There are some other outdoor flies which are not very dissimilar to the common blue-bottle, but they are more soberly coloured, ranging from bluish black to speckled and tinted greys; some of these have a pattern on the shiny upper surface of the abdomen which is conspicuously and beautifully chequered. Closely akin to these latter is the large grey blow-fly, or flesh-fly, Sarcophaga carnaria; it is much referred to in entomological books as of marvellous fecundity. The female deposits not eggs in a few hundreds, but already hatched maggots to the number of many thousands. Amongst half-a-dozen rarer kinds of smaller grey blow-flies the females differ in their striped markings, but their respective males seem quite indistinguishable apart.
Notwithstanding the prodigious fecundity of the grey blow-fly, the credit of being a practically useful scavenger of carrion must be given only to the blue-bottle, which is of a more robust habit, and which so promptly monopolises available matter that Sarcophaga carnaria and her congeners are sometimes, perforce, compelled to give their larvæ a mere vegetarian diet.
The yellow cow-dung fly, Scatophaga stercoraria, is inoffensive, and one of the commonest flies observable in the course of a country-side ramble. It and its congeners are distinct in habits and appearance from any of the other flies above-mentioned. In this species the male is larger than the rather more smooth and dull-coloured female. Its body is furry but slender; it has small eyes and head parts. In repose it holds its wings parallel close above the abdomen, more like the "breeze-flies," or true "gad-flies," than the ordinary muscid flies. Although its proboscis does not seem as formidable as that of more insectivorous flies, yet it may sometimes be observed pouncing upon some small fly, which it holds with its powerful legs. This fly does not appear to be very predaceously inclined; perhaps it is only "acting a part," like some other creatures, including the amorous male of the common frog, which, failing to secure a more natural and complacent "partner in the dance," will in springtime seize upon and very persistently cling to an astonished carp.
Amongst many flies with bodies favoured with a brilliant metallic sheen, several species of green-bottle flies (Luciliæ) are notorious. Of these latter L. Cæsar is the most common, but L. Sericata is by far the worst in England, not uncommonly laying eggs upon sheep; many are of a brilliant golden green, but some vary towards a coppery green; all have red eyes and silvery faces. In summer-time these flies seize every opportunity of depositing their eggs upon any sores or skin wounds of animals; their larvæ normally feed on carrion and dung. The green-bottle, like the blue-bottle flies, are fond of both sweets and filth, but they do not pester wholesome animals as do the sweat-flies.
Next to the Muscidæ the most often observed and easily recognisable as a distinct family of flies are the Syrphidæ, which include the "hover-flies," the drone-flies (often mistaken for the male of the hive-bee), and a number of other very common flies of a generally similar full-bodied shape, in most of which colour stripes and bands more or less suggest a comparison with wasps. The numerous species native to Great Britain are widely distributed, and, excepting the rare and very hairy Merodon narcissi, of which the larvæ feed on liliaceous bulbs, none is injurious and some are beneficial. Nearly all the flies of this family frequent flowers. The habit of many to hover for hours about a favoured spot, as if for mere pleasure, is remarkable; but it is not generally recognised that some of these hover-flies (of the genus Syrphus) are hawking for winged aphides and other small insects, which they quickly suck dry and drop whilst still on the wing. Many of the flower-frequenting Syrphidæ are great devourers of pollen; all have strongly developed suctorial mouth parts.
The larvæ of the various syrphid flies differ greatly in appearance and habit; some are terrestrial; some aquatic; some semi-aquatic; some feed on decadent vegetation; some on sewage and filth, and some are insectivorous. Most useful to the horticulturist are those of the genus Syrphus, which feed on green-fly and other aphides. The most curious in shape are the "rat-tail" maggots of the common drone-fly, Eristalis tenax (also others of allied genera), which can extend their long tubular tails and breathe atmospheric air through the same whilst lying under water. The larvæ of the genus Volucella are found dwelling in the nests of bumble-bees and wasps; it is rather uncertain how far they are commensal, or parasitic, or devourers of dead matter. Some of the syrphid flies are single-brooded, but some at least are double or treble-brooded in the year; records are wanting about many, and which, if any other than the common drone-fly, are multi-brooded. Anyhow, none appears to breed in Great Britain as rapidly as do the house-fly, the blue-bottle, and other muscid flies.
The larvæ of Conops flavipes are parasitic in the body of the adult bumble-bee, and they pupate therein.
The small family of the Stratiomyidæ contains a few fairly common species called soldier-flies; these are interesting as linking Orthorrhapha with Cyclorrhapha; their larvæ are some aquatic (the star-tailed maggots), others terrestrial, and some have hard shell-like skins; but they are not so curiously like a creeping marine limpet as are those belonging to the genus Microdon (of the Syrphidæ), which are rare and wonderful dwellers in ants' nests.
There is a curiously shaped race of parasitic flies which cling to the host like a louse, called Hippoboscidæ; these have more than the usual provision of claws to their feet, both in the number (normally two) and size of the claws. The forest or spider-fly attaches itself to some part of the body out of reach of the horse's tongue. The ked, tick, or sheep louse-fly has a similar mode of life, and, after selecting its host, it becomes wingless. These flies, strange to say, nurse and nourish their larvæ within the oviduct, and, when one might think that they were laying their eggs, they are depositing pupæ or larvæ just ready to pupate. There are some species of the family of the louse-flies which infest birds.
The true gad-flies of the family of Tabanida were, and sometimes still are called "blinden breeze-flies," and sometimes dun-flies; by a very easy mistake the countryman's word "blinden" (blind) has got changed by authors in books to "blinding," which is nonsense, and misses a wonderful instance of old-folk knowledge; the females are amongst the most inveterate blood-sucking flies, but the males are mere idle loiterers in summer sunshine on flowers; the eggs are laid on herbage in moist situations; the maggots and pupæ of many of these species are said "to be found in the soil," and some, if not all the larvæ, are predaceous, attacking worms and underground larvæ of various insects. They are more or less midsummer flies and are single-brooded. There are several largeish species (of the genera Tabanus and Therioplectes) found in Great Britain, and they are diversely distributed, being respectively woodland, moorland, lowland, and highland inhabitants. The great ox-gad-fly is as large as a bumble-bee, though more long than broad in body, but the term gad-fly is often wrongly given to the worble-fly, which is really more bee-like, being furry and rounder in body. The genus Hæmatopota comprises three smaller sized extra vicious blood suckers, H. pluvialis, rather common, H. italica, very local, and H. crassicornis, darker in colour and with spotted and dark tinted wings. Several of the large gad-flies have dull-tinted wings. They have large, shallow, brightly shining and curiously banded compound eyes, but no "ocelli"; they all seem to be at least semi-blind, and the females are rather sluggish, except between the hours of 11 a.m. and 5 p.m. in bright midsummer sunshine. The females hunt entirely by scent and are easily captured when attacking human beings; they alight on their victims with a stealthy silent approach. They appear unable to discriminate between clothing and bare skin as suitable spots for feeding. Amongst a band of mountaineering pedestrians, on a sunny day, it was observable that there would be a dozen or more "blinden breeze-flies" settling on the back of one, whilst the rest of the party were only favoured now and then by one or two apiece. It was apparently the smell of the "home-spun" coat which attracted; the colour of the garment did not seem to be the cause of the selection. Sunshine loving flies prefer white and pale colours. If a dog could speak, he would explain the smell of some "finished" cloth, but, for the sake of the fastidious, the secret is not here disclosed.