The present study of contact chemoreception in Pieris brassicae
L. is divided into three major parts, viz. 1. behavioural analyses, 2. the identification and description of sense organs and 3. investigations concerning the sensory physiology. In a separate section some of the results were put into a coherent discussion.
In the behavioural studies the food selection behaviour was investigated as well as the behavioural responses to single or combined chemical stimuli (Chapter 3 and 4). It was found that the preference order for certain food plants can be strongly influenced by previous experience. Herein the time of experience, the species of food plant concerned and the choice-situation itself are playing an important rôle. The various experiments indicated that modifications in preferences were related to learning processes rather than to changes in the physiology of the relevant sense organs. In extirpation experiments it was found that the very subtle discriminative capacity and the possibility to induce preference modification remain unaffected by bilateral maxillectomy.
With respect to the above phenomenon the studies of the behaviour to chemical stimuli were performed with larvae reared on a meridic diet. The food intake behaviour of the larvae on this diet was used for deriving several response parameters, among which were frequency and time intervals of mouth part movement and measurement of quantities of food ingested. Only sucrose and D-glucose appeared to be able to induce an actual food ingestion. Fructose and other mono-, di- or trisaccharides were completely ineffective. From the stimulus- response relationships investigated it was deduced that sucrose is a much more effective stimulus than glucose. The relationship between stimulus intensity and behavioural response was also studied in relation to starvation of the experimental animals.
Other types of chemical stimuli, such as mustard oil glucosides, amino acids, salts and ascorbic acid are unable to induce food intake but show a positive interaction when mixed sub-optimally stimulating concentrations of sucrose. It was found that the stimulus-response relationships established for mustard oil glucosides differed in an essential way from those established for the remaining compounds. Several indications were found that the maxillary palpi exert an endogenous inhibiting effect on the onset of food intake.
The larvae demonstrated a high sensitivity to alkaloids and steroids which inhibited food intake. For these inhibitors the relationships between stimulus intensity and behavioural response were quantitatively analysed in relation to different types of feeding substrates. In ablation experiments it was found that bilateral maxillectomy or galeaectomy resulted in a loss of responsiveness to mustard oil glucosides. However, this was not the case after bilateral palpectomy. The sensitivity to sucrose, glucose, alkaloids or steroids was not affected by bilateral maxillectomy.
Chapter 5 concerns the identification and description of the structure of a number of sense organs which play an important rôle in food discrimination. The structure of the maxillary sensilla styloconica was electron-microscopically investigated. The medial and the lateral sensilla styloconica contain five bipolar neurons each. Four neurons possess distal processes which stand in contact with the external environment by a pore opening of about 190 mμdiameter. The fifth cell, which is a mechanoreceptor, has a dendrite which terminates without modification at the base of the flexible papilla at the end of the sense organ. In all five dendrites the transitional region from inner and outer segment is characterized by the presence of a modified ciliary connecting structure with two basal bodies. The axonema contains peripheral doublets according to a '9 + 0' configuration. The base of the scolopale, the proximal segments of the dendrites, the perikarya and a part of the axons are enveloped by the trichogen cell. The distal parts of the trichogen cell is surrounded by two tormogen cells.
On the basis of the results of several behavioural experiments investigation was made of the localisation of sense organs in the buccal cavity. In the epipharynx three types of sense organs were found, viz. setae, sensilla campaniformia and papilla-like organs. Each seta and sensillum campaniformium is innervated by one sense cell. Each of the papilla-like organs contains three bipolar neurons of which the dendrites terminate in a small papilla of approximately 0.5 mμdiameter and 0.5 0.7 μlong. Of both sensilla campaniformia and papilla-like organs one pair is present in the epipharynx.
Other sense organs which possibly could contribute to the highly developed discriminative capacity were found in the incisor cusps of the mandibles. Herein a number of pore canals are present which each are associated with one pair of bipolar sense cells. Electronmicroscopic investigation showed that the dendrites run to the distal termination of the pore canals. Several ultra-structural features of the pore canal organs suggested a resemblance to the structure of scolopidia.
In the investigations on the sensory physiology (Chapter 6) the epipharyngeal papilla-like organs were identified as chemoreceptive sense organs. Of the three bipolar neurons of each sensillum two were electrophysiologically characterized as a salt sensitive cell and a sugar sensitive cell. The third receptor responded specifically to stimulation with solutions of some alkaloids and steroids. The papilla-like organs do not contain any receptors sensitive to mustard oil glucosides. On the basis of responses to stimulation with different stimulus qualities and intensities it was concluded that the salt sensitive cells in the medial sensilla styloconica and the papilla-like organs are functionally almost identical. This was also true for the receptors sensitive to stimulation with alkaloids and steroids which were present in the same sense organs. To a certain extent the sugar sensitive cells in the papilla-like organs appeared to correspond functionally to the receptors in the lateral sensilla styloconica. Although both types of receptors were specifically sensitive to sucrose and glucose the stimulus-response relationships were quantitatively somewhat different. In both cases, however, it appeared that sucrose provided a more effective stimulus than glucose. In view of theoretical considerations it was assumed that the cell membrane of the sugar sensitive receptors in the lateral sensilla styloconica and the papilla-like organs possess one type of receptor site with two subunits. Each receptor site would be occupied by one molecule of sucrose or two molecules of glucose. In the medial sensilla styloconica a sugar receptor was identified having a broader sensitivity spectrum, which included sucrose, glucose, fucose and fructose as effective stimuli.
The presence of sodium or calcium chloride in the stimulating solution could strongly influence the responsiveness of the sugar sensitive receptors. In contrast to sodium chloride the effect of calcium chloride was exclusively inhibitory. A corresponding effect of calcium chloride was observed with regard to the responsiveness of the receptors sensitive to alkaloids and steroids.
Concerning some receptor cells in the medial sensilla styloconica and the papilla- like organs it appeared that an absolute receptor specificity could not be assumed under all conditions of stimulation.
In Chapter 7 the possible causal relations between quantitative electrophysiological events in chemoreceptor cells and various measurable behavioural responses to chemical stimuli are dealt with. Several aspects and problems related to such a study were discussed briefly. In situations in which chemical stimuli were presented individually a reasonable qualitative and quantitative correspondance was determined for distinct behavioural responses and the electrical activity of the relevant chemoreceptors. This, however, was valid only for the behavioural response to sucrose and glucose and the response characteristics of the sugar receptors in lateral sensilla styloconica and the papillalike organs. At lower concentrations a deviation from a direct proportionality between both stimulus-response relationships was found, which was ascribed to the endogenous inhibiting influence exerted by the palpi. A good correspondance was apparent when the quantitative stimulus-response relationships of maxillectomized larvae were compared with the electrophysiological response characteristics of the sugar receptor in the papilla-like organs. The fundamental significance of the sugar sensitive receptors in inducing food intake was, among others, also deduced from the fact that a suppression of the responsiveness of the sugar receptors by calcium chloride resulted in a proportional reduction in food intake. On the basis of the results of different experiments the hypothesis was advanced that the actual ingestion of food into the foregut is controlled by the sense organs located in the epipharynx, whereas biting responses are under control of the sense organs in the maxillae. Herein the mustard oil glucosides are belonging to stimuli which exclusively elicit biting responses.
The functional significance of the various chemoreceptors located in the maxillae can show large relative differences. Thus the sugar receptor which was electrophysiologically identified in the medial sensilla styloconica was inconsequential in influencing biting responses. Some receptors, such as the salt sensitive receptors, may evoke a stimulation as well as an inhibition of behavioural responses, depending on the stimulus intensity.
In situations in which, combinations of different stimulus qualities were presented the influence of the integrating mechanisms in the central nervous system on the quantitative behavioural responses was studied. Apart from the existence of positive interactions demonstrated for certain stimulus qualities, it was found that negative interactions were apparent in behavioural responses to specific inhibiting stimuli relative to the stimulating value of the feeding substrate. This means that the larvae reacted more sharply to small differences in stimulus intensities of specific inhibitors in the presence of a highly stimulating substrate than when the substrate possessed a lower stimulating value. An electrical activity in the alkaloid sensitive receptors approaching saturation level was related to the occurrence of an absolute inhibition of food intake.