Fig. 24.—Larva of F. canicularis. (From Gordon Hewitt’s Report to Local Government Board, 1912.) Magnified.
The larva of Fannia is a flattened-looking grub with distinct segments, decorated by numerous feathery processes. It lives amongst decaying vegetation and fruit, and also amongst fermenting animal matter and dejecta. Sometimes it is found in rotting grass. As we shall see later, it frequently passes into the human alimentary canal. F. scalaris, usually known as the ‘latrine-fly,’ is even commoner than its congener, and the external structural differences are minute. As its name indicates, it is found as a rule breeding in human dejecta, and is, therefore, as a typhoid carrier, much more dangerous than F. canicularis. Its larva is also more commonly found in the human intestine.
Then there are two species of large flies known as blue-bottles or blow-flies—Calliphora erythrocephala and C. vomitoria. The former of these is the more common. The sides of its face are golden yellow, set with black hair; whereas in C. vomitoria the sides of the face are black, but the hair is golden. Both are handsome, sturdy-looking diptera, with bluish-black thoraces, and abdomens of a dark metallic gun-metal sort of colour.
Blow-flies deposit their eggs on fresh or decaying flesh, and this is one of the great sources of trouble to the officers of the Army Service Corps. But they are not content with killed flesh. They will lay their eggs on any living flesh which is exposed, or in sores or tumours, and here their larvae will thrive. Dr. Graham-Smith tells us he once found the exposed muscles of the broken leg of a living rabbit seething with a mass of small blow-fly larvae, which were nourishing themselves upon the living tissues.
Fig. 25.—Blow-fly or blue-bottle, Calliphora erythrocephala, female (× 3). Antenna. Male head, dorsal view. Side view of head. Natural size, resting position. (From Graham-Smith.)
The eggs of the blow-fly hatch out in from ten to twenty hours in normal British temperatures; the larval life, in its three stages, lasts from seven to eight and a half days; the pupa state lasts a fortnight, so that the total development extends a day or two over three weeks. The maggots are unusually voracious; and Linnaeus used to say that the progeny of three blow-flies will dispose of a dead horse as quickly as three lions.
C. erythrocephala is essentially an outdoor fly and enters houses only in search of a nidus on which to deposit its eggs. C. vomitoria resembles its congener in size and habits, but it is not so abundant. Occasionally its eggs have been known to be deposited in the nostrils of animals and men.
But there are:—