Xo. 04 I DIPTERA or COXMXTICTT 3 



Aside from the habit of feediiiiir on cadavers, a characteristic 

 of blow-flies, there are many species of Diptera which parasitize 

 livinjj: warm and cold-blooded animals. Spiders form the sole hosts 

 of one family of fiies, the Cyrtidae, and some other species are found 

 in spider cocoons where they prey upon the eggs. Some phorid larvae 

 live in the lieads of ants. Almost all insect orders contain some species 

 wliich are the prey of certain flies. The Tachinidae are noted as para- 

 sites of caterpillars and sawfly larvae. Rodents, ungulates, bats, and 

 birds serve as hosts for internal and external parasites. Some of 

 these parasites live as maggots in the intestinal tract, nasal passages, 

 or flesh of mammals, and are known as bot-flies and w^arbles. The 

 larvae of Protocalliphora live on nestling birds. Adult pupiparous 

 flies live as external parasites on sheep, bats and birds. 



The life cycle of the Diptera involves a complete metamorphosis, 

 that is, the immature stages difter markeiUy from the adults and a 

 transitional more or less quiescent stage, the pupa, intervenes between 

 the tAvo. Duriiifo; this stage a transformation occurs, the larval struc- 

 tures disapi^ear, and the adult structures are formed. There are thus 

 four stages in the typical life cycle of a fly — egg, larva (usually called 

 a maggot), ])upa. and adult. These four stages may occur in different 

 places. Warble flies show a great diversity in this respect. The eggs 

 are laid on the hair of the legs of cattle, the larvae spend the last 

 part of their lives under the skin of the back, and the pupae are found 

 in the soil. Even in the case of the housefly, which may spend its 

 entire existence in and around a manure ]ule, the difl'erent stages of 

 the life C3'cle occur in different parts of the pile. The eggs are laid 

 on or close to the surface, the larvae live in the wet parts, the pupae 

 are found in the drier sections or in the earth about the pile, and the 

 adults hover in the air above. The northern house mosquito is more 

 restricted in its habitat. The eggs are laid on the surface of the water 

 and the larvae and pupae live in it. Among the pupiparous flies, 

 such as the sheep tick and bird flies, the habitat is confined to the 

 body of the host. The larvae are retained within the abdomen of the 

 adult female until fully grown and the pupae adhere to the wool or 

 feathers. 



The al)un(hince of flies depends, as does the abundance of any 

 animal, on the reproductive power of the species and on the piessure 

 of the environment which restricts its realizaticm. The reproductive 

 power is limited by the number of females present, the number of 

 progeny per fenuile. and the number of generations a year. Most 

 species reproduce bisexually, so that an average population includes 

 an equal number of males and females. A notable exception occurs 

 in the case of certain cecidomyids, as Miastor, which reproduce in the 

 larval stage, a phenomenon known as paedogenesis. Paedogenesis is 

 not continuous, however, but is interrupted by a generation which 

 reproduces bisexually. Most adult flies deposit eggs, but many species 

 of the family Metopiidae produce young maggots, and the Pupipara 

 produce fully grown maggots. 



