Fig. 387.—A nearly mature embryo of Xiphidium ensiferum: o, o, œnocyte clusters seen from the surface through the integument; a, pleuropodium of the right side (appendage of the first abdominal segment); s, styli; c, cercopods.—This and Fig. 386 after Wheeler.
These cells (Fig. 386), with the exception of the eggs, are the largest in the body, and occur in most if not all winged insects. They were called œnocytes (oinos, wine; kustis, cyst), by Wielowiejski in allusion to their wine-yellow color. These cells are arranged segmentally (Fig. 387) in clusters, held in place by tracheæ, and are situated mostly on each side of the abdomen, rarely being found in the adjoining parts of the thorax. They are more or less intimately associated with the blood and fat-body. Unlike the fat-body, however, they arise in embryonic life from the ectoderm, either by delamination or by immigration, just behind the tracheal involutions.
The separate cells of each cluster are usually separate, but in rare cases may fuse in pairs or form smaller clusters. In shape they are round or oval, often sending out pseudopodia-like processes, by which they are attached to the tracheal twigs or to each other. “The cytoplasm, which is very abundant, is full of yellowish granules and is sometimes radially situated towards its periphery. The large spherical or oval nucleus contains a densely wound and delicate chromatic filament.” (Wheeler.)
Graber first pointed out the identity of these clusters of cells with certain metameric cell-masses in insect embryos, observed by Tichomiroff in those of the silkworm, and by Korotneff in the embryo mole-cricket.
Although they resemble the blood corpuscles in some insects, they are always much larger, and do not seem to be amœboid, while they are never seen to undergo self-division, or to exhibit any appearance of giving rise to the blood-cells (Wheeler). They have not yet been detected in Thysanura (Synaptera) or in Myriopoda.
d. The phosphorescent organs
Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms, and has been observed in the bivalve Pholas, in a few abyssal Crustacea, in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain deep-sea fishes.
Fig. 388.—A, sagittal section through the hinder end of a male Luciola, the organs above the phosphorescent plate only drawn in outline: s, integument of the last segment, somewhat removed by the section-knife from the phosphorescent tissues; d, dorsal layer of the phosphorescent plate penetrated by irregular tracheal branches, and rendered opaque by numerous urate concretions imbedded in it; v, ventral phosphorescent layer of the plate, with perpendicular tracheal stems whose branches, where they pass into capillaries, bear lumps which stain brown with osmic acid; n, structureless substance (coagulum?) filling the end of the last ventral segment. B, isolated portion of the ventral layer of the phosphorescent plate; tr, tracheal stem surrounded by a cylindrical lobe: p, parenchym cell attached to the cylinder; c, capillary, without the spiral threads; m, coagulum stained brown. C, a tracheal stem of the ventral layer: at the fork of the brown-stained capillaries are lumps stained brown with osmic acid. D, a part of C, more highly magnified, showing the remains of the tracheal end-cells (tc) enveloping the brown lumps (m).—After Emery.
In insects luminosity is mostly confined to a few Coleoptera, and besides the well-known fireflies, an Indian Buprestid (Buprestis ocelata) is said to be phosphorescent; also a telephorid larva. Other luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera (Culex, Chironomus[[60]] and Tyreophora), and an ant (Orya).