Blowflies - Family Calliphoridae

Blowflies often are an attractive blue-green, metallic colour, leading to the common English names, blue-bottles and green-bottles - and the common Australian name, blue-arsed flies. They also come in a non-metallic, brown form, but all blowflies usually relatively large flies. Blowflies can pick up faint traces of the odour of decay and can fly up to 20 km from their birth-place in search of a suitable corpse in which to lay their eggs.

Female blowflies will often use their tongue-like mouthparts to feed on the protein secretions oozing from a corpse, prior to laying their eggs through their pointy ovipositor. Although they race around like blue-arsed flies when locating a suitable corpse, their behaviour changes dramatically when they are ready to lay, and they become quite lethargic and persistent. Blowfly eggs are 2 mm long and are laid in clumps that resemble miniature rice balls. A single female can lay nearly 2,000 eggs during her life, and 5,645 eggs have been counted from a small piece of meat (150 g) after five hours exposure. The eggs hatch after between 12 hours and 2 days, depending on the temperature.

Blowfly maggots are of two forms: smooth maggots, and 'hairy' maggots. The smooth maggots belong to pioneer flies that are purely corpse feeders. 'Hairy' maggots will often feed on corpses, but they are also active predators that feed on smooth maggots. Although they look hairy, the 'hairs' are really papillae (protrusions of the body), which deter other predators from consuming them. Because they are predators, the arrival of the secondary flies that produce hairy maggots is normally later that of the pioneer maggots.

Blowfly larvae feed using their mouth hooks and rapidly increase in size. They shed their skins (moult) three times during their development. Each stage of development is called an instar and the time it takes to develop between instars is fairly constant, although it is dependent upon temperature. The identification of larvae of a particular instar, combined with a knowledge of recent weather conditions, allows forensic entomologists to determine the time of death in murder investigations.

When the third-instar larva has finished growing, it leaves the corpse and burrows into the ground where it develops into a pupa. It takes around 14 days for the pupa to reorganise itself and emerge as an adult fly, but can take much longer in cold weather.