1. The site of the photoperiodic receptors controlling the production of sexual and parthenogenetic females in the aphid Megoura has been identified by exposing adult apterae to supplementary periods of localized illumination.

2. The microilluminators devised for this purpose could be attached to the insects. In one type, the light beam was contained in a metal capillary; in another, light was conducted to the required site by means of a fine plastic filament.

3. Photosensitivity is confined to the head of the aphid, the central region of the dorsum being particularly important as a light pathway. The compound eyes are not involved, although stimulation of the eye by a relatively intense beam can elicit the response, probably because of light scattering.

4. The response is unimpaired if the eyes are covered with opaque paint, and often persists after they have been cauterized.

5. The photoperiodic mechanism in the embryo is actuated by light transmitted through the body wall of the parent.

6. The apparent absence of distinctive morphological features in the cuticle and epidermis of the dorsum suggests that the photoperiodic receptors are located in the underlying protocerebrum. Neurosecretory cells, possibly in the pars intercerebralis, may be implicated both as receptors and as humoral effectors.

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