endeavoring to ascertain the limits of natural groups, as the internal anatomy and the embryology and metamorphosis of insects, before we can hope to obtain a natural classification. The family of bird lice is a very extensive one, embracing many genera, and several hundred species. One or more species infest the skin of all our domestic and wild mammals and birds, some birds sheltering beneath their feathers four or five species of lice. Before giving a hasty account of some of our more common species; we will give a sketch of the embryological history of the lice, with special reference to the structure of the mouth parts. [Illustration: 114. Embryo of the Louse.] [Illustration: 115. Mouth Parts of the Louse.] The eggs (Fig. 114, egg of the head louse) are long, oval, somewhat pear-shaped, with the hinder end somewhat pointed, while the anterior end is flattened, and bears little conical micropyles (_m_, minute orifices for the passage of the spermatozoa into the egg), which vary in form in the different species and genera; the opposite end of the egg is provided with a few bristles. The female attaches her eggs to the hairs or feathers of her host. [Illustration: 116. Mouth Parts of the Louse.] [Illustration: 118. Mouth Parts of Louse.] [Illustration: 117. Mouth Parts of Louse.] After the egg has been fertilized by the male, the blastoderm, or primitive skin, forms, and subsequently two layers, or embryonal membranes, appear; the outer is called the amnion (Fig. 114, _am_), while the inner visceral membrane (_db_) partially wraps the rude form of the embryo in its folds. The head (_vk_) of the embryo is now directed towards the end of the egg on which the hairs are situated; afterwards the embryo revolves on its axis and the head lies next to the opposite end of the egg. Eight tubercles bud out from the under side of the head, of which the foremost and longest are the antennae (_as_), those succeeding are the mandibles, maxillae, and second maxillae, or labium. Behind them arise six long, slender tubercles forming the legs, and the primitive streak rudely marks the lower wall of the thorax and abdomen not yet formed. Figure 115 represents the head and mouth parts of the embryo of the same louse; _vk_ is the forehead, or clypeus; _ant_, the antennae; _mad_, the mandibles; _max_1, the first pair of maxillae, and _max_^2, the second pair of maxillae, or labium. Figure 116 represents the mouth parts of the same insect a little farther advanced, with the jaws and labium elongated and closely folded together. Figure 117 represents the same still farther advanced; the mandibles (_mad_) are sharp, and resemble the jaws of the Mallophaga; and the maxillae (_max_^1) and labium (_max_^2) are still large, while afterwards the labium becomes nearly obsolete. Figure 118 represents a front view of the mouth parts of a bird louse, Goniodes; _lb_, is the upper lip, or labrum, lying under the