what are aquatic macroinvertebrates?

Aquatic macroinvertebrates are visible organisms with no backbone that spend all or most of their life in rivers, streams, lakes, and ponds (AKA water bugs!). They are a necessary part of a healthy aquatic ecosystem serving as the primary processors of organic material while also being prey to larger organisms, such as fish and other predatory macroinvertebrates.

Above: Caddisflies in cases attached to a river rock!

habitat

Certain types of macroinvertebrates require different habitat conditions. Rivers and streams, with faster flowing waters, usually have a colder temperatures and higher levels of dissolved oxygen which are ideal for stoneflies, mayflies and caddisflies. Lakes and ponds, with slower or more stagnant waters, usually have warmer temperatures and lower levels of dissolved oxygen which are tolerated by leeches, mosquitoes and midges.

Macroinvertebrates also require adequate shelter and food sources that can be provided in a diverse environment of submerged logs and woody debris, rocks and sandy substrate, riparian vegetation, decomposing leaves and other organic material that falls into the water.

the 5 c's of a healthy river

Cold

 

Clean

Clear

Complex

Connected

Life Cycles

Most aquatic invertebrates will live the majority of their life time in the water and only emerge as terrestrial adults.  There are two types of life cycles that an invertebrate will go through as it grows:

 

Complete metamorphosis has 4 distinct phases in which the organism does not resemble its previous form.

​​

1.     Eggs are deposited in the water and hatch into larvae.

2.     Larvae eat and eat and eat until they grow enough begin their transformation.

3.     Pupae enclose themselves in cases, sacs, or their outer skin hardens into a protective shell. Inside they develop into their adult form.

4.     Adults emerge from their cases and fly off to mate and lay more eggs. 

Examples are caddisflies, midges, and crane flies. 

Incomplete metamorphosis has three phases.​

1.     Eggs are deposited in the water and hatch into nymphs.

2.      Nymphs eat and grow but periodically shed their rigid exoskeleton during their molting phases so they can grow some more.  Molting occurs repeatedly during this stage of development. Wing pads develop gradually with each molt. 

3.     Adults emerge after the last molt, dry their wings and fly off to mate and lay more eggs. 

incomplete_morph.png

Examples are mayflies, stoneflies, and dragonflies.

Anatomy

anatomy.png

Anterior: Refers to the head end of the body/structure.

Lateral: Refers to the side of the body/structure.

Posterior: Refers to the tail end of the body/structure.

Dorsal: Refers to the upper or top part of the body/structure.

Ventral: Refers to the lower or bottom part of the body/structure. 

Head: Usually capsule-like and contains the feeding apparatus of the organism.  

Thorax: Composed three segments and the location of the legs or leg-like appendages. 

Abdomen: Composed of several segments (often 8-11).

Antennae: A pair of slender movable sensory organs located on the head. 

Mandibles: A pair of appendages near the mouth used to grab, cut and chomp.

Tarsal claws: Claws located at the end of the leg.

Cerci: Appendages located on posterior end of some insects.

BioIndicators

Bioindicators are organisms that effectively indicate the condition of the environment because of their limited tolerance to environmental variability.  We can use the presence of certain macroinvertebrates (stoneflies, mayflies and caddisflies) to help determine the likely health status of a river! For example, stoneflies need cold, clean and well-oxygenated waters so if you find some stoneflies in a stream then that's a good sign that the water is in good health. 

Factors governing aquatic insect distribution:

  • Oxygen availability

  • Temperature

  • Sediment and substrate type

  • Presence of pollutants such as pesticides, acidic materials and heavy metals

Learn more about macroinvertebrate orders - key characteristics for identification and stress tolerance levels! 

Please give credit to Watershed Education Network for anything you reproduce or use from our website. Thank you in advance.

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water@montanawatershed.org

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Missoula, MT 59807

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