Feeding, nutrient flow, and digestive enzyme release in the giant milkweed bug, Oncopeltus fasciatus

Abstract.  The giant milkweed bug, Oncopeltus fasciatus , is an intermittent feeder, spending most of its time in compact groups away from the food, with feeding bouts lasting 30–60 min. They macerate a small area of the seed with their stylets, secreting large amounts of saliva (which suspends and emulsifies the seed tissue), and quickly suck the emulsion into the gut. Therefore, little digestive activity occurs in the seed tissue. Freshly ingested food in the first gut region (V1) consists of 50% water, 46% lipid, 5% protein, and 1% soluble carbohydrates. The junction of the midgut and hindgut lacks a lumen in the larvae stages, and a 3 μL large oil drop (> 85% triacylglycerides) accumulates in the third gut region (V3). Approximately 24 h after the imaginal moult, this large volume of oil and faeces is defecated, and thereafter an oily faeces is defecated several times daily. It is postulated that the delayed egestion in larvae protects them against suffocation due to self-soiling with oily faeces. Serine proteases are absent in the gut and, instead, a cystein proteinase, inhibited by E64, is present (gut pH 6.0). High lipase activity was found throughout the gut, but only limited amylase activities. Basal levels of all three enzymes occurred throughout the gut, but at lower levels in starved bugs. Salivary gland extracts contain a cysteine proteinase and a lipase, but no amylase.