Light-induced and circadian changes in the compound eye of the haematophagous bug Triatoma infestans (Hemiptera: Reduviidae).

We analysed dynamic changes in the ommatidial structure of the compound eyes of Triatoma infestans. This nocturnal insect possesses open-rhabdom eyes, in which a ring of six rhabdomeres from retinula cells 1-6 (R1-6) surrounds a central pair of rhabdomeres from retinula cells 7 and 8 (R7-8). Screening pigments are located in all the photoreceptors and in the primary (PPC) and secondary (SPC) pigment cells. During the day, pigments within R1-6 and the PPCs form a small 'pupil' above the rhabdom and pigments within R7-8 are clustered around the central rhabdomere, allowing light to reach only the central rhabdomere. At night, the 'pupil' widens, and pigments inside R7-8 concentrate in the proximal region of the cells, allowing light to reach the peripheral rhabdomeres. In addition, the distance between the cornea and the rhabdom decreases. These rhythmic changes adapt the sensitivity of the eye by controlling the amount of light reaching and travelling within the rhabdom. Furthermore, the rhythm persists under conditions of constant darkness (DD), i.e. it is controlled by an endogenous oscillator. Remarkably, there are differences in pigment movements between the retinula cells of a single ommatidium. The migration of pigments in R1-6 is regulated by a circadian input, while that in R7-8 is regulated by both direct light and circadian inputs. The rhythm vanishes under constant-light conditions (LL). In this species, the circadian rhythm of photonegative behaviour persists in both DD and LL conditions, suggesting that these two rhythms, in retinal morphology and visual behaviour, may be generated by different circadian oscillators.