Legging it

Butterflies are insects. That means they have three body segments – the head, thorax and abdomen.  At a more detailed level the thorax is comprised of three body segments fused together. To each of these is attached a pair of legs. The abdomen is made up of 10 segments. Young butterflies i.e. caterpillars also have a three-segmented thorax and a 10 segment abdomen.

The caterpillar pictured above shows this typical structure. Even though they may seem to have more, caterpillars only have three pairs of legs. As in the adult they are attached to the three thoracic segments near the head (see picture above). What appear to be legs further down the body are known as prolegs. These are hydraulically powered appendages used for gripping plant stems

Butterfly and moth caterpillars have a maximum of 5 pairs of prolegs. They are positioned on segments 3-6 and (always) 10 on the abdomen. The caterpillar pictured above has two pairs of prolegs which are characteristic of moths of the Geometridae family. The Emperor Gum Moth caterpillar (Opodiphthera eucalypti), pictured left has the full complement of five pairs of prolegs.

The number of prolegs is diagnostic for insect type. The larvae of sawflies (e.g. spitfires) have between 12 and 16 prolegs in addition to the 6 true legs. Scorpionfly caterpillars top the list with 18 prolegs.

It’s lucky they don’t have to buy shoes.