- Thoracic crop (see Petersen-Braun, M.; Buschinger, A. 1975: Entstehung und Funktion eines thorakalen Kropfes bei Formiciden-Königinnen. Insectes Sociaux 22: 51-66 (Development and function of a thoracic crop in ant queens);
Casadei-Ferreira, A., Fischer, G., Economo, E.P. 2020. Evidence for a thoracic crop in the workers of some Neotropical Pheidole species (Formicidae: Myrmicinae). Arthropod Structure, Development 59, 100977 (doi:10.1016/J.ASD.2020.100977)
During independent, claustral colony foundation of ant queens the flight muscles degenerate. The then „empty“ space within the thorax can be filled with a considerable swelling of the esophagus, which may serve as a „thoracic crop“, in addition to the usual crop in the gaster. Hölldobler & Wilson 1990, p.157: „More recently, it has been found that the esophagus of the queen expands into a "thoracic crop" in which the converted tissues are temporarily held in liquid form. In Pharaoh's ant (Monomorium pharaonis), the esophagus diameter widens from 7-10 micrometers to 265 micrometers. The thoracic crop has been demonstrated in five genera of Myrmicinae and Formicinae so far (Petersen-Braun and Buschinger, 1975).“
Here is a couple of microscopic photos and schemes of the thoracic crop in the Pharao's Ant. The photos are of suboptimal quality, but were taken in 1975, and they demonstrate convincingly that this additional "crop" develops with age of the queens, when the flight muscles degenerate. The growing ovaries need space in the gaster, competing with the gastral (ordinary) crop. In case of reduced food supply the thoracic crop may shrink, its space then is filled out by inflated lateral tracheal stems.
Monomorium pharaonis: Cross section through the esophagus of an unmated gyne less than one week old. bm = ventral nerve cord; oes = esophagus; thm = cross section of longitudinal flight muscles. Note the folded dorsal wall of the esophagus.
Monomorium pharaonis: Sagittal section through the esophagus of an unmated gyne less than one week old. bm = ventral nerve cord; l = lumen of esophagus; thm = longitudinal flight muscles.
Monomorium pharaonis: Sagittal section through thorax with transition from esophagus (left = anterior) to thoracic crop. (From Petersen-Braun, PhD-thesis 1973)
Monomorium pharaonis: Cross section through the thorax of a mated queen, five weeks old. The flight muscles are almost completely replaced by fat body tissue.
Monomorium pharaonis: Schematic cross section of the thorax of a 12 weeks old mated queen. In addition to the esophagus, the lateral tracheal trunks have expanded. A = Adipocytes; B = Ventral nerve cord; M = leg muscles; O = esophagus; T = tracheae. (From Petersen-Braun, PhD-thesis 1973)
Monomorium pharaonis: Schematic longitudinal section of a four weeks old mated queen. 1 = pharynx; 2 = thoracic crop; 3 = gastral crop; 4 = stomach; 5 = ovaries; 6 = hindgut.
Thoracic crops were also found in other ant species, e.g. in Temnothorax interruptus, and in Lasius niger. Probably this system is common in ant queens with nuptial flights and claustral colony foundation.
Temnothorax interruptus (Schenck, 1852): Median-sagittal section through a reproductive, mated queen in hibernation. thK = thoracic crop; gK = gastral crop.
Lasius niger (Linnaeus, 1758: Scheme of thoracic and gastral crop of a fertile queen (from B. Mura, thesis 1973)