Each pygidial gland consists of a variable number of secretory lobes (acini), collecting duct, reservoir chamber, reaction chamber, and efferent duct (Figure 1). These glands (class 3 according to the classification of Noirot and Qhennedey
[45][46]) are variable in structure and have been described in several species
[27][39]. The lobe or acinus, which is spherical or elongated and enveloped in a thin basal lamina, is a cluster of secretory units, connected to the collecting duct by a conducting duct that drains secretions outward. The secretory unit consists of two parts, an elongated, cube-shaped secretory cell surrounding a receiving duct and a duct cell surrounding the conducting duct
[27][28][29][43]. The receiving duct is a porous tube composed of one or more layers of epicuticle located in its extracellular space and bounded by microvilli. The collecting duct has an epithelial wall of flattened cells, lined by endocuticle, and a thin layer of epicuticle that is regularly folded into spiral ridges, annular arrays, or pointed peg-like projections, that reduce the volume of the lumen to control the free flow of secretion to the reservoir chamber
[27][28][43]. The entrance of the collecting duct to the reservoir chamber is of great variability. It is located at the anterior or middle position in Scaritinae, Brachininae, and some Bembidiini, Pterostichini, Amarini, Carabini, Nebriini, Metriini, and Paussini
[25][27][28][29][32][34][38]. While it is located near the entrance of the efferent duct in Harpalini, Agonini, Chlaeniini, Dryptini, Anthiini, Lebiini, Trachypachini, Omophronini, Loxandrini, Catapieseini, Galeritini, and Zuphini
[23][25][27][42]. The reservoir chamber is a spherical, elongate, or bilobate compartment of variable size. Interwoven muscle bundles cover the outer wall and are connected to tracheal branches. The basal membrane supports flattened epithelial cells covered by a thin uniform layer of endo- and epicuticle. The muscular contraction regulates secretion through a valve that separates the reservoir from the reaction chamber. In Paussinae, Brachininae, and Carabinae, an accessory gland is located below the valve
[27]. Secretions from the reservoir chamber are mixed with secretions from the accessory glands in the reaction chamber. The efferent duct leads from the reservoir chamber to the external orifice. The close association of the pygidial glands with the tracheal branches suggests a high aerobic metabolism.
The external orifice is located dorso-laterally in the posterior part of the abdomen, near to the antero-lateral margin of the ninth tergite, and close to the tergo-sternal suture in Carabinae, Scaritinae, Paussinae, Elaphrinae, Broscinae, and Brachinini, or at the posterolateral margin of the eighth tergite in derived lineages, e.g., Trechinae and Harpalinae, and including Licinini, Chlaeniini, Panagaeini, Anthiini, Zabrini, Oodini, Pterostichini, and Agonini
[27][47]. Differences in pygidial gland morphology between sexes have been reported in
Cicindela campestris [30].