Our next Bug of the Month comes from one of the richest and most diverse groups of aquatic insects—the midges (the Family Chironomidae). Midge larvae and pupae are aquatic, and the adults are terrestrial—visible as swarms (called clouds) flying over the surface of the water during breeding season. Worldwide, there are over 4,000 species of midges, most of which are less than 10 millimeters long; hence, they are the bane of fly fishermen. In order to use them, fishermen need to tie the imitations in hook sizes that range from 18 to 28 (that’s tiny in case you were wondering). They try this in any event because midges (although small) tend to be very numerous, forming a major component of the forage base for nearly all fish species (adult and juvenile), be it a stream, river, lake, or pond.
Because this is such a large and diverse group, they have undergone a high degree of specialization, colonizing pretty much every habitat type available in aquatic systems. The family is divided into nine Subfamilies and/or Tribes, five of which are common:
- The subfamily Diamesinae prefers cold running water and are common in trout streams with cobble-gravel substrates.
- The subfamily Orthocladiinae likes cold or warm running water and prefers cobble-gravel substrates.
- Tanytarsini, a tribe, are mostly found in slow current climbing on aquatic vegetation.
- Another tribe, Chironomini, prefers sandy or muddy bottoms in lakes and slow-moving rivers. Some species in this tribe are adapted to anoxic substrates (low oxygen) and can colonize areas where many other invertebrates are excluded.
The term for all of this is called resource partitioning—a process that reduces direct competition, allowing many more species to colonize a water body than would otherwise be the case.
- Most midges are herbivores, but a subfamily called Tanypodinae are predators and can be found in a lot of different places.
A popular way to characterize water quality using a biotic component like midges is first to assign a Tolerance Value to each species. These values range from 0 to 10 and are published in the state protocols we follow to provide laboratory services to our clients. Low values (0–3) indicate low tolerance, meaning that these species are sensitive and require clean water. Those with intermediate values (4–6) indicate facultative species, which can tolerate a moderate amount of pollution. The most tolerant (7–10) are at an advantage where they can survive even when the water and/or the substrate is contaminated. Most midges fall within a range of 4 to 10, so they are almost entirely either facultative or tolerant. Their presence in large numbers, combined with an absence of other invertebrate forms, often indicates some level of poor water quality, sediment quality, or habitat degradation.
A fascinating midge species that recently made it into our lab is Saetheria tylus. This specimen was collected from Little Silver Creek, a small stream in New Jersey. This is an uncommon species specialized to live on sandy substrates subject to a lot of shifting during high-flow events. This was of particular interest as we do not see this species all that often. The Tolerance Value is 4 so they are facultative, on the sensitive side. Because of its uniqueness, Saetheria tylus is our Normandeau “Bug of the Month.”
