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The feeding behaviour of Oncopeltus fasciatus has been investigated. Exploration of food materials is performed by means of watery saliva which is secreted onto surfaces of substrates and sucked back again. Histological tests indicate that the animal preferably pierces the conducting system of the Vicia faba plant. Oncopeltus fasciatus secretes sheath material which coagulates and forms a lining to the path of the stylets during plant feeding. Feeding on seeds (Asclepias syriaca and Helianthus annuus) is not accompanied by stylet sheath formation. The function of this stylet sheath is discussed. Under extreme conditions the phytophagous animal shows cannibalism in spite of its marked subsocial behaviour. Penetration of the cuticle seems to be effected mainly by mechanical forces. No chitin-splitting enzyme could be detected in the saliva. The feeding activity over a photoperiod of 12 hrs on plants (uptake of water) and seeds (uptake of nutritive materials) is determined. It is suggested that on a green plant the animal is saturated more rapidly than on dehydrated seeds. The considerable amounts of watery saliva emulsify and suspend the contents of the seeds. Into seeds of Asclepias syriaca saliva is injected at a rate of approximately 1.14 mg/animal/hr.