Lithobius forficatus, (Lithobiomorpha), 1758

Müller, C. H. G., Rosenberg, J. & Meyer-Rochow, V. B., 2003, Hitherto undescribed interommatidial exocrine glands in Chilopoda, African Invertebrates 44 (1), pp. 185-198 : 189-191

L. forficatus (Lithobiomorpha)

In L. forficatus somewhat similar interommatidial exocrine glands can be found. However, they differ from the type described above by forming clusters of two or three glandular units. Secondly, there is no evidence for the existence of an intermediary cell and, therefore, every glandular unit includes only two different cell types, a proximal secretory gland cell and a distal canal cell ( Fig. 1B ).

Secretory gland cell

Secretory gland cells, varying in number from one to three, can be observed in the proximal regions of several interommatidial triangles always made up by three surrounding ommatidia. However, a consistent arrangement was not apparent. The number of secretory gland cells apparently depends on their position within the Lithobius - eye, whereas the clustered ones seem to be limited to the central region.

Each secretory gland cell is similarly drop-shaped as in S. coleoptrata . Its voluminous part is located most proximally, reaching the level of the nuclei of the proximal retinula cells. The small kidney-shaped nucleus is also located at this level. The rather electrondense cytoplasm is enriched by dictyosomes, small osmiophilic vesicles, and an often circularly arranged rough ER ( Fig. 4A ). The lobe of the cell decreases continuously in volume, leading to distal regions, and usually contains a peripherally displaced rough ER, dictyosomes, and large, electron-dense and seemingly coalesced vesicles. The deeply infolded cell apex surrounds the basal lobe of the canal cell, which houses a small, cuticular duct ( Fig. 4B ).

A wide extracellular cavity could, however, not be determined. Distally the secretory gland cell is increasingly ensheathed by numerous thin processes of the interommatidial pigment cells (covering cells).

Canal cell

Up to three clustered canal cells are noticeable in a triangular space surrounded by three ommatidia (see fig. 4C). No regular pattern was apparent here either, but eye glands located in more marginal areas show mostly only a single canal cell.

The formation of a common cuticular duct, the occurrence of thin apical infoldings surrounding the cuticular duct, the cytoplasmic consistency, its content of organelles, its shape, as well as the proximal position of the nucleus, closely resemble the situation described above for the canal cells of S. coleoptrata ( Fig. 4D ). The proximal part of the canal cell envelopes, as well as penetrates, the apex of at least one secretory gland cell ( Fig. 4B ). The oblong nucleus lies beneath the transition zone of the distal and proximal retinula cells.

Figures

Fig. 1. Semi-schematic reconstruction of two different types of scutigeromorph and lithobiomorph interommatidial exocrine gland. A. A longitudinal section through one interommatidial exocrine gland present within the compound eye of Scutigera coleoptrata. B. The probably more advanced lithobiomorph type, where up to three glandular modules are fused to one single interommatidial exocrine gland (Lithobius forficatus, cut-away view). cc = canal cell, co = cornea, du = cuticular gland ductule, gc = secretory gland cell, igc = intermediary gland cell, ipc = interommatidial pigment cell (covering cell), om = ommatidium.

Fig. 4. Transmission electron micrographs (TEM), showing fine structural organisation of the interommatidial exocrine gland within the lateral compound eye of Lithobius forficatus. A. Transverse section through proximal region of one secretory gland cell containing typical glandular organelles. The gland cell lobe is surrounded by dense layers of covering and proximal (basal) retinula cells. B. Transverse section through distal region of two neighbouring, unequally sized secretory gland cells; the upper one being penetrated by a small proximal process of the canal cell with the cuticular gland ductule. C. Oblique-transverse section through a triangular interstitium in distal eye region.Two neighbouring, but not yet fused canal cells fill the interommatidial space and are surrounded by covering cells and distal retinula cells. D. Transverse section through middle part of one canal cell containing axially placed cuticular gland ductule. ax = nerve bundle, cc = canal cell, co = cornea, coc = covering (interommatidial pigment) cells, dic = dictyosome, du = cuticular gland ductule, gc = secretory gland cell, mt = mitochondrium, nu = nucleus, pro = cuticular projections, rc = retinula cell, rER = rough endoplasmatic reticulum, sv = secretory vesicle.