The structure and postembryonic development of antennae and deutocerebrum in various insect orders are reviewed. First, the number and/or size of system components, i.e. antennal sensilla, neuroreceptors, deutocerebral neurons and synaptic complexes (glomeruli), are compared in adult insects. Second, the neuronal organization of the system is examined. Evidence of projection of the neuroreceptors from the 2 basal antennal segments (scape and pedicel) in the antennal mechanosensory and motor center and from the distal segments (flagellum) in the antennal lobe is discussed. Third, the types of cerebral neurons found in the antennal lobe are described. Fourth, all synaptic contacts between neurons in the antennal lobe take place in discrete glomeruli whose ultrastructure and neurotransmitters are examined. Evidence of individual identifiability of glomeruli is given. Fifth, various sexual dimorphisms present in the antenna and antennal lobe, related or not to sex pheromone perception, are described at sensillar, neuronal, and glomerular levels. Sixth, the postembryonic development of neurons and glomeruli is analyzed in holo- and hemimetabolous insects. In conclusion, the columnar organization of the system is emphasized, alternative models of antennal-neuron projection into glomeruli are considered, and the functional significance of identified glomeruli is discussed.

Structure and development of the insect antennodeutocerebral system

Distribution, morphologie fonctionnelle et morphogenese des chemo-, hygro-, thermo- et mecano-sensilles des larves et des nymphes d'insecte, en fonction du comportement d'alimentation et de la selection de la nourriture

Sensilla of Immature Insects

The morphology of the antennal hair-sensilla of Periplaneta americana, their distribution and frequency on the antennal flagellum have been examined by transmission and scanning-electron microscopy. The types of sensilla were distinguished with respect to physiologically relevant criteria such as wall structure and number of sensory cells. Among the sensilla of the antenna of the adult male, long, single-walled sensilla with four sensory cells (type sw B), probably responsible for reception of sexual pheromones, are most frequent, representing about 54% of the antennal sensilla. About half of these sensilla are newly-formed at the imaginai ecdysis; the other half are derived from the shorter type sw B sensilla of the nymphal antenna. Short type sw B sensilla are present in all larval stages of both sexes and in adult females as well. During the imaginal ecdysis of males, however, the length of these sensilla increases to double that found in nymphs. Dendritic branches also increase in number. During postembryonic development, the number of sensory fibers in the antennal flagellum increases nearly 20-fold, from 14,000 in the first larval instar to about 270,000 in the adult male. The greatest increase, approximately 90%, occurs during the last developmental stage.

Antennal sensory system of Periplaneta americana L.: distribution and frequency of morphologic types of sensilla and their sex-specific changes during postembryonic development.

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 antennal flagellum. Olfactory and contact chemoreceptive sense organs are the most numerous receptor organs on the antenna at all stages of postembryonic development. The sensory complement of male and female antennae is identical during the nymphal period, but a large difference between sexes appears at 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.

Antennal sensory system of the cockroach, Periplaneta americana: Postembryonic development and morphology of the sense organs

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 chemoreceptors 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.

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The nature and development of sex attractant specificity in cockroaches of the genus Periplaneta. I. Sexual dimorphism in the distribution of antennal sense organs in five species

In the flagellum of the antennae of the German cockroach, Blattella germanica (L.), new annuli are formed lie fore each molt within the meriston (3rd segment) and by division within the next several annuli distal to the meriston (meristal annuli). Doublets produced by division of meristal annuli do not themselves divide nor do the 17 or 18 terminal annuli (singletons) of the 1st instar. The latter are gradually lost during nymphal development. The overall result is that a 1st-instar flagellum having 24 or 25 annuli may become an adult flagellum having as many as 95 annuli.

Meristal annuli range from 4 to 12. The mode changes during development, being least in the 2nd instar (6) and greatest (9) in 4th, 5th, and 6th instars.

Only in the 1st instar do the annuli distal to the meriston increase gradually in length without discontinuities.

In all subsequent instars there are more or less obvious points in the flagellum at which the progression in length of annulus changes. These saltation points occur between groups of annuli; i.e., between the meristal annuli and the adjacent doublet group, between adjacent doublet groups, and between the distal doublet group and the group of singletons. These discontinuities make it possible to determine the instar of flagella that have singletons. The 2 annuli comprising a doublet are conspicuously unequal in length when first formed. In subsequent instars the difference in length diminishes and may undetectable when the doublet group of the 2nd instar reaches the 6th instar.

The growth in length of the flagellum as a whole like that of other hard parts of insects, increasing about 1.3 times at each molt. But within the flagellum the growth ratio ranges from about 4.0 in the meriston 1.0 at the apex, declining continuously from base apex, not only between groups of annuli but within groups. The observations summarized above apply to both sexes.

June D. Priestley

Papers

Flagellar Annuli of Blattella germanica (Dictyoptera: Blattellidae).–Changes in Their Numbers and Dimensions during Postembryonic Development