In association with this movement in a definite direction the body has become vermiform and bilaterally symmetrical.
But in general we find that elaboration of imaginal structure is associated with degradation in the nature of the larva, cruciform and vermiform larvae being characteristic of the highest orders of the Hexapoda, so that unlikeness between parent and offspring has increased with the evolution of the class.
In all the remaining orders of the Endopterygota the larva is eruciform or vermiform.
The eruciform larva of the Orthorrhapha leads on to the headless vermiform maggot of the Cyclorrhapha, and in the latter sub-order we find metamorphosis carried to its extreme point, the muscid flies being the most highly specialized of all the Hexapoda as regards structure, while their maggots are the most degraded of all insect larvae.
In some species (Spengelidae) there is a long capillary vermiform extension of the stomochord in front.