Using a Dichotomous Key

Marney Pratt

Using a Dichotomous Key

  • Identifying organisms using a technical key can be tricky.

  • This is a simplified demonstration of how to use a dichotomous key to practice identifying a macroinvertebrate to prepare for doing this same activity with real organisms during lab.

  • Use the arrow keys on your keyboard to move back and forth through these slides and click on images to make them larger.

Mystery Macroinvertebrate

Photo of an aquatic larva viewed from the dorsal side.

Dorsal view of macroinvertebrate. Click on the image to zoom

This is an image of a macroinvertebrate collected from the Mill River in Northampton, MA. We want to identify it down to the family level.

Dichotomous Key Step 1

A dichotomous key gives pairs of options to help you ID the organism. Select the option that describes the organism (click on photos to zoom in)

dorsal view of an aquatic larva.

dorsal view of macroinverterate with three main body parts labeled head, thorax, abdomen. 6 legs are present and 2 long tails.

  • 1 organism without distinct head, thorax, and abdomen …. 2
  • 1’ organism with distinct head, thorax, and abdomen …. 3

dorsal view of an aquatic larva.

dorsal view of macroinverterate with three main body parts labeled head, thorax, abdomen. 6 legs are present and 2 long tails.

  • 3 organism without three pairs of jointed legs attached to the thorax …. 4
  • 3’ organism with three pairs of jointed legs attached to the thorax …. Class: Insecta …. 5

dorsal view of an aquatic larva.

dorsal view of macroinverterate with three main body parts labeled head, thorax, abdomen. 6 legs are present and 2 long tails.

  • 5 Three long tails (but sometimes two) and gills on the abdomen …. Order: Ephemeroptera (mayflies) …. 6
  • 5’ Two long tails and no gills on the abdomen …. Order: Plecoptera (stoneflies) …. 7

close up of head showing notched labrum.

  • 6 Labrum notched …. 8
  • 6’ Labrum smooth and straight (not notched) …. 9

dorsal view of an aquatic larva.

  • 8 antennae relatively long (two to three times longer than the width of the head) …. Family: Baetidae
  • 8’ antennae relatively short (less than two times longer than the width of the head …. 9

Photo of an aquatic larva viewed from the dorsal side.

You got it! This macroinvertebrate is in the family Baetidae, also called the small minnow mayfly.