Morphology of defense glands of the opilionids (daddy longlegs) Leiobunum vittatum and L. flavum (Arachnida: Opiliones: Palpatores: Phalangiidae)

Abstract

Opilionid defense glands consist of 0.5 × 0.9-mm sacs attached to the underside of low tubercles located on the dorsal side of the cephalothorax, posterior to the first pair of legs. Each gland opens via an elongated slit, located in the posterior floor of a crater that is situated at the summit of the tubercle. The center of the sac, called the reservoir, is lined by a cuticle consisting of epicuticle and endocuticle which is continuous through the slit with the exoskeleton. The layers of cuticle vary in thickness with different locations in the gland. A hemocoelomic (basement) membrane, 0.5-1, μ thick, forms the boundary between glandular cells and hemocoel. The gland has a nonsecretory portion consisting only of cuticle-supporting cells and a secretory portion consisting of secretory and cuticle-supporting cells. The cuticle lining the reservoir in the secretory area is broached by many cuticle-lined ductules, each of which drains an isolated intercellular space called the intercalated cistern. This in turn drains microvilli-lined canaliculi located between and extending into secretory cells. The cisterns are devoid of microvilli. Secretory cell cytoplasm contains a Golgi apparatus, many free ribosomes, rough endoplasmic reticulum (RER), two types of granules (speckled and dense), and mitochondria. Speckled granules are partially filled with fairly large particles and are found in association with the Golgi apparatus. They also surround canaliculi into which they empty. Dense granules are packed with very small particles, have a gray homogeneous appearance, and are scattered throughout the cytoplasm. Mitochondria containing matrix granules tend to scatter throughout the cytoplasm but are concentrated around canaliculi.