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Rat tail maggots (Syrphidae)

Taxonomic group

Family: Syrphidae

Diagnostic features

The syrphid larva has a maggot-like body with a long tail-like breathing tube.

Typical habitats

The breathing tube allows syrphids to live in waters with almost no oxygen. Such conditions might exist immediately below long-term discharges of organic waste such as silage pit leachate, dairy waste or sewage.


They eat decaying organic matter, hence their occurrence in waters with a long term supply of organic waste.

Indicator value

  • Hard bottom: 1
  • Soft bottom: 1.6

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

The presence of syrphids in any stream indicates firstly that you shouldn’t be coming into contact with the water, and secondly you should be calling your local pollution hotline as syrphids are a sign of a serious pollution problem. Syrphids have tolerance values of 1 (hard bottom sites) and 1.6 (soft bottom sites) reflecting their association with heavily polluted waters.