An Introduction to Invertebrate Anatomy
Identifying macroinvertebrates is easier if you are familiar with the terminology used in basic invertebrate anatomy.
You will come across some terms that indicate orientation or direction: Dorsal means the upper side, or top (analogous to our back or spine); ventral refers to the underside or bottom; lateral refers to the left and right sides. Anterior structures are located towards the front of the body, or head; posterior structures are towards the "tail" of the body.

Click on the links below to learn about invertebrate anatomy

Insects: Adult insect | immature insect: nymph | immature insect: larva
Non-insects: freshwater mussels

Remember that development, or metamorphosis, varies between taxa. As an organism molts and progresses through its life stages, some body parts may change in size but remain similar in appearance (i.e. larval vs. adult Hemiptera), which other structures may change appearance or dissapear completely (i.e. larval vs. adult Coleoptera).

All insects have 3 body regions (head, thorax, abdomen). The thorax consists of 3 segments, which carry the true legs and wings (in adults), and the abdomen consists of 11 segments, which may carry gills, cerci, and in adults, the genitalia.

Immature insect: dorsal view of a mayfly nymph (Ephemeroptera) Note: All insects have 3 body regions (head, thorax, abdomen). The thorax consists of 3 segments, which in nymphs carry the true legs. The abdomen consists of 11 segments, which may carry gills and cerci.         wing pads   gills antennae head   thorax     abdomen         cerci             Dorsal View of adult true bug (Hemiptera:Belostomatidae Note: All insects have 3 body regions (head, thorax, abdomen). In adults, the thorax consists of 3 segments, which carry the true legs and wings, and the abdomen consists of 11 segments, which may carry cerci and the genitalia.               foreleg   pronotum mid leg scutellum   femur tibia hind leg     tarsi tarsal claws Freshwater mussel(Illustration ©Ethan Nedeau, Biodrawversity)

Lateral view of a fly larva (Diptera: Dixidae)