Insects with Incomplete Metamorphosis


The three insect orders with incomplete metamorphosis (hemimetabolous) are Ephemeroptera, Odonata, and Plecoptera.  The Odonata are further divided into two suborders, Anisoptera (dragonflies) and Zygoptera (damselflies).  All of these insects have immature stages that are found in aquatic environments, such as ponds and streams, and posses gills.  The immatures, known as naiads, must come to the surface of the water or crawl out of the water for their final molt.  The Ephemeroptera and Odonata are the only extant insects with a paleopterous flight mechanism and are often classified as infraclass Paleoptera.

Order Ephemeroptera (Mayflies)

Mayflies are medium-sized soft-bodied insects.  They are common near aquatic environments, where the immatures develop.  The adults have membranous wings, with the front wings being large and somewhat triangular while the hind wings are typically smaller and rounded.  Unable to fold their wings over their back, they belong to the Paleoptera infraclass.  The adults have one pair of small bristlelike antennae.  The adults do not feed and have vestigial mouthparts.  The "tails" on the abdomen are key characteristics of mayflies.  Some mayflies have only a pair of cerci, others have a pair of cerci and a median caudal filament.

Order Odonata (Dragonflies and Damselflies)
 The Odonata are relatively large insects.  As immatures, they live in aquatic habitats and often remain near water as adults.  They are predaceous as both immatures (naiads) adults.  Like other aquatic insects, they are often used as ecological indicators.  Both suborders have large compound eyes, three ocelli, short bristlelike antennae, chewing mouthparts, and membranous wings.  The variations among these characteristics often separates the two suborders.
 

 Suborder Anisoptera
The dragonflies have hind wings that are wider than the front wings, especially at the base.  Also, the wings are held horizontally at rest.  Typically, the compound eyes of dragonflies are touching at the top of the head.

 

           Suborder Zygoptera
Damselflies are usually more slender and smaller than dragonflies.  Their eyes are separated at the top of the head and their wings, which are of equal size and narrow at the base, are held vertically when at rest.