Workers: the undeveloped females in the social Hymenoptera; also those sexually undeveloped Termites that are not soldiers. X 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. Y Yellow: used without modification is sulphur or lemon yellow. Yolk: the nutritive matter of an egg as distinguished from the living, formative material; = deutoplasm. Z Zona: a belt or zone; as of distribution. Zonite: = arthromere or somite; q.v. Zooenite or Zooenule: = zonite. Zygoptera: those Odonata, having the fore and hind wings subequal in width, venation comprising a quadrilateral, not a triangle; nymphs with caudal tracheal gills. ADDENDA. Calacobiosis: see symbiosis. Cleptobiosis: see symbiosis. Dulosis: see symbiosis. Coxal file: in some aquatic Coleoptera a series of striations just above the hind coxa of male and, perhaps, a stridulating organ. Coxal plates: plate-like expansions or dilations of the coxa: specifically in aquatic Coleoptera on the posterior pair. Ecto-parasite: one that is attached to the external surface of the host. Ento-parasite: one that feeds within the body of the host. Embioptera: an ordinal term proposed for the Neuropterous family Embidae. Hamabiosis: see symbiosis. Heliophobic: loving darkness: applied to species that shun the light, like, e.g. Termites. Heliotactic: light loving: applied to species that live in the open and in daylight. Lestobiosis: see symbiosis. Meron: in Neuroptera, a sclerite posterior to the coxa and below the epimeron: corresponds to the trochantine in Lepidoptera. Metasternal wing: in some aquatic Coleoptera a leaf-like expansion above the coxal plates. Myrmecophily: is the relation existing between ants and those guests that seek their company primarily for their own individual advantage. Phylacobiosis: see symbiosis. Prosternal process: in aquatic Coleoptera a modification of the prosternum used in the differentiation of species. Sub-clypeal pump: in some Diptera, the enlarged, more or less bulb- like structure at the anterior entrance of the oesophagus. Sub-clypeal tube: in Diptera: see pharynx. EXPLANATION OF PLATES. PLATE 1. Structures of the External Body Wall. 1. Harpalus caliginosas showing the underside, and the head from above, to show the regions and the position of the sclerites. 2. Thorax of a Dipteron to show location of bristles. 3. Lateral view of a denuded Lepidopteron to show arrangement of sclerites. 4. Abdominal segment of a caterpillar to show the position of the tubercles. 5. Lateral view of a dragon fly to show the body sclerites. All the abbreviations used in this plate are readily understood.