1. Pupae of the privet hawk moth Sphinx ligustri are equipped with defensive devices known as gin traps.
2. The function of the traps depends on a discrimination between receptors inside and outside their margins. The traps close when the sensilla within them are disturbed.
3. There is no consistent difference in the response of the trichoid sensilla inside and outside the traps. Discriminations between them must depend on a difference in their central connexions.
4. There is no fusion of the afferent axons from individual receptors in the region of the gin trap.
5. Maps of the triggering and non-triggering receptors show that the pupa does not discriminate between receptors inside and outside the trap, but between receptors at different points on the antero-posterior axis.
6. The distinctive feature of the triggering receptors may be their position in a segmental antero-posterior gradient.