Light microscopy

Semithin sections confirmed that the reservoir has two different parts. The unpaired sac-like part is located ventrally of the pharynx whereas the two branches of the glove-like part are located laterally somewhat above the pharynx, with the main branches extending laterally and the ‘fingers’ extending dorsally. Some of the ‘fingers’ of the gland are located directly subjacent to the front cuticle and cuticular epithelium of the upper head and are separated from the brain by air sacs. The wall of the gland reservoir is formed by a monolayered epithelium [Fig. 7.3A,B,C]. The cells are 15-30 µm wide and 10-25 µm high. The shape of the cells varies considerably within the epithelium; they are either flattened quadrangular, pentangular or nearly triangular with the tip pointing towards the lumen of the reservoir. Noteworthy, the inner sides of these cells bear several (Fig. 7.3A,B,C, apparently about 2-5 in the semi-thin sections) hairs (in the triangular and pentangular cells the hairs are located on the 'tips'). These hairs occur in both, the sac-like and the glove-like parts of the reservoir and resemble the

Fig. 7.1. Dissected postpharyngeal gland of A: a female European beewolf, scale bar = 1 mm, and B: a worker of the ant species Camponotus floridanus scale bar = 0.25 mm. The basic similarities of the glove-like gland reservoir and the location relative to the hypopharynx are obvious. fi, finger; hp, hypopharyngeal plate; oe, oesophagus ; res, reservoir; s, suspensorium.

Fig. 7.2. 3D-reconstruction of the PPG in the head of a female of the European beewolf (frontal view). The glove-like part of the gland (yellow) extends dorso-laterally from the hind-pharynx to the inner border of the compound eyes. The sac-like part that extends ventrally from the pharynx is not visible in this view (see supporting online material, movie 1). (Note: due to cutting off of the outer parts of the head, these parts could not be reconstructed). as, air sac (grey); b, brain and nerves (white); ce, compound eye (black); mm, mandibular muscles (green); oc, ocelli (black); pha, pharynx (grey); res, reservoir of the PPG (yellow); scale bar = 1 mm.

hairs on the inner walls of the pharynx. However, in the pharynx the hairs are longer (about 1.5 x), more regularly arranged, and are associated with a thick layer of connective tissue.

No typical class III gland cells with duct cells (Noirot and Quennedey, 1974; Quennedey, 1998) occur in the epithelium and no such cells are connected to the gland reservoir [Fig. 7.3A]. Furthermore, some epithelial cells contain a few conspicuous large vesicles. The nuclei are oval and large (8-10 µm) in relation to the amount of cytoplasm. The epithelium of the gland reservoir mostly borders on air sacs that resemble the gland reservoir when viewed at low magnifications. At higher magnifications air sacs and the gland reservoir can be easily distinguished since the former show the typical taenidia on the inner surface, whereas the latter bears the already mentioned hairs on the inner surface. In some regions the epithelium borders a tissue of large rounded or slightly oval fat cells (diameter: 28-35 µm) some of which contain large numbers of colourless (and rarely yellowish) vesicles [Fig. 7.3A].

The opening of the ventral sac-like part of the reservoir to the pharynx bears a fringe that contains a ring muscle [Fig. 7.3B,C]. This muscle is more pronounced in the anterior part of the fringe (about 50 muscle cells about 150-200 µm long, 80-100 µm wide) compared to the posterior part (about 10 muscle cells, 20-25 µm long, 8-10 µm wide). Furthermore, the anterior part of the opening is terminated by a brim (50-150 µm long, 25-50 µm wide, Fig. 7.3C). Possibly, the sac like part of the reservoir can be closed and opened by contraction and relaxation of this ring muscle. The anterior brim might cover the remaining opening when the ring muscles are contracted.

The two branches of the glove-like part of the reservoir open to the pharynx laterally and dorsally to the middle axis of the pharynx. These openings also show thin brims both at their anterior and their posterior side but no ring muscle is visible. However, there are several muscles connected to the hypopharynx. A set of muscles that runs upwards from the sclerotised elongations of the hypopharyngeal suspensorium to the cuticle of the head capsule might close the gland when the fibres contract. Another set of muscles that runs more or less downwards to the cuticle of the head capsule might be involved in the closing of the gland. Several other muscles that run transversally might also contribute to the control of the gland opening. However, since these muscles probably have additional other functions and might interact in a complex way, we cannot yet establish how the opening and closing of the gland is accomplished.

B

Fig. 7.3. A Semithin section of the glove like part of the PPG of female beewolves showing a branching point of two fingers surrounded by air sacs. scale bar = 50 µ m. B Semithin section of the lower part of the PPG of a female European beewolf and surrounding tissue. Note the nearly triangular shape of the cells of the epithelium and their apical hairs. scale bar = 150 µ m. C anterior rim of the opening of the lower part of the PPG with muscle fibres. scale bar 50 µm. ar, anterior rim of the opening of the lower part formed by a brim; as, air sac; ep, epithelium of the PPG; h, hairs; hp, hypopharynx; fc, fat cell; lu, lumen of the PPG; mf, muscle fibres; nu, nucleus; op, opening; ph, pharynx; pr, posterior rim of the lower part. scale bar = 50 µm.

7.4.4 Ultrastructure

SEM investigation of dissected glands revealed that the outer wall has a reticulate surface [Fig. 7.4A]

that apparently enlarges the surface area. The inner walls bear the hairs that were already visible in the semithin sections [Fig. 4B]. The length and structure of the hairs varies somewhat within the reservoir.

Near the opening the hairs are about 30 µm long, 2 µm thick and their tips are spliced. In the more distal parts of the glove-like part, the hairs are located on more or less regularly spaced scale like structures in groups of 15-20 [Fig. 7.4B] and are about 20-40 µm long.

Ultrathin sections revealed the typical organisation of a cuticle lining on the inner side of the reservoir [Fig. 7.5A], showing that the reservoir represents an exocrine gland. The scales that bear the hairs are extensions of the cells of the epithelium. The cells of the epithelium showed well developed smooth endoplasmatic reticulum, Golgi-apparatus, and some mitochondria. Microvilli appear in the apical region of the cells. Some conspicuous multilamellar bodies of different sizes are visible and most of them are localized in the apical region, associated with

the microvilli. Both the microvilli and the multilamellar bodies are not evenly distributed over the epithelium but seem to be concentrated in some areas. Furthermore, the microvilli are not as numerous and not as regularly shaped as in the epithelium of the PPG of C. floridanus workers and as reported in some earlier publications of the PPG of ants (e.g. Fig. 9 in Billen, 1991). Some electrolucent vesicles can be seen in the epithelium. Furthermore, septate desmosomes are visible. The basal part of the epithelium shows numerous invaginations [Fig. 7.5B] some of which seem to have extensions into the cell. At some places there are large rounded cells filled with numerous large electrolucent vesicles adjacent to the epithelium. These are the fat cells that can also be seen in the semithin sections.

Fig. 7.4. A SEM picture of the outer wall of a dissected PPG of a female European beewolf showing the reticulate surface structure. scale bar = 2 µ m. B SEM picture showing the inner surface of the reservoir of a dissected PPG of a female European beewolf showing a regular arrangement of scale like structures bearing a number of hairs. h, hair; sc, scale; scale bar = 20 µm.

A B

Fig. 7.5. A TEM picture of the apical region of the epithelium of the PPG of a female European beewolf, showing the well developed cuticle that lines the lumen of the gland. scale bar = 1 µ m. B TEM picture of the basal region of the epithelium, showing multiple invaginations and electrolucent vesicles; scale bar = 1µm. bin, basal invaginations; bl, basal lamina; enc, endocuticle; epc, epicuticle; exc, exocuticle; ga, Golgi apparatus; lu, lumen of the gland; M, mitochondria; mv, microvilli; sd, septate desmosome; sER, smooth endoplasmatic reticulum; v, vesicle.

B