioural responses of both in respect of humidity, temperature, light and gravity, are such as to keep them in conditions within the range of their physiological limitations. From these results it is concluded that the Machiloidea can survive outside the shelter of forests, provided that water is readily available in some form in which it can be absorbed by the animals. The general implications of the results are such as to permit the erection of an hypothesis explaining the distribution of the Thysanura in South Africa in terms of t he availability of wate. The results also lead to speculations on the evolution of the Pterygota.
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- Date Issued: 1963