Aquatic Macroinvertebrate Identification

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 four 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 the develop into their adult form.

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

 

Examples of invertebrates that go through complete metamorphosis 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. 

Examples of invertebrates that go through incomplete metamorphosis are

mayflies, stoneflies, and dragonflies.

Anatomy

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 taxa that effectively indicate the condition of the environment because of their limited tolerance to environmental variability. 

Many taxa differ with regard to their sensitivity to environmental change and habitat requirements so we can choose the taxon according to the needed resolution. 

Factors governing aquatic insect distribution:

  • Oxygen availability

  • Temperature

  • Sediment and substrate type

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

Levels of Sensitivity or Stress Tolerance

Very Intolerant: Usually found only in nearly pristine environments; quickly eliminated if large disturbances occur; do not occur in high numbers.

Somewhat Intolerant: Will be in pristine environments but can also withstand a limited amount of disturbance; usually do not occur in high numbers.

Facultative: Occur in environments with conditions ranging from pristine to moderate levels of disturbance; often occur in high numbers under conditions of moderate disturbance. 

Somewhat Tolerant: Similar to very tolerant but they cannot survive in severely disturbed environments; occur in high numbers but they do not dominate the community as completely as the very tolerant kinds. 

Very Tolerant: Seldom found in pristine environments; occasionally found in moderately disturbed environments; exceptionally high numbers in environments with severe disturbance; can withstand almost anything; flourish where conditions are so bad that they probably have only one or two competitors, or none. 

Identification

Curious about how to identify the macroinvertebrates living in your stream or pond? Learn more about the different orders and families by clicking the button below! 

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