Larval anatomy and structure of absorbing epithelia in the aphid parasitoid Aphidius ervi Haliday (Hymenoptera, Braconidae)

The present work describes Aphidius ervi Haliday (Hymenoptera, Braconidae) larval anatomy and development, focusing on time-related changes of body st ructure and cell ultrastructure, especially of the epithelial layers involv ed in nutrient absorption. Newly hatched 1st instar larvae of A. ervi are c haracterised by gut absence and a compact cluster of cells makes up their b ody. As the parasitoid larva develops, the central undifferentiated cell ma ss becomes hollowed out, leading to the formation of gut anlage. This sugge sts that absorption of nutrients at that stage may take place through the b ody surface, as more directly demonstrated by the occurrence on the epiderm is of proteins associated with transepithelial transport, such as Na+/K+-AT Pase and alkaline phosphatase (ALP). Second instar larvae show the presence of the gut with a well-differentiated brush border and a peritrophic membr ane. Gut cells are filled by masses of glycogen granules and lipid droplets . The tracheal system starts to be visible. The haemocoel becomes evident i n late 2nd instar, and contains large silk glands. Mature 3rd instar larvae are typically by hymenopteriform. The midgut accounts for most of the body volume and is actively involved in nutrient absorption, as indicated by th e well developed brush border and by the presence of Na+/K+-ATPase and ALP on the basolateral and luminal membrane respectively. At this stage, large lipid droplets have gradually replaced the cellular glycogen stores in the midgut cells. The tracheae are completely differentiated, but their interna l lumen still contains fibrillar material, suggesting that they are not fun ctional as Iona as host fluids bath the parasitoid larva. In late 3rd insta r larvae, silk glands, structurally similar to Malpighian tubules, show a v ery intense vesicular traffic toward the internal lumen, which, eventually, results in being filled by secretion products, suggesting the possible rec ycling of metabolic waste products during mummy formation. (C) 2001 Elsevie r Science Ltd. All rights reserved.