Xylem feeding by spittlebug nymphs: some observations by optical and cryo-scanning electron microscopy

The feeding of spittlebug nymphs (Philaenus spumarius) from mature xylem vessels was studied by optical and cryo-analytical scanning electron microscopy. Feeding did not produce xylem embolisms and vessels remained 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 stylets as they enter a vessel, and it extends into the vessel and along its periphery beyond the breach. The sheath is heterogeneous, with a thin outer layer adjoining the plant tissues and a thicker layer that contacts the stylet 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 plant (which is lost during tissue preparation), and other phenyl propanoid compounds, which are retained and which may produce the intense reaction of the periodic-acid-Schiff's-positive inner layer. It is concluded that the salivary sheath allows the insects to feed from functioning vessels without embolizing them or losing xylem fluid to the surrounding tissues. These findings and others in the entomological literature indicate low daytime tensions in the xylem conduits of the host plants.