Changes in the corpora allata and epidermal proliferation along the fourth instar of the Chagas disease vector Triatoma infestans

Triatoma infestans, a blood-feeding insect, synchronises physiological mechanisms leading to moult with food intake. Since the corpora allata are important in moult and metamorphosis regulation, we have studied morphological changes in 4th instar nymphs (gland size, cell density, percent of animals showing mitoses and cell size). Changes were correlated with the effect of precocene II, epidermal proliferation, and with the extent of the "head critical period". Based on morphological grounds, three stages can be defined in the gland along the 4th instar: Stage 1 (days 0-2 after feeding) showed small corpora allata, composed by a small number of cells, and in which mitoses were absent; Stage 2 (days 3-9) showed growing corpora allata, in which cell number was increasing and proliferation was apparent; and Stage 3 (days 10-13) showed no mitotic activity, and a sharply diminishing size of the gland, as a consequence of the diminishing size of their cells. The ability of precocene II to induce abnormal moulting disappeared during stage 2 correlating with the termination of the head critical period and suggesting that corpora allata are essential during days 3 to 5 to determine normal growth. Epidermal cell number was increasing as a consequence of more frequent mitotic activity, beginning after the finalization of the head critical period and after a first increment in the size of the gland.