Structure and functional morphology of the acid-secreting pygidial glands in the whipscorpion Mastigoproctus giganteus (Lucas, 1835) (Arachnida: Thelyphonida)

Abstract

The order Thelyphonida (Chelicerata: Arachnida) contains more than 100 species of whipscorpions that live in warm tropical and xeric environments. All whipscorpions possess a pair of pygidial glands in their opisthosoma that secrete and actively discharge a defensive spray composed mostly of acetic acid, hence their common name, vinegaroon. To date, the structure of the glands remains poorly understood and has not been subject to detailed analysis for more than a century. Here, we provide the first description of gland structure in the North American vinegaroon, Mastigoproctus giganteus. We used paraffin histology to study general gland structure in juveniles (2nd instar) and adults. We supplement these observations with histochemical stains, confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We found the glands to be composed of numerous long strips of multinucleated syncytium that rest on a basal lamina that lacks an external muscle coat. The gland syncytia contain abundant glycogen granules and secretion drops of different sizes and staining qualities. The adult epithelium is organized into three general patterns: as simple epithelia, as simple acinar glands, and as compound acinar glands. The juvenile gland epithelium is less distinctly organized. A cuticular intima lines the apical (lumenal) surface of adult and juvenile glands, is multilaminate, contains chitin, and may be permeated by irregular pores. The basal side of the intima consists of cuticular, finger-like projections that protrude into a subcuticular space. The underlying epithelium possesses an apical brush border of microvilli that appears to engage in Kurosumi’s type III extrusion, but the contents (cuticle or acids) of the vesicles are unknown. Curiously, the cells possess apical nuclei that also suggest a basal route of secretion, but details on this process are undetermined. Together, these results reveal that vinegaroon pygidial glands are unlike most other arachnid exocrine glands in their structure, methods of secretion, and mechanisms of discharge.