484 MISCELLANEOUS NOTES ON AUSTRALIAN DIPTERA. VII. ON body-colour; and on species of tabanidae, cyrtidae and asiloidea. By G. H. Hardy. [Read 30th October, 1940.] A theory of body coloration. — Williston (1908, 43-5), discussing the vestiture found on flies, concludes: ". . . tlie tomentum can only correctly be used as the designation for flattened, scale-like or stubble-like, more or less recumbent hairs, which gradually merge into dust or pollen, which is so generally represented in Diptera, and upon which the determination of many species must largely depend." This expression is responsible for clouding the issue concerning the gradual change of the vestiture from pubescence, to tomentum, to dust (herein called the pulverulent overlay), all being regarded as having a common basis. Actually I have found no point where pubescence, tomentum and the like, ever merge into the pulverulent overlay, and so conclude they have had different origins. To give a more satisfactory account of this is the purpose of the present note. The new theory has, of course, to be based on the view that the pulverulent overlay arises directly from the surface of the cuticle and is actually part of the cuticle that has crumbled into dust instead of being in homogeneous continuity with the body wall. It is a superlayer that has developed, perhaps, from an extra development of the normal cuticle. It is also necessary to note that, when arranged phylogenetically, some large genera show that there is a gradual merging from the normal pigmented and somewhat shining cuticle to a highly polished one, and even to iridescence, and it is upon this bare surface that first comes a slight covering that is the beginning of a pulverulent overlay. The thin transparent layer that forms the polished surface and the powdery covering both show white in the primitive state, both being without pigmentation; in transition from one to the other, it appears as if the glass-like surface crumbles to a white powder just as transparent glass would crumble to white. It is assumed that an unpigmented layer which gives the high polish is superimposed on the normal pigmented part of the cuticle and in continuity with it. Then it is assumed that this unpigmented layer breaks down into a pulverulent overlay. It must be noted that the overlay might be developed by the failure of the layer to form a homogeneous unit with the cuticle and thus be derived directly from the body-wall. It is not yet determined which of these actually takes place — both may do so. Development of the coloration in the cuticle depends upon the supply of materials from waste products, and there seems to be a very definite limit to coloration found in the body-wall, ranging from yellow, which is the primitive coloration, through red, green and blue, each colour separately or in combination. A red-green gives copper, and a red-blue gives purple, but there are limits to shades developed this way. The process goes on by a deepening of these pigments in the cuticle until black, the ultimate, is reached. It is the pulverulent overlay that shows the wider development in colours and tones. the reason being obvious when one accepts the view that the overlay is developed from the surface of the cuticle. As already stated, the clear glass-like surface layer of the cuticle breaks up into a silvery-white, but if the layer be impregnated with a yellow-red pigment, then the overlay is golden, and a black pigment there would give an ashy-white. The vast array of powdery browns met with doubtless is due to mixed pigments containing red.
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