Sciarid gnats
| Sciarid gnats | ||||||||||
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A fungus gnat |
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| Sciaridae | ||||||||||
| Billberg , 1820 |
The sciarid gnats (Sciaridae) are a family of the two-winged birds (Diptera) and belong to the suborder of the mosquitoes (Nematocera). Around 1,800 species have been described worldwide , which is probably only a fraction of the total number of species. More than 600 species are known in Europe . They owe their German and scientific name to the dark color of their bodies and their darkly clouded wings ( Greek σκιαρός skiaros , shadowed, dark).
features
The mosquitoes reach a body length of one to seven millimeters. They have a slim body and are dark in color. They have characteristically dark wings, on which the central vein divides into a bell shape (see fig.). The females of some species, however, are wingless. Like most mosquitoes, they have long legs and 8 to 16-link antennae . In addition to the compound eyes, they also have point eyes ( ocelli ). Your maxillary palps consist of three segments. The thorax protrudes over the head in many species.
The larvae have a slim build, are grayish white in color and have very small tracheal openings . They have a completely formed, chitinized black head capsule, so they are eucephalic .
distribution
The fungus gnats are common worldwide. They also colonize extreme habitats far north or south of the Arctic Circle, such as islands around the Antarctic , tundra areas and mountain regions above 4,000 m above sea level. There are also troglophilous species, some of which even live exclusively in caves . But you can also find them in the hot regions of the world. Species of the genus Parapnyxia burrow in the desert sand both during hot hours and on cold nights . Most species can be found in moist habitats such as forests , moors , wet meadows , on pastures , fields and also in gardens. There they live hidden in leaves and plants. They also occur in houses and apartments and develop there in flower pots. In humid and shady biotopes , the fungus gnats can make up 70% of all two-winged species .
Their distribution is decisively influenced by wind and other drift such as driftwood. Humans also make a major contribution. The trade in agricultural products, and in particular the transport of humus , potting soil and peat , brings in numerous species that can settle permanently. The ideal conditions for rearing non-biotope species in greenhouses, in which the animals have optimal temperature and humidity conditions and there are no enemies, also contribute to this.
Fossil evidence
Fossil fungus gnats are known from various amber deposits from the Cretaceous and Tertiary periods . The oldest evidence comes from the amber of the Lower Cretaceous Lebanon (about 130 million years old); with at least ten genera, the Baltic amber family ( Eocene , approx. 40 to 50 million years old) is most strongly represented. Here the fungus gnats make up about a fifth of all Nematocera found in Baltic amber. The fungus gnats are such a typical and central element of the fauna of the Eocene- Oligocene amber forest that some authors refer to them as the Sciara zone.
Way of life
The adults only ingest fluids and die about five days after hatching. They only live to mate and reproduce. Their prancing flight is characteristic.
The larvae saprophagus or mycetophagus either eat organic material such as leaves, bark and dead wood or fungi . There are also species that mine in parts of plants and roots . The larvae are among the most important foliage-decomposing organisms in forests. They are also an important link in the food chain, both for the predators who eat them and for the microorganisms that eat their excrement.
Under the name "Heerwurm" (also Ascarides militares , hazel worm, starvation worm, snake worm, worm snake, Dragfae, Orme-Drag, Gärds-Drag [= house move], Härmask, Grynorm, Luskung [= Lauskönig], Armyworm, war worm, war snake, army worms , Army snake, worm dragon or dragon worm) are popularly known as conspicuous collections of sciarid gnat larvae. These trains can reach up to 14 meters in length and contain several hundred thousand larvae. You can cover about one meter per hour in wet weather. Armyworms were previously attributed to the species Sciara hemerobioides and Sciara militaris (which was named after them), and breeding experiments have shown that numerous other species such as Sciara analis , Bradysia bicolor , Cratyna perplexa (syn. Sciara gregaria ) and Ctenosciara hyalipennis can build such larvae. Armyworms appear mainly in Europe, almost only in Central and Northern Europe, especially in the low mountain ranges. The reasons for the mass increase that lead to the army worm trains are not known. In the Middle Ages, army worm trains were often associated with war, famine, disease and death. The first documented report of such larval movements dates from 1603.
Since 90 percent of the larvae are females and the male adults can only cover short distances due to their short life and their very limited flight skills, finding a partner is only guaranteed if the larvae remain in a group. The high proportion of female animals causes a much higher potential for reproduction compared to species with a balanced sex ratio. In view of the low mobility of the adults, the common larval migration also serves to spread.
development
The females lay up to 200 transparent eggs in the moist soil. The larvae hatch after about seven to eight days. These live in large groups of up to 2,500 animals per square meter. The larval migrations can be observed in Central Europe from May to June. Pupation, which takes place in a mummy doll , takes place in July and August.
genetics
The genetics of the sciarid gnats have some peculiarities, most of which were researched in the 1920s and 1930s by Charles W. Metz and colleagues in the USA.
The males of the sciarid gnats - as with the related gall gnats - only pass on to their offspring those genes that they had received from their mother. While the female sex reproduces normally and forms a continuous chain of ancestors and descendants, the males only act as mediators between these purely female lines of inheritance. Cytologically, this is based on a special process of spermatogenesis : In meiosis I, the homologous chromosomes do not pair (as is the rule), but separate without prior pairing. The structure of the spindle apparatus is also unusual : there is only one half spindle with one pole, to which the maternal chromosomes (i.e. those of maternal origin) are drawn. The paternal chromosomes (of paternal origin), on the other hand, move in the opposite direction to the cell periphery and are pinched off there in a small plasma bud that perishes.
This separation of the homologous chromosomes without prior pairing is possible because the two sets of chromosomes are not mixed with each other in the intact cell nucleus before the nucleus division (as is usual). Instead, the four chromosomes of each set are arranged in a ring, and both rings are roughly parallel to each other. While in the usual course of the first meiotic division homologous chromosomes are randomly distributed to both daughter nuclei after mating (an essential aspect of genetic recombination ), here they are already separated according to their origin before meiosis.
The course of the second meiotic division is also unusual . In this case, the X chromosome (the only sex chromosome present in males ) does not gather with the other chromosomes ( autosomes ) in the metaphase plate between the spindle poles in order to be divided there, but moves to a pole prematurely and only divides there or on the way there. Since here too (as already after meiosis I) a highly inadequate cell division follows and a sperm emerges only from the daughter structure which has ultimately received the double X chromosome, so all sperm contain two X chromosomes. After fertilization , therefore, there are three X chromosomes in the zygote .
The determination of the sex of fungus gnats is also carried out in an extraordinary way. As with many other insects, the sex determination is basically the X0 type, i.e. females have two X chromosomes, males only one. However, the number of X chromosomes is not determined here (as is normally the case) by the chromosome configuration of the germ cells, because the zygote always contains three X chromosomes for the reasons mentioned above. At an early embryonic stage (usually at the 7th cleavage division), one or two X chromosomes are eliminated because they do not divide during mitosis and remain in the metaphase plate. How many X chromosomes are eliminated depends on the genome of the mother; therefore every female sciarid gnat has either only female or only male offspring.
Another peculiarity is the presence of special chromosomes only in the germ line , i.e. in germ cells and their potential predecessors (but not in all species of sciarid gnats). They are called germline-limited chromosomes. germ line limited chromosomes or L-chromosomes derived therefrom. In the meiotic divisions in the course of spermatogenesis, they are also not randomly distributed, but all get into the single sperm without reduction. Similar to the previously mentioned surplus X chromosomes, however, all but exactly two L chromosomes are excreted from the cell nuclei in an early embryonic stage of the offspring. At the transition from the germline to the soma they are completely eliminated, but they are indispensable in the germline cells: If they are lost, then these cells lag behind in development and do not enter into meiosis.
Types (selection)
Harmful effect and control
Some species of sciarid gnats can cause damage to agriculture when seen in large numbers. In addition to various types of vegetables and mushrooms , ornamental plants are also infested, both in the greenhouse and in the field. The larvae damage the roots and other parts of the plant from eating. This can cause seedlings and young plants to die.
While in the past you had to resort to synthetic insecticides (for example dimethoate , commercially available as Bi-58) to cope with the plague, there are now biological agents that work just as well and, unlike chemical preparations, do not affect young plants . These agents include Steinernema feltiae - nematodes (SF-nematodes) and Bacillus thuringiensis israelensis (BTI). Poured into the substrate, they fight the mosquitoes in different ways: While the nematodes are actively searching for the larvae, the larvae first have to eat the BTI. Accordingly, the former is better for severe infestation and the latter for prevention.
Yellow boards are often offered against fungus gnats, but are only used to a limited extent for combating them, as they only reduce short-lived adults and leave the larvae undisturbed.
Web links
- Fam. Sciaridae fungus gnats ( Memento from July 5, 2008 in the Internet Archive )
- Sciarid gnats as pests on indoor plants
Individual evidence
- ↑ George O. Poinar, Jr .: Life in Amber . 350 pp., 147 figs., 10 plates, Stanford University Press, Stanford (Cal.) 1992. ISBN 0-8047-2001-0
- ↑ Wolfgang Weitschat and Wilfried Wichard: Atlas of plants and animals in Baltic amber , 256 p., Numerous. Fig., Pfeil-Verlag, Munich 1998. ISBN 3-931516-45-8
- ^ Sven Gisle Larsson: Baltic Amber - a Palaeobiological Study. Klampenborg (DK) 1978. ISBN 87-87491-16-8 .
- ^ Frank Menzel (1999): Revision of the Palearctic fungus gnats (Diptera, Sciaridae) with special consideration of the German fauna. Dissertation, Department of Environmental Sciences at the University of Lüneburg. therein chap. 4.2: The phenomenon of the army worm.
- ↑ Frank Menzel & Ulrich Schulz (2007): The fungus gnats in Germany - ecosystem significance, cenological coincidences and bioindicator potential. Contributions to entomology 57 (1): 9-36 doi : 10.21248 / contrib.entomol.57.1.9-36 .
- ↑ a b c C. W. Metz, Chromosome behavior, inheritance and sex determination in Sciara. In: American Naturalist. Volume 72, 1938, JSTOR 2457532 , pp. 485-520
- ↑ a b c S. A. Gerbi: Germ Line - Soma Differentiation . Ed .: Wolfgang Hennig. tape 13 . Springer, Berlin, Heidelberg 1986, ISBN 978-3-662-21958-4 , chapter Unusual Chromosome Movements in Sciarid Flies , p. 71-104 , doi : 10.1007 / 978-3-540-39838-7_2 .
- ↑ DF Kubai: Nonrandom chromosome arrangements in germ line nuclei of Sciara coprophila males: the basis for nonrandom chromosome segregation on the meiosis I spindle. In: The Journal of Cell Biology . tape 105 , no. 6 December 1987, pp. 2433-2446 , doi : 10.1083 / jcb.105.6.2433 .
- ↑ Melvin L. Zilz: In vitro male meiosis in the fungus gnat (Sciara), with analysis of chromosome movements during anaphase I and II . Detroit, MI 1970, OCLC 16865909 (PhD thesis).
- ↑ Clara Goday, M. Rosario Esteban: Chromosome elimination in sciarid flies . In: BioEssays . tape 23 , no. 3 , March 2001, ISSN 1521-1878 , p. 242-250 , doi : 10.1002 / 1521-1878 (200103) 23: 33.0.CO; 2-P .
- ↑ B. de Saint Phalle, W. Sullivan: Incomplete sister chromatid separation is the mechanism of programmed chromosome elimination during early Sciara coprophila embryogenesis . In: Development . tape 122 , no. December 12 , 1996, ISSN 1477-9129 , pp. 3775-3784 ( abstract ).
- ^ A b Sally M. Rieffel, Helen V. Crouse: The elimination and differentiation of chromosomes in the germ line of sciara . In: Chromosoma . tape 19 , no. 3 , September 1966, ISSN 1432-0886 , p. 231-276 , doi : 10.1007 / BF00326917 .