Pear blight. (a) (Bot.) A name of two distinct diseases of pear trees, both causing a destruction of the branches, viz., that caused by a minute insect (Xyleborus pyri), and that caused by the freezing of the sap in winter. A. J. Downing. (b) (Zoöl.) A very small beetle (Xyleborus pyri) whose larvæ bore in the twigs of pear trees and cause them to wither. -- Pear family (Bot.), a suborder of rosaceous plants (Pomeæ), characterized by the calyx tube becoming fleshy in fruit, and, combined with the ovaries, forming a pome. It includes the apple, pear, quince, service berry, and hawthorn. -- Pear gauge (Physics), a kind of gauge for measuring the exhaustion of an air-pump receiver; -- so called because consisting in part of a pear-shaped glass vessel. -- Pear shell (Zoöl.), any marine gastropod shell of the genus Pyrula, native of tropical seas; -- so called from the shape. -- Pear slug (Zoöl.), the larva of a sawfly which is very injurious to the foliage of the pear tree.