OBSERVATIONS ON GROWTH, MORPHOGENESIS AND PROGRAMMED OBSOLESCENCE OF SCENT GLANDS IN THE MILKWEED BUG, ONCOPELTUS-FASCIATUS (HETEROPTERA)

Observations have been made on growth, morphogenesis, and programmed o bsolescence in the scent glands of the milkweed bug Oncopeltus fasciat us. The dorsal scent glands of larvae cease to function in the adults. The results of treatments with allatotoxin (precocene-II) and a juven oid suggest that the attainment of obsolescence is instar dependent an d not dependent on hormonal changes at metamorphosis. Development of t he adult metathoracic scent gland has been traced back to day 1 in lar val instar IV. A cuticular intima forms around day 3 in instar V by wh ich time all cell division has ceased and moulting to the adult commen ced, Tubuligenesis and ventral closure of the future median reservoir occur early in the instar-V larva. Growth is continuous and the estima ted cell-doubling time 19 h. In precocious adults, ventral closure fai ls to occur. As a result, the intima of the median reservoir evaginate s to reveal the secreting faces of the accessory glands. The findings on development are discussed in relation to evolutionary speculation o n the generation of the diversity of metathoracic scent structures to be seen in the Heteroptera.