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During a study of the morphology of the orthopteroid Hexapoda between the years 1939-1941, it appeareded to the writer that further investigation into the structure of the insect tentorium was necessary. The early entomologists laid the foundation or our knowledge or the tentorium but during the latter half of the last century little was added in this particulart field. Comstock and Kochi (1902), as far as the writer is aware are the only workers who have published a paper devoted entirely to the skeleton of the bead of insects, and they did much to clarify both nomenclature and existing knowledge of this structure at that time. Berlese (1909) described a typical tentorlum as observed in orthopteroid Hexapoda and then considered the variations of the tentorium which occur in other groups of insects. Some confusion then appears to have arisen amongst workers, over the origin of the dorsal arms which were erroneously considered by some to be cuticular invaginations and not secondary outgrowths of the anterior tentorial arms. Certain morphologists amongst them, Hoke (1924) Denis (1928), Snodgrass (1928, 1935), Hansen (1930), Walker (1933) Imms (1934), and Symmons (1952), have done much towards improving our knowledge of the tentorium, but all too frequently workers offer brief and inadequate descriptions of this structure in otherwise comprehensive investigations of the insect head. Themain objects in this study are threefold, vlz. (1) The claification of nomenclature (2) The examination and the interpretation ot the tentorium in certain groups of insects. (3) The deliberation as to whether the tentorium is of phylogenetic significance, and thus of use in assessing various schemes of insect classification which have been presented. Introduction, p. 1-2.

It is well known that high potassium ion concentrations depolarize nervous tissue and it has been suggested that the nerve sheath surrounding the peripheral nerves of insects serves as a protective barrier for the exclusion of potassium ions, in the haemolymph, from the immediate environment of axons. Further it is known that the concentration of potassium ions in the haemolymph of phytophagous insects is far higher than that in predatory forms; this has led to the suggestion that the nerve sheath in plant feeding insects should be more highly developed than that of entomophagous insects. In this work the structure of the nerve sheath in phytophagous and predatory insects has been studied and this assumption has been shown to be groundles. However, preliminary experiments on the effects of ions and drugs on the peripheral nerves of phytophagous and predatory insects have shown that there is a definite difference in susceptibility between the nerves of these two forms and this has led to the postulate of a diffusion barrier beneath the level of the nerve sheath, which is more highly developed in phytophagous than in predatory forms. The properties of this second barrier are discussed. Part 1. / During the course of the work which has been described in Part 1- an outbreak of large saturniid moths Nudaurelia cytherea capensis Stoll.) occurred in the Grahamstown area. It was felt that an investigation into the properties of the flight motor of this moth, which has an extremely low wing beat frquency, might be rewarding as our knowledge of the flight motor in insects is limited to those with very much higher wing beat frequencies than that of this moth. The anatomy, innervation and histology of the flight muscles of Nudaurelia are described and it is shown that the flight motor of this moth is functionally different to that of other insects which have been investigated. Further, Nudaurelia shows a characteristic warm-up fluttering of the wings prior to flight - this phenomenon has also been examined in the following investigation. This study has yielded information about the location of a warm-up centre in the central nervous system of this moth. Part 2.

The basic components and subsidiary factors comprising predation have been demonstrated in field and laboratory studies that analyzed the predation by small mammals of the European pine sawfly, Neodiprion sertifer (Geoff.). Predation was restricted to the cocoon stage of the insect and only three species of small mammals were important predators -- Sorex cinereus cinereus Kerr, Blarina brevicauda talpoides Gapper, and Peromyscus maniculatus bairdii Hoy and Kennicott. The characteristics of the prey and of the field conditions were so simple that prey density and predator density were the only variables affecting predation. There were two basic responses to changes in prey density: the functional response, where the number of cocoons consumed per predator changed and the numerical response, where the density of predators changed. These responses differed for the three different predators. When the two basic responses were combined to reveal the relation between per cent predation and prey density, predation increased initially with increase in prey density (concurrent density dependence) and thereafter decreased (inverse density dependence). The forms of these peaked predation curves was different for each species of predator, for they reached different maxima and peaked at different prey densities. The functional and numerical responses are the basic components of predation. Laboratory experiments, however, showed that subsidiary factors can exert an effect through these responses. Prey characteristics, by altering the strength of stimulus from prey, can change both the functional and numerical responses. Similarly, increase in the number or palatability of alternate foods can decrease the functional response and increase the numerical response. A hypothetical predator - prey model showed that under certain conditions the peaked type of predation can regulate the numbers of a prey and can damp oscillations of prey numbers. The scheme of predation revealed in this study can explain all types of predation and types of parasitism as well. It seems, however, that there are four major types of predation conceivable, each type having different functional and numerical responses. / Science, Faculty of / Zoology, Department of / Graduate

The secretion of the segmented Malpighian tubules of Cenocorixa bifida (Hungerford) was studied in vitro to determine if the Malpighian tubules of an aquatic predator function in a similiar manner to those of terrestrial insects, and to determine the importance of the different morphological segments in the ion transport of the whole tubule. Fluid secretion in the Malpighian tubules of C. bifida appears to be governed by the same in vitro factors found important in other insects. As in other insects, the secreted fluid is isosmotic to the bathing medium, while the potassium is hypertonic and sodium hypotonic over a wide range of bathing medium sodium-potassium ratios. At bathing medium potassium concentrations close to that of the insect's haemolymph, potassium and sodium are isotonic in the secreted fluid. The Malpighian tubules of C. bifida produce alkaline secretion when rate of secretion is increased by addition of cyclic AMP. Differences in ion and fluid transport between the segments of the Malpighian tubules of C. bifida are statistically significant, but only slight. This correlates with the lack of morphological differences in apical and basal infoldings between the segments. The major exceptions to this trend are (1) the high pH of Segment II fluid, (2) the transport of dyes by Segment III and (3) the production of 'secretory granules' by Segment III. Segment III is the most distinct ultrastructurally. / Science, Faculty of / Zoology, Department of / Graduate

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