The compound eye of cockroaches is obligatory for entrainment of the Madeira cockroach's circadian clock, but the cellular nature of its entrainment pathways is enigmatic. Employing multiple‐label immunocytochemistry, histochemistry, and backfills, we searched for photic entrainment pathways to the accessory medulla (AME), the circadian clock of the Madeira cockroach. We wanted to know whether photoreceptor terminals could directly contact pigment‐dispersing factor‐immunoreactive (PDF‐ir) circadian pacemaker neurons with somata in the lamina (PDFLAs) or somata next to the AME (PDFMEs). Short green‐sensitive photoreceptor neurons of the compound eye terminated in lamina layers LA1 and LA2, adjacent to PDFLAs and PDFMEs that branched in LA3. Long UV‐sensitive compound eye photoreceptor neurons terminated in medulla layer ME2 without direct contact to ipsilateral PDFMEs that arborized in ME4. Multiple neuropeptide‐ir interneurons branched in ME4, connecting the AME to ME2. Before, extraocular photoreceptors of the lamina organ were suggested to send terminals to accessory laminae. There, they overlapped with PDFLAs that mostly colocalized PDF, FMRFamide, and 5‐HT immunoreactivities, and with terminals of ipsi‐ and contralateral PDFMEs. We hypothesize that during the day cholinergic activation of the largest PDFME via lamina organ photoreceptors maintains PDF release orchestrating phases of sleep–wake cycles. As ipsilateral PDFMEs express excitatory and contralateral PDFMEs inhibitory PDF autoreceptors, diurnal PDF release keeps both PDF‐dependent clock circuits in antiphase. Future experiments will test whether ipsilateral PDFMEs are sleep‐promoting morning cells, while contralateral PDFMEs are activity‐promoting evening cells, maintaining stable antiphase via the largest PDFME entrained by extraocular photoreceptors of the lamina organ.

Circadian clocks control rhythms in physiology and behavior entrained to 24 h light-dark cycles. Despite of conserved general schemes, molecular circadian clockworks differ between insect species. With RNA interference (RNA i) we examined an ancient circadian clockwork in a basic insect, the hemimetabolous Madeira cockroach Rhyparobia maderae. With injections of double-stranded RNA (dsRNA) of cockroach period (Rm´per), timeless 1 (Rm´tim1), or cryptochrome 2 (Rm´cry2) we searched for essential components of the clocḱ s core negative feedback loop. Single injections of dsRNA of each clock gene into adult cockroaches successfully and permanently knocked down respective mRNA levels withiñ two weeks deleting daytime-dependent mRNA rhythms for Rm´per and Rm´cry2. Rḿ per RNAi or Rm´cry2 RNAi affected total mRNA levels of both genes, while Rm´tim1 transcription was independent of both, also keeping rhythmic expression. Unexpectedly, circadian locomotor activity of most cockroaches remained rhythmic for each clock gene knockdown employed. It expressed weakened rhythms and unchanged periods for Rm´per RNAi and shorter periods for Rm´tim1 RNAi and Rm´cry2 RNAi .As a hypothesis of the cockroach´s molecular clockwork, a basic network of switched differential equations was developed to model the oscillatory behavior of clock cells expressing respective clock genes. Data were consistent with two synchronized main groups of coupled oscillator cells, a leading (morning) oscillator, or a lagging (evening) oscillator that couple via mutual inhibition. The morning oscillators express shorter, the evening oscillators longer endogenous periods based on core feedback loops with either PER, TIM1, or CRY2/PER complexes as dominant negative feedback of the clockwork. We hypothesize that dominant morning oscillator cells with shorter periods express PER, but not CRY2, or TIM1 as suppressor of clock gene expression, while two groups of evening oscillator cells with longer periods either comprise TIM1 or CRY2/PER suppressing complexes. Modelling suggests that there is an additional negative feedback next to Rm´PER in cockroach morning oscillator cells.

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