A caterpillar's body plan

The External Morphology of Larval Lepidopterans

I came across this really good diagram of a caterpillar's body plan so I decided to share it here. It outlines the important body features, many of which are useful for identification. All insects share a few basic features in their body plan, and since caterpillars are insects (they are the larval stage of butterflies and moths) you can see all these features on their body plan.

Image created by Michał Komorniczak (Poland)  - (Click image to enlarge) 

A – head, B – thorax, C – abdomen

1 – prothoracic shield

2 – spiracle

3 – true legs

4 – midabdominal prolegs

5 – anal proleg

6 – anal plate

7 – tentacle (anal horn)

a – frontal triangle

b – stemmata (ocelli)

c – antenna

d – mandible

e – labrum

Insect bodies are broken down into 3 main parts: the head (A), the thorax (B), and the abdomen (C).  The Class Insecta, which contains all insects, is within the Subphylum Hexapoda which literally means six-legged  and contrary to what some might think caterpillars only have 6 "true legs". Just like other insects the thorax of a caterpillar (B, and labelled in blue) has 3 segments, each with a pair of legs (3 x 2 = 6!). The body segments posterior to the thorax are part of the abdomen (C, labelled in orange), some of which have leg-like appendages called prolegs (4). The anal horn (7) is a prominent feature of Sphingidae caterpillars, but is not present in many other groups.