Antennal growth in the cockroach, Periplaneta americana, takes place by division of the meriston (third antennal segment) and the meristal segments at the base of the ;antenna1 flagellum. Olfactory and contact chemoreceptive sense organs are the most numerous receptor organs on the antenna at all stages of postembryonk development. The sensory complement of male and female antennae is identical during the nymphal period, but a large difference between sexes appears ai: the adult stage. The adult male has nearly twice as many olfactory sensilla as the female, and more than half of these sensilla appear at the adult stage. These morphological data are consistent with the hypothesis that sensory receptors specific to the female sex attractant appear in the male only at the adult stage.
Sexual dimorphism in the distribution of antennal sense organs is common among adults of the genus Periplaneta. In three out of the four strains of Periplaneta americana examined, adult males had more contact chemoreceptros than females. In the fourth strain of P. americana and in P. australasiae, P. brunnea, P. fuliginosa, and P. japonica, no statistically supportable sexual dimorphism of contact chemoreceptors was found. However, in all strains and species of Periplaneta examined, sexual dimorphism was found in the total number and/or density of olfactory sensilla. Male adults had nearly twice as many olfactory sensilla as female adults. These observations are consistent with the behavioral observation that males within the genus Periplaneta rely on the reception of an airborne pheromone for the initiation of courtship behavior. In P. americana, where sexual dimorphism was found in the contact chemoreceptors, contact stimuli release the full wing raising display and presentation in males during courtship.
Sexual dimorphism of antennal sense organs appears only at the adult stage during normal development of the cockroach, Periplaneta arnericana. Adult males acquire approximately twice as many olfactory sensilla as females at the terminal ecdysis. When terminal instar larvae are subjected to unilateral antennectomy, most ecdyse to supernumerary larvae rather than adults. Sexual dimorphism is not evident in the intact (unamputated) antenna during the extra larval stage, but appears at the following ecdysis which leads to the adult stage. Allatectomy of male and female larvae in the penultimate instar produces adultoids which show antennal sexual dimorphism. Whole-body treatment of terminal instar larvae with exogenous juvenile hormone-mimic (JH-M) results in supernumerary larvae which lack antennal sexual dimorphism. When these superlarvae are removed from the influence of JH-M, they ecdyse to adults with antennal sexual dimorphism. Topical application of JH-M to male antennae early in the terminal larval instar results in the emergence of adults which lack the total male complement of antennal sensilla, but are otherwise normal-appearing. These results indicate that an inhibitory action of juvenile hormone prevents the appearance of antennal sexual dimorphism during normal larval development.The postembryonic development of antennal olfactory receptors in cockroaches is regulated, in part, by the juvenile hormone (JH) (Schafer and Sanchez, '74). In the African woodroach, Leucophaea maderue, the density of antennal olfactory sensilla remains constant at about 400 sensilla per mm2 during each stage of larval development, because olfactory sensilla are added at each ecdysis in direct proportion to the increase in antennal surface area. However, at the terminal ecdysis leading to the adult stage, the density of olfactory receptor organs increases to 620 sensilla per mm2 in both males and females. Exogenously applied juvenile hormonemimic (JH-M) prevents this increase; or, the increase can be induced precociously by removing the corpora allata, endocrine organs which produce juvenile hormone in larval insects (Schafer and Sanchez, '74).J. EXP. UXIL., 198: 323-336.We chose to perform similar experiments on the American cockroach, Periplaneta americana, since the antennae show distinct sexual dimorphism related to the pheromone-dependent sexual behavior of males of this species. For example, the initial step in courtship behavior of P. americam is the male's reception of a femaleproduced sex attractant (Roth and Willis, '52; Barth, '70; Simon, '71). Of particular interest was the possibility that the differentiation of sex attractant receptors is inhibited by juvenile hormone during larval development. There are approximately twice as many olfactory sensilla on adult male P. americana antennae as on adult female antennae (Schafer and Sanchez, '73). In contrast, no sexual dimorphism is seen in the antennae of Leucophaea at any
The density of olfactory sense organs on the antenna of the cockroach, Leucqphaea maderae, is relatively constant throughout larval development (average 400 sensilla/mnP), but undergoes a substantial increase at the adult stage (to about 620 sensilla/mm"). Experimental manipulations of juvenile hormone (JH) activity result in either supernumerary larval instars (induced by unilateral antennectomy or addition of exogenous JH), or premature adulthood (induced by allatectomy). The density of antenna1 sensilla remains at the larval level during the extra instars, but increases to the adult level or surpasses it at the terminal ecdysis following the induction of extra instars. Adultoids resulting from allatectomized sixth instars also have the high density of antenna1 olfactory sensilla characteristic of the normal adult. These data suggest that an interplay of surface area effects and an inhibitory action of JH controls the pattern of postembryonic development of antenna1 olfactory sensilla. Limited behavioural observations of the insects resulting from these experiments are consistent with the hypothesis that sex attractant-specific olfactory receptors appear only at the adult stage. However, electrophysiological data will be needed to confirm or negate this hypothesis.
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