Localization of subrhabdomeric haemolymph lacunae in the retina of Drosophila melanogaster and Calliphora erythrocephala.

The haemolymph of flies and other insects contains tyrosinase (EC 1.14.18.1), the rate-limiting enzyme in mammalian melanogenesis. After incubation with 5 mM L-DOPA for several hours the endogenous tyrosinase of the haemolymph forms an electron dense reaction product. This method was used to localize spaces in the retina of the wild type and the white (w) mutant of Drosophila melanogaster that are filled with haemolymph. A network of subrhabdomeric haemolymph lacunae was found. Moreover it was found that these haemolymph lacunae also form extensions into the photoreceptor cells and are connected with small haemolymph lacunae that cross the retinal basement membrane. In a second set of experiments L-DOPA was injected into the thoraces of Calliphora erythrocephala. Half-an-hour after injections the flies were killed and the eyes were embedded for electron microscopy. The small molecule of DOPA or its product dopachrome, surprisingly, penetrated the retinal basement membrane and reached the subrhabdomeric haemolymph lacunae and the ommatidial cavity.