FLIES
Part II
THE BLUE-BOTTLE (Calliphora erythrocephala), AND OTHERS
Who fills our butchers’ shops with large blue flies?
(Rejected Addresses.)
But there are other flies: first amongst which may be mentioned Fannia canicularis and F. scalaris. These belong to the family known as Anthomyidae, and are distinguished from the house-fly by being smaller in size, and by many other small details in the imago stage hardly to be appreciated except by trained dipterologists. For a short time at the beginning of the summer, during part of May and June, specimens of F. canicularis are more abundant than M. domestica, and, when seen on the window-panes of our living-rooms, are apt to be thought, by the uninformed, to be young specimens of the latter. But, as has been said, flies, when they are once flies, do not grow; all the growing they do is done in the larval stage.
Fig. 23.—Latrine-fly, Fannia scalaris, male (× 3). Antenna. Head of female, dorsal view. Natural size, resting position. (From Graham-Smith.)
As the days lengthen the common house-fly becomes vastly more common than F. canicularis, the ‘lesser house-fly,’ and the latter now tend to aggregate in those rooms of our houses not devoted to cooking, and may frequently be noticed flying in a jerky and disconcerting manner around the chandeliers or bedposts in unfrequented living- or bed-rooms. The relative proportion of these two genera in full summer varies in different localities. Roughly speaking, out of 100 flies collected in a house there is something between 90 and 99 per cent. of M. domestica, but the numbers not only vary with locality, but with temperature.
On the other hand, there is a curious disproportion between the number of sexes found ‘at home’ in the lesser house-fly. For every 100 F. canicularis taken indoors seventy to seventy-five are males, the numbers being evened by an equal preponderance of females who have remained out of doors.