Segment number one has only a back piece, you see.

All the other segments have four pieces.

Segments two, three, four, five, six, seven, and eight are alike.

Segments nine and ten are modified to form the ovipositor.

The segments are fastened together by skin. The skin is soft so the segments can move back and forth.

The segments can be crowded close together to shorten the abdomen.

The segments can be separated from each other to lengthen the abdomen.

There is no chitin in the skin between the segments. It is soft so the segments can move.

Do you know how a telescope is made?

The abdomen of the insect can lengthen and shorten somewhat like a telescope.