CHAPTER XII
THE SERVICE AND UTILITY OF FLIES
It is often asked—have not house-flies some use in Nature? The only true answer is that they are warning signals.
They certainly do join with a multitude of other flies in promiscuous scavenging services, and they can be very active agents therein; but this work only aggravates the fact of their dangerous partiality to mankind, together with all his belongings and surroundings. These creatures may well be imagined to have developed out of some primæval species by reason of the increase of mankind upon this planet. The mere presence of the house-fly denotes some nuisance more or less remote; the local density of the brood indicates the degree and the proximity of unsanitary conditions. Under present circumstances the visitation of the house-fly is Nature's intimation that peril of a very insidious character is about. Very properly, Nature's messenger will not be denied, and pertinaciously manifests herself to us indoors!
It has already been explained that the scavenging service of the house-fly can be altogether dispensed with, inasmuch as there is a sufficiency of other less noxious flies and creatures devoted to such work. Reflecting on the Story of Creation, and the mission of man as first a gardener, and then, when expelled from Paradise, destined to more laboriously cultivate the earth, it may be held to be man's allotted duty not only to wage war against weeds, but likewise to distinguish friends and foes of all kinds, and treat accordingly creatures even of all branches of the animal kingdom, whether insects, reptiles, birds, or mammals, favouring one and exterminating another. This will be to rule the earth and "subdue it" (Genesis i. 28). Nemesis will inevitably chastise man unless he rectifies the consequences of his own delinquencies, whether they be direct or indirect. The "good service" of the house-fly is comparable with that of the flea, which performs an unwelcomed and indirect "service," inciting the housewife to have well-swept floors and clean bedding.
The unalloyed good service of insectivorous flies is quite apparent. The details of their life-history and a description of their different characteristics would make an interesting volume, but the limitations of the present work preclude such enlargement of its scope.
It certainly stands to the credit of the blue-bottle that she is by far our best native scavenger of carrion; so good, indeed, that none other as an assistant is needed. She may just possibly sometimes convey germs and contaminate food, but she does not so directly assail man. Her larger size makes her easy for exclusion from the domestic food store. So eager and alert is the female in searching for meat, dead animals, and fish, that our other native carrion-feeding flies are at a great disadvantage in the struggle for existence; so prompt is she in monopolising carrion, that would-be competitors often have to be content with laying their eggs in less rich food materials. The blue-bottle can withstand temperatures a few degrees colder than can those flies to which she is nearest akin; and she seems capable somehow of sheltering herself better. Even as late as the middle of winter, sometimes, a dead bird may be found to be fly-blown under circumstances which indicate that the eggs must have been laid in frosty weather, when flies are not seen at large. The explanation is that the blue-bottle will for a short time awake, and venture outside her retreat in a sheltered south wall, warmed by a few hours of winter sun. She does not hibernate in early winter in such a continuous state of rigid torpor as, for instance, does the queen wasp; probably few or none survive the winter by a real hibernation, the progenitors of the first spring broods emerging at a date no earlier than do their rivals and congeners.
As a bird food or a fishing bait gentles are superior to the maggots of any other dipterid insect. So-called ants' eggs and meal-worms are more highly appreciated by the fancier for bird food, but they are expensive. The pupæ called "ants' eggs" can only be had fresh for a short period of summer, and dried stock for the rest of the year requires much labour to obtain and prepare. Meal-worms are the cleanest and the easiest to propagate of all similar larvæ; but they are very slow growing compared with gentles; in the natural state the life-cycle from the egg to the feeding worm, then the pupa or chrysalid, and finally the beetle, Tenebrio molitor, occupies a twelvemonth's time; but, like many other insects which have accommodated themselves to human surroundings, they can increase more rapidly. By rearing a number of broods, each in a separate vessel, and by hastening the development and propagation of some broods more than others, meal-worms may be obtained fresh throughout the year; the amount of trouble and attention required is accordingly great.
The artificial rearing of gentles is easy, rapid, and cheap. Generally it is not well or methodically managed, and consequently it is then a horribly malodorous nuisance. However, with proper care the process can be managed without offensiveness, and it will immensely repay all trouble.
A more restricted variety of birds relish gentles, but no insect food is more wholesome for any of the galinaceous tribe.
A spot in the open air, preferably sheltered from the north or east wind and from mid-day sun, should be chosen. There an inverted earthenware sea-kale pot or a similar vessel should be fixed on a stand or table twenty inches high from the ground; the table should have a central hole corresponding to that in the sea-kale pot. The hole in the pot should be obstructed with a wire-cage strainer, or a piece of perforated wood or metal, above which a few sticks or a bunch of straw may be placed.