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Wednesday, June 26, 2013

Dazzling Dragonflies

Halloween Pennant, Celithemis eponina 

With their large, multi-faceted eyes, two pairs of strong, transparent wings, and an elongated body, the dragonfly is an ancient insect that inspires myth and lore in many world cultures.  For Native Americans, their form can represent swiftness and energy, pure water, and even symbolize renewal after a time of great hardship.

Usually found around lakes, ponds, streams, and wetlands, dragonflies typically eat mosquitoes, midges, and other small insects such as flies, bees, and even some butterflies.  They capture their prey by clasping them with their spike-studded legs, and their prey cannot use their usual form of escape by diving away, since dragonflies always attack from underneath!  Normally, dragonflies do not bite humans, but if you grasp one by the abdomen, it will bite in order to escape.

Exuvia of Flame Skimmer, Libellula saturata

The lifecycle of a dragonfly consists of three stages: egg, nymph, and adult.  The female will lay her eggs directly on aquatic plants or merely drop them in water.  Once hatched, these nymphs begin their life living underwater, eating other aquatic creatures.  Nymphs of larger dragonflies will even eat the nymphs of smaller species!  This nymphal stage can last as long as four years in some species, but most overwinter in ponds and marshes and emerge in the spring as adults. Once fully grown, the nymph will crawl out of the water on the stem of a plant, break through its’ skin (called the exuvia) and enlarge its’ body and wings by pumping fluids into them.

Mature dragonflies are known for their aerial acrobatics, capable of hovering and rapid acceleration, and can both hunt and mate on the wing.  They need to make the most of their time as aerial predators, since adults live only up to two months. Adult dragonflies are often confused with damselflies, but they are two distinct insect families.  When at rest, damselflies often hold their wings together or slightly open above their body, whereas most dragonflies hold their wings fully open horizontally or slightly down and forward.  Additionally, the eyes on a damselfly are separated, while the eyes of most dragonflies touch.  

Both damselflies and dragonflies are members of the scientific order Odonata, so their lifecycles are very similar.  The mating behavior of dragonflies is a multi-step process because the male has both a primary and secondary set of genitalia.  He will first transfer sperm from the tip of his abdomen to the secondary structures on his second or third abdominal segment.  Then, using the special structures at the tip of his abdomen, he will grab a receptive female by the eyes (or by the 'neck' behind the eyes if a damselfly).  The pair is now considered to be in 'tandem', and the male tows the female in flight.  To complete the reproductive act, the female bends her abdomen beneath the male and touches the tip to the male's secondary structures.  This position is called the 'copulation wheel' and it is at this point when sperm is transferred to the female to fertilize her eggs.

Roseate Skimmer, Orthemis ferruginea
Dot-winged Baskettail, Epitheca petechialis 
Jade Clubtail, Arigomphus submedianus
Neon Skimmer, Libellula croceipennis
Twelve-spotted Skimmer, Libellula pulchella

The common names for dragonfly species that occur in Texas are as colorful as the insects themselves – Twin-Spotted Spiketail, Roseate Skimmer, Eastern Pondhawk, Black Saddlebags, Blue-Faced Meadowhawk, Halloween Pennant, and Jade Clubtail to name a few.  To some extent, the presence of dragonflies may be taken as an indicator of ecosystem quality.  Local populations and diversity may be strongly affected by changes in water flow, turbidity, and in aquatic or waterside vegetation.  Not surprisingly, the greatest number of species  are found at sites with natural water flows, high water quality, native plants, and a variety of microhabitats.

When you see dragonflies this summer, admire their maneuverability, enjoy their jewel-like colors, appreciate their mosquito-eating, and be thankful that we no longer have the “giant dragonflies” from the Jurassic & Cretaceous geologic periods, when their wingspans were up to six times larger than those we have today!