Morphogenesis and Cytology of the Salivary Gland of the Hessian Fly, Mayetiola destructor (Diptera: Cecidomyiidae)1

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Abstract
The salivary glands of the Hessian fly, Mayetiola destructor (Say), are composed of two distinct regions: the anterior basal region and the posterior filament region. These regions were distinguished on the basis of cell and nuclear morphology and of differential growth rates during salivary gland morphogenesis. The basal region is composed of 18–24 cells that are larger than the 80–100 cells of the filament region during the feeding period of larval development. However, the basal-region cells collapse and are significantly smaller than the filament-region cells, which increase in size, during the nonfeeding period of larval development. Four distinct polytene chromosomes were identified in basal-region cells, which established that the haploid somatic chromosome number of the Hessian fly is four. Nuclei in the filament region become progressively endopolyploid during larval development, although chromosomes maintain low levels of polyteny. The basal region is the probable source of plant growth-inhibiting substances produced during the larval feeding period. Morphogenesis of the filament region indicated that its activity occurs primarily during the nonfeeding period of larval development and may be associated with the sclerotization of the third-instar cuticle, which is retained by the fourth instar for protection during estivation and diapause and for pupal and adult development. The comparative morphology, cytology, and function of salivary glands of the Hessian fly and other cecidomyiids are discussed.