Xanthophyll: the yellow of autumn leaves; one of the substances found in the blood of insects.
Xenobiosis: see symbiosis.
Xerophilous: applied to species living in dry places.
Xylophaga: wood-eaters: applied in several orders.
Xylophagous: feeding in or upon woody tissue.
Xyphus: a spinous or triangular process of the meso-sternum in many Hemiptera, and some other insects.
Yellow: used without modification is sulphur or lemon yellow.
Yolk: the nutritive matter of an egg as distinguished from the living, formative material; = deutoplasm.