Fine structure of the prothoracic mycangium, a chamber for the culture of symbiotic fungi, in the southern pine beetle, Dendroctonus frontalis.

The ultrastructure of the prothoracic mycangium of female Dendroctonus frontalis is examined. The mycangium consists of a cuticular invagination within which symbiotic fungi are cultured by the pine beetle and transported to new host trees. Secretions from two types of gland cells pass into the mycangial lumen. The plasma membrane of type-1 cells is invaginated to form an enclosed extracellular cavity. The secretory product passes into the cavity, then through fine cylindrical channels into an end apparatus and finally via an efferent cuticular ductule to the lumen of the mycangium. Secretion of the type-2 cells is released into a cavity just beneath the mycangial cuticle. The cuticle over this cavity is quite thin (1-2micro), consisting mostly of inner epicuticle riddled with irregular canals through which the secretion reaches the lumen. Beneath the patches of porous cuticle are ribs (up to 10micro in thickness) which flank the cavities and presumably provide structural support for the porous secretory zones.