The Reproductive Biology of the Rabbit Flea Spilopsyllus Cuniculi (Dale) and The Dependence of this Species Upon the Breeding of Its Host

1. Unlike any other species of flea which has been cultured successfully Spilopsyllus cumculi (Dale) is entirely dependent for its own reproduction upon its host becoming pregnant. 2. The ovaries of fleas kept on male or non-pregnant female rabbits remain immature whereas those on a pregnant host are mature at the time of parturition. Vitellogenesis commences at a critical point approximately 7 days pre-partum, irrespective of how long the fleas have been on the host. It is postulated that a factor required by the flea for ovarian development is only available during the final week of pregnancy, and not at all in male or non-pregnant rabbits. The factor disappears from the adult after parturition but is present in her nestlings for at least 7 days. 3. Most fleas desert the adult doe shortly after the young are born and enter her nest, where copulation and oviposition occur. 4. The factor does not act solely as a trigger initiating in the flea developmental processes which then continue in its absence; ovarian regression occurs among maturing fleas transferred to hosts that do not supply the factor. 5. The processes of vitellogenesis, not those of oogenesis, fail in the absence of the factor. It is suggested that the ‘yolk-forming hormone’ normally secreted by the corpus allatum of the adult insect may only be produced by rabbit fleas when they can obtain the postulated factor. Variations in the quantity of this factor available to fleas on different hosts could explain the observed variations in ovarian activity. Some parallels between the factor and steroid hormone levels in the host's blood are discussed.