The insect order Odonata is an old insect order having evolved in the
Pennsylvannian and dominated in the Permian. Today, there are about
5,000 species grouped into three extant suborders: Zygoptera -
damselflies (Figure 1); Anisoptera - dragonflies (Figure 2); and
Anisozygoptera, a small suborder of two Asian species morphologically
intermediate to the former two groups. Two of the world's three
suborders - Anisoptera and Zygoptera - are found in Michigan.
Distinguishing between the two suborders is rather easy, based on body shape and posterior abdominal features mentioned in the key below. Both the larvae and adults of all Odonata are carnivores. Larvae of almost all species are aquatic, respiring by means of gills (internal with Anisopera, external with Zygoptera, the later that may also cutaneously respire). The modified labium, used by all larval odonates to capture food, characterizes the larval stage of this order from from all other insects. It is held underneath the head, and, when prey is detected, thrusted out in a lightning-like strike. Prey is then seized with palpal lobes at the end of the labium, brought to the mouth and finally macerated with the larva's mandibles.
1a. Body slender, head wider than thorax and abdomen (Figure 1a1); abdomen terminating in three caudal lamellae (gills) (Figure 1a2 and 1a3) - Zygoptera
Fig.
1a1
Fig. 1a2
Fig. 1a3
Fig. 1a1: Enallagma ebrium
larva, (6x, dorsal view).
Fig. 1a2: Ibid., (6x, lateral view).
Fig. 1a3: Ibid., (12.5x, dorsal view).
1b. Body stout, head usually narrower than thorax and abdomen (Figure 1b1); abdomen terminating in 5 short, stiff, pointed appendages (Figure 1b2) - Anisoptera
Fig. 1b1
Fig. 1b2
Fig. 1b1: Cordulia
shurtleffi larva, (3x, dorsal
view).
Fig. 1b2: Erpetogomphus
designatus larva, (6x, dorsal
view). Dark spots on the epiproct are tubucles that normally develop
on maturing male larvae, and are particularly evident on larvae of
Gomphidae, Aeshnidae, Corduliidae and Macromiidae as well as some
Libellulidae.