Our cut (Fig. 28, _a_) also represents the pupa, or chrysalis, as seen lying in its cell. The limbs are folded close to the body in the most compact way possible. On the head of the semi-pupa, _i.e._, a transition state between the larva and pupa, there are two prominent tubercles situated behind the simple eyes, or ocelli; these are deciduous organs, apparently aiding the insect in moving about its cell. They disappear in the mature pupa. [Illustration: Fig. 29. Fig. 30. Fig. 31. Fig. 31. Larva of Halictus parallelus. Fig. 29. Larva of Andrena vicina. Fig. 30. Pupa of Halictus parallelus seen from beneath.] To those accustomed to rearing butterflies, and seeing the chrysalis at once assuming its perfected shape, after the caterpillar skin is thrown off, it may seem strange to hear one speak of a "half-pupa," and of stages intermediate between the larva and pupa. But the external changes of form, though rapidly passed through, consisting apparently of a mere sloughing off of the outer skin, are yet preceded by slow and very gradual alterations of tissues, resulting from the growth of cells. An inner layer of the larva-skin separates from the outer, and, by changes in the form of the muscles, is drawn into different positions, such as is assumed by the pupa, which thus lies concealed beneath the larva-skin. But a slight alteration is made in the general form of the larva, consisting mostly of an enlargement of the thoracic segments, which is often overlooked, even by the special student, though of great interest to the philosophic naturalist. From Mr. Emerton's observations we should judge that the pupa state lasted from three to four weeks, as the larvae began to transform the first of August, and appeared during the last week of the same month as perfect bees. The Andrena is seen as late as the first week in September, and again early in April, about the flowers of the willow. It is one of the largest of its genus and a common species. Having, in a very fragmentary way, sketched the life history of our Andrena and had some glimpses of its subterranean life, let us now compare with it another genus of solitary bee (Halictus), quite closely allied in all respects, though a little lower in the scale.