XYLEM FEEDING BY SPITTLEBUG NYMPHS - SOME OBSERVATIONS BY OPTICAL ANDCRYOSCANNING ELECTRON-MICROSCOPY

The feeding of spittlebug nymphs (Philaenus spumarius) from mature xyl em vessels was studied by optical and cryo-analytical scanning electro n microscopy. Feeding did not produce xylem embolisms and vessels rema ined liquid-filled during the day. Saliva secreted by the insect forms a hardened lining (salivary sheath) between the stylet bundle and the plant tissues. This sheath is continuous through the hole made by the stylers as they enter a vessel, and it extends into the vessel and al ong its periphery beyond the breach. The sheath is heterogeneous, with a thin outer layer adjoining the plant tissues and a thicker layer th at contacts the styler bundle. Both layers give positive histochemical reactions for proteins and, in fresh tissues, contain a red, strongly autofluorescent pigment, possibly condensed tannin derived from the p lant (which is lost during tissue preparation), and other phenyl propa noid compounds, which are retained and which may produce the intense r eaction of the periodic-acid-Schiff's-positive inner layer. It is conc luded that the salivary sheath allows the insects to feed from functio ning vessels without embolizing them or losing xylem fluid to the surr ounding tissues. These findings and others in the entomological litera ture indicate low daytime tensions in the xylem conduits of the host p lants.