Fig. 18.—‘Nymph’ of M. domestica dissected out of pupal-case about thirty hours after pupation. an, Swellings of nymphal sheath marking bases of antennae; cx, coxa of leg; lb, labial portion of proboscis sheath; lbr, labral portion of same; n.sp, spiracular process of nymph; w, wing in nymphal alar sheath. (From Gordon Hewitt.)
Fig. 19.—Pupal-case or puparium of M. domestica from which the imago has emerged, thus lifting off the anterior end or ‘cap’ of the pupa; ventro-lateral aspect. a.sp, Remains of the anterior spiracular process of larva; l.tr, remains of the larval lateral tracheal trunk; n.sp, temporary spiracular process of nymph; p.sp, remains of the posterior spiracles of larva. (From Gordon Hewitt.)
Flies become sexually mature in a week or ten days after emerging from the chrysalis-case, and are capable of depositing their eggs four days after mating, so that if the conditions be indeed favourable the whole development from the egg to the perfect fly may be accomplished in nine or ten days, and the second generations are able to lay their eggs ten days later. The appalling fecundity of such an insect explains the fact that in the hotter parts of the world nearly every edible thing seems to be covered with them.
The proboscis of a fly can only suck up liquid food; and when we see it feeding on solid substances, such as sugar, it has really dissolved the sugar by depositing some saliva on it, and is sucking up the sugary solution so produced. It not infrequently regurgitates its food in a spherical drop, which it generally re-absorbs.
As we have seen, flies are very susceptible to temperature, and with the approach of cold weather they seem to die. We used to think that some, in a state suspended animation, ‘carried on’ through the winter months. This is, however, ‘non-proven.’ Many of them undoubtedly die in the autumn, as bees die, of old age. They are literally worn out. But a great number fall victims to a parasitic fungus called Empusa. Flies killed by this fungus are frequently to be seen in autumn, hanging dead on windows, &c., surrounded by a little whitish powdery ring of spores formed by the fungus.
Flies, like many other insects, are extremely difficult to keep alive in captivity, and few have succeeded in rearing them for more than a month or two. At one time, as we have said, it was thought that those flies which survive the winter were fertilised females of the younger broods, and that during the winter they subsisted on their ‘fat bodies.’
Fig. 20.—M. domestica in the act of regurgitating food. × 4½. (From Gordon Hewitt.)