The experiments of Strickland on copulation and oviposition in the rat flea showed that fleas do not copulate until they are sexually mature and that, at least in the case of _Ceratophyllus fasciatus_, the reproductive organs are imperfectly developed for some time (more than a week) after emerging from the pupa. When mature, copulation takes place soon after the fleas have fed on their true host--the rat--but not if they have fed on a facultative host only, such as man. Copulation is always followed by oviposition within a very short time. The effect of the rat's blood on the female with regard to egg-laying, Strickland concludes, is stimulating rather than nutritive, as fleas that were without food for many months were observed to lay eggs immediately after one feed. Similarly, the male requires the stimulus of a meal of rat's blood before it displays any copulatory activity. Mitzmain (1910) has described in detail the act of biting on man, as observed in the squirrel flea, _Ceratophyllus acutus_. "The flea when permitted to walk freely on the arm selects a suitable hairy space where it ceases abruptly in its locomotion, takes a firm hold with the tarsi, projects its proboscis, and prepares to puncture the skin. A puncture is drilled by the pricking epipharynx, the saw-tooth mandibles supplementing the movement by lacerating the cavity formed. The two organs of the rostrum work alternately, the middle piece boring, while the two lateral elements execute a sawing movement. The mandibles, owing to their basal attachments, are, as is expressed by the advisory committee on plague investigations in India (_Journal of Hygiene_, vol. 6, No. 4, p. 499), 'capable of independent action, sliding up and down but maintaining their relative positions and preserving the lumen of the aspiratory channel.' The labium doubles back, the V-shaped groove of this organ guiding the mandibles on either side." "The action of the proboscis is executed with a forward movement of the head and a lateral and downward thrust of the entire body. As the mouth-parts are sharply inserted, the abdomen rises simultaneously. The hind and middle legs are elevated, resembling oars. The forelegs are doubled under the thorax, the tibia and tarsi resting firmly on the epidermis serve as a support for the body during the feeding. The maxillary palpi are retracted beneath the head and thorax. The labium continues to bend, at first acting as a sheath for the sawing mandibles, and as these are more deeply inserted, it bends beneath the head with the elasticity of a bow, forcing the mandibles into the wound until the maxillæ are embedded in the skin of the victim. When the proboscis is fully inserted, the abdomen ceases for a time its lateral swinging."