Philornis downsi (Dodge & Aitken), the avian vampire fly, is the main threat to land bird conservation in the Galapagos Islands. However, very little is known about the reproductive morphology, physiology, and behavior of P. downsi adults impeding progress on developing effective methods for its control. Here, we provide the first preliminary description of the female and male reproductive systems of a Philornis species and compare the activity of lab-reared and wild flies. In mature females, the accessory glands are connected from the common oviduct toward the ovaries, and there are 3 spermathecae (1 double and 1 singlet). For males, we found 2 peanut-shaped testes, one possible accessory gland, an apodeme, and an aedeagus. Both wild and lab-reared individuals had similar patterns of movement, although lab-reared adults were more mobile at dusk. We observed higher locomotion in the morning and dusk with a lull at midday, and increased activity as flies aged from 1 to 10 days. Females became more active during the morning at 10 days of age compared to males. Knowledge on the morphology of P. downsi's reproductive system will give us a better understanding of its reproductive physiology and will aid in efforts at establishing a self-reproducing colony. Mobility trials can be used as an easy and practical quality control method, as well as providing essential information on peak activity times, which may be related to foraging in the morning for females and sexual activity and host searching at dusk.

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