Please refer also to the following links for details on this group:
As with the Hymenoptera, by the 4th decade of the 20th Century there already existed an extensive literature concerning the exceptional diversity in habits and forms within the order Diptera (Clausen 1940). Their economic importance judged from the attack on other insects, ranks Diptera next to Hymenoptera in the number of species and in the effectiveness of attack. The Tachinidae are outstanding among the parasitic families followed by others that consistently have this habit being Cyrtidae, Pipunculidae, Nemestrinidae, Conopidae and Pyrgotidae. The Bombyliidae are mostly parasitic but there are many predaceous species also. Parasitic representatives occur in many families that are predominantly not entomophagous, such as the Agromyzidae, Phoridae, Cecidomyiidae, Calliphoridae and Anthomyiidae. Predaceous families known for their attack on crop pests are Asilidae, Tabanidae, Syrphidae and Ochthiphilidae, but many others also have species with predaceous habits.
Most parasitic Diptera are primary parasitoids of plant pests and are considered very beneficial. However in the Tachinidae exceptions are those that attack adult Carabidae and Apidae and some spiders. The Pipunculidae, Pyrgotidae and Nemestrinidae are almost entirely beneficial. Parasitic Bombyliidae are most harmful because they attack larvae and pupae of beneficial Tachinidae and Hymenoptera, which counteracts their value as infrequent parasitoids of Coleoptera and Lepidoptera. The Conopidae may be considered injurious because they attack Vespoidea, and the Cyrtidae similarly are harmful because of their spider parasitization. Groups that are predaceous as larvae are usually beneficia, as is the case with adults having predaceous habits. Many different kinds of insects are prey of asilids and other large flies, which ranges from scarab beetles to small flies, and thus it is difficult to evaluate their effect. Generally, the food sources of adult predaceous Diptera are determined by size and ease of capture and by their relative abundance in the predators' environment.
Host preferences of parasitic species vary, with larvae of Lepidoptera being preferred followed by larvae and adults of Coleoptera. These two orders are attacked primarily by Tachinidae. Other hosts that are infrequently attacked include dipterous larvae, hymenopterous larvae and adults, hemipterous, homopterous and orthopterous adults and occasionally nymphs, nymphs and adults of Dermaptera, nymphs of Trichoptera and the Isopoda and Arachnida (Clausen 1940/1962).
Predators are commonly found in the Syrphidae attacking Aphididae, Coccidae and other Homoptera; Asilidae, the larvae of which prey on various insects in soil; and Bombyliidae, Calliphoridae, Sarcophagidae and Anthomyiidae, many of which are predators of acridid egg masses. Occasionally species of Drosophilidae, Phoridae and Sarcophagidae develop in the egg sacs of spiders. Predaceous aquatic types feed on a variety of insects and other minute animals in their surroundings. The habits of these were already known in great detail by the 1930's as shown by the memoirs of Johannsen (1934-1937), published under the title "Aquatic Diptera." Clausen (1940) provided a general statement on the habits of entomophagous dipterous larvae, grouping them into either predominantly parasitic or predatory and predatory only. He noted that there may be a difference of opinion as to which habit predominates in a particular family. In the Bombyllidae, e.g., the number of species parasitic larvae of Hymenoptera and others probably exceeds that which is predaceous in egg masses, although the population of the latter may be larger.
Food requirements of adult Diptera is extremely variable. Parasitic species feed generally at blossoms and on various plant exudations, as well as on honeydew secreted by other insects, and a few species are known to imbibe the blood of their hosts upon which they lay their eggs. Adults of groups with predaceous larvae may themselves be predaceous, as is found in Asilidae, Rhagionidae, Therevidae and Dolichopodidae. However, those of other groups where the larvae are not highly entomophagous, may feed almost entirely on insect food. Among the latter are the Empididae, Mydaidae, Ceratopogonidae and Scatophagidae.
The first attempt to systematize the subject of dipterous reproduction was by Townsend (1908), who proposed five groups for Tachinidae, based on placement of the eggs or larvae with respect to the host. Pantel (1910) followed with studies on the parasitic species of the order, but with special emphasis on Tachinidae. He increased these groups to 10, using as a basis the reproductive system of the female, the type of egg laid, the stage of incubation at the time of oviposition, and the placement of the egg or larva. These groupings still largely hold and are summarized as follows:
1. Egg macrotype, broadly oval, flattened ventrally, the chorion thick and rigid dorsally and thin ventrally, size proportioned to that of female; deposited on the host body; posterior uterus of gravid female short and broad, occasionally long and narrow (Thrixion). Example = Meigenia floralis Meig.
2. Egg microtype, chorion as previous (#1), size largely independent of that of female; laid on food of the host and ingested by latter; posterior uterus moderately to very long, adapted for partial incubation of a large number of eggs. Example = Gonia atra Meig.
3. Egg large, elongate, not flattened or pedicellate, the chorion thin and flexible; posterior uterus an incubating organ containing a moderate number of eggs; females lacking chitinized piercing organ; larviporous. Example = Miltogramma spp., Sarcophaga spp.
4. Egg with very thin, uniform chorion; ovarioles 50-150, posterior uterus long and coiled, with eggs lying transversely in several series; 1st instar larva with cuticular armature for protection, indicating a free-living period; larva laid by female near host, usually on its food plant. Example = Echinomyia fera L.
5. Egg and larva as in previous (#4), but the ovaries less numerous, the posterior uterus very long, slender and coiled (Glaucophana, Bigonicheta) or moderately long and distended, the eggs lying transversely in regular series (Bigonicheta) or longitudinally and irregularly; larva laid in host vicinity. Example = Bigonicheta setipennis Fall.
6. Egg and larva as in #4, with the chorion slightly thicker dorsally; larva without a specialized cuticular armature; ovarioles 15-55, posterior uterus of medium length, in 1-2 corkscrew coils, and somewhat distended, with eggs lying transversely or longitudinally; fully incubated egg laid on host body. Example = Cryptophlebia ruricola Meig.
7. Female with piercing organ, distinct from ovipositor, for perforating skin of host; egg not narrowed at posterior pole; posterior uterus slender, elongate, intestiniform, serving as an incubating organ, the eggs lying transversely in a single series. Example = Compsilura concinnata Meig.
8. As previous (#7) except that ovipositor itself serves as the piercing organ. Example = Cercomyia curvicauda Fall.
9. Female with piercing organ variably formed and functional; egg much narrowed at posterior pole; posterior uterus short and does not serve for incubation. Example = Hyalomyia, Oxyptera, Conops.
10. Egg with a pedicel at posterior pole, serving for adhering to host; ovarioles of moderate number and posterior uterus intermediate between the simple and incubating forms. Example = Carcelia cheloniae Rond.
Townsend (1934) further extended this classification through the Muscoidea, enumerating 39 habit groups, the majority of which comprise some entomophagous species. He includes additionally characters of the 1st instar larva. However, this arrangement is of greater value to a taxonomist and to the specialist of insect parasitology than to researchers working on biologies of specific parasitoids or predators. Pantel's earlier arrangement is simpler and quite satisfactory. Clausen (1940) conceded that this classification would need revision and extension it if were to include all parasitic groups of Diptera, and further still if it were to include predaceous forms. Considering only parasitic species, the Conopidae, Pipunculidae, Pyrgotidae and Agromyzidae (Chryptochaetum) seem to fall into groups 7-9, for the species consistently insert their eggs into the host body. However, most Conopidae have stalked eggs. Most parasitic Phoridae have the same oviposition habit, but some deposit eggs externally; yet females are still able to puncture the host with the ovipositor. The parasitic Cecidomyiidae that have stalked eggs do not fall into any of the 10 groups, nor do the Cyrtidae, Nemestrinidae and Bombyliidae.
The parasitic species show a wide range in reproductive capacity, this being related to hazards encountered by the eggs and young larvae before the latter gain access to the host body cavity. Species depositing their eggs or larvae in or directly on the host body usually are limited to a few hundred, and several species are known to produce less than 100. Those which lay them in the general area of the host produce 1-2,000, the latter including most Bombyliidae and many Tachinidae. When oviposition is entirely apart from the host or where the eggs must be consumed by the host, the hazards are increased and thus a higher reproductive capacity is required. In this way the Tachinidae and Cyrtidae deposit 2-6,000 eggs. An extreme is shown by Echinomyodes which produces ca. 13,000 maggots (Clausen 1940/1962).
Life cycles among entomophagous Diptera range from ca. 10 days in Metagonistylum and an almost equally short time for various other Phoridae, Tachinidae, Syrphidae and Sarcophagidae, to the annual cycle found in a great many of both the parasitic and predaceous species and to a possibly obligatory 2-yr. cycle in some Nemestrinidae. The egg stage is almost always short, often owing to partial or complete uterine incubation, and in many species hatching occurs in the uterus or immediately on deposition of the egg. The shortest larval period occurs among the parasitic species, some of which complete feeding in 2-4 days, this brief period also being found in a few predators. However, there is no uniformity in this respect for many species remain inactive within the host for long periods of time. The pupal stage similarly shows much variation, ranging from one week to almost a year. The normal cycle of many species may not be completed, being interrupted by adverse conditions. This results in some or all individuals entering diapause, that may extend to over several years. The adult life is correlated with the kind of reproduction, but in most species it extends to 1-2 months (Clausen 1940/1962).
For a detailed discussion of immature stages of Diptera, please refer to Clausen (1940/1962).
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