Morphology of the Alimentary Canal of Chrysoperla rufilabris (Neuroptera: Chrysopidae) Adults in Relation to Microbial Symbionts

A study of the internal morphology of the alimentary canal of Chrysoperla rufilabris (Brumeister) adults in relation to yeast symbionts was conducted using light, scanning, and transmission electron and epißuorescence microscopy. The alimentary canal of Þeld-collected adults possessed a single large ( 300Ð400 m in length) diverticulum at the posterior end of the foregut. Although yeast cells (4.0Ð6.5 m) were distributed throughout the alimentary canal, large numbers of blastically dividing cells (i.e., yeast) were observed within the diverticulum. The diverticulum interior was highly convoluted and folded transversely and longitudinally, and yeast cells were observed to accumulate within the folds. Large tracheal trunks were attached to the lateral side of the diverticulum, suggesting ahighdemand for gas exchangewithin this organ.Thediverticulumwas lined with cuticle, and the underlying tissues did not contain large amounts of endoplasmic reticulum, mitochondria, or Golgi complex, indicating that minimal absorption occurred within this gut region. This suggested that the high potential for gas exchange in the diverticulum by the tracheal trunks was primarily to support yeastmetabolic activity. All size classes (i.e., 0.1, 4.0, and 10.0 m)of ßuorescence particles ingested by newly eclosed adults eventually ended up in the midgut and hindgut regions, indicating that the foregut and/or diverticulum do not possess an absolute mechanism for retaining particles based on size. However, all size classes of the ßuorescent particles typically persisted within the diverticulum. The evident conßuence between the diverticulum lumen and the gut lumen suggested a free exchange or ßow of ßuids between these regions. The proventriculus (proximal to the diverticulum) was pronounced and consisted of a series of long “hairs,” short “hairs,” and small spine-like structures projecting into the midgut. Large numbers of yeast cells were observed in associationwith the proventricular hairs, and these hairsmay play a role in the retention of yeast cells. The midgut possessed typical absorptive structures (i.e., microvilli), and large numbers of mitochondria, rough endoplasmic reticulum, and Golgi complex were observed in midgut epithelial cells. Because evidence indicated no or minimal absorption of nutrients within the diverticulum, it was concluded that nutritional factors provided by the yeast must be transferred to the midgut where absorption occurs. Large numbers of yeast cells enclosed within a well-developed peritrophic matrix were observed in the midgut, suggesting that the yeast themselves may serve as a source of nutrients. Whereas the exact mechanism by which yeast contribute to the nutrition of C. rufilabris adults was not determined, morphological evidence obtained in this study supported the hypothesis that chrysopids form amutualistic symbiosis with yeast and that the esophageal diverticulumwas a specialized structure for housing them.