Dragonfly Biology

Head

Like all insects, the dragonfly is made up of three main body parts: head, thorax and abdomen. The head is a tough, rounded capsule, hollowed out at the back to allow efficient attachment of the neck and to increase head mobility. The mouth is a complex hodgepodge of structures that you would not want to encounter in a dark alley. The upper lip, or labrum, is often considered part of the face. The lower lip, the labium (sometimes called the chin), is made up of three lobes. The labrum and labium function together to capture and secure prey while the jaws do the chewing. The jaws, which work from side to side, are made up of one pair of upper mandibles and two pairs of lower maxillae. These jaws, a series of incurved meat hooks, are worth a close inspection and should be approached with caution in larger species. Species such as dragonhunters and larger darners can drawn blood when they bite.

The face is a conglomeration of plates separated by seams called sutures. The sutures are often darkened into stripes. The upper half of the face is the frons, and the upper surface of the frons is a shelf-like protuberance on which various diagnostic markings may be found. The compound eye is composed of nearly 30,000 lenses, which work in consort to provide a rich visual image to the dragonfly. They are sight-based creatures who, with a quick turn of the head, are able to scan 360 degrees as well as above and below. Their vision probably allows them to discern individual wing beats, which to us would appear as a blur. They can see ultraviolet and polarized light. Many species also see well in dim light.

Their two short bristly antennae are thought to function as windsocks or anemometers, measuring wind direction and speed, thereby giving them a method with which to assess their flight. By the way, dragonflies have no sense of hearing, only a rudimentary ability to smell and are unable to vocalize.

Thorax

The thorax is the center for locomotion. It is a muscular powerhouse, controlling head, wing and leg movements. Dragonflies are unusual in their wing movements. Most insects’ wings are attached to plates of the chitonous exoskeleton that are, in turn, attached to muscles that move the plates that move the wings. Dragonfly wings, on the other hand, are directly connected to large muscles within the thorax. The interior of the thoracic exoskeleton is massively braced and strengthened to withstand the pressures of these large flight muscles. This bracing can be seen through the exoskeletons of lightly-pigmented individuals such as the Wandering Glider, the Four-spotted Skimmer and the Common Green Darner.

Thoracic stripes are present in many species. In order to easily communicate the positions of these stripes, the thorax can be separated into three sections: top, shoulder and sides. The top stripes of the thorax will be found in the region between the head and the wings and are best viewed from the front of the dragonfly. The side stripes of the thorax are found below the hindwing attachment point and back toward the abdomen. The shoulder stripes are found below the forewing attachment point, in between the top stripes and the side stripes. Back to the top

Wings

The anatomy of wings and their venation can be very complicated, and one could make a life’s work of just studying them. Most dragonflies can be identified to the level of genus and many to the level of species by just knowing the wing venation. The veins in the wings of dragonflies start as flattened tubes in the compact, tightly folded wings hidden inside the skin of the aquatic nymph. During transformation to adulthood, the veins fill with hemolymph, or insect blood, causing the wings to unfurl. Most of the hemolymph is drawn back into the body after the wings have been fully expanded. The empty tubes and the membranes dry, leaving crisp, tough wings.

The most obvious feature of a clear, unpatterned wing is the stigma, located on the leading edge of each wing out towards the wingtips. It is thought that the stigma may be used for signaling mates or rivals and may also act as a tiny weight that dampens wing vibrations. The nodus, located at the shallow notch midway down the leading edge of each wing, is an intersection of several large veins and is a point of both strength and flexibility. Because of the structure of the venation around the nodus, the wing is allowed to bend downward (during an upward stroke of the wing) but not upward (during a downward stroke of the wing), resulting in a powerful flight stroke without losing much energy on the return stroke. The wing triangles are located about twenty percent of the way from the wing base toward the tip. The relative size and orientation of these triangles on a dragonfly’s wings can be a clue as to the dragonfly’s family. Originating from an inner, rear corner of the hindwing triangle, the anal loop reaches down into the expanded base of the hindwing. The degree to which the anal loop is present varies from one family to the next. Back to the top

Abdomen

The abdomen always has ten segments. Segments 1 and 2 appear to be integrated into the thorax and are sometimes difficult to tell from the thorax. To find a particular segment, it is usually best to start with segment 10, far out at the tip, and count backwards. Because of its segmented nature, the abdomen is very flexible and is able to arch up or down (but not side to side). Learn to count abdomen segments as many of our descriptions are based on them.The male abdomen is often narrower (“waisted”) at segment 3, whereas the female abdomen is almost always more robust. Back to the top

Reproductive System

The male testes are located in segment 9. Due to the unique nature of dragonfly copulation, the male must transfer sperm to his secondary genitalia, called the hamulus, located in the underside of the second and third segments. The hamulus is a complicated set of “surgical tools” that the male uses for removing the reproductive “investment” made by other males during previous matings. Other parts of the hamulus are then used by the male to fertilize the female with his own sperm. The terminal abdominal appendages of the male are called claspers. The claspers are formed by a pair of upper appendages, called cerci, and a single lower appendage, an epiproct. In some species, the males possess auricles on the sides of segment 2 whose function is to help direct the female’s genitalia to a proper fit with the male’s secondary genitalia during copulation.

The female terminal appendages consist of a pair of cerci, which have little or no function. In some species, namely the Shadow Darner, they are very brittle and tend to break off. Underneath segment 8 there is either an ovipositor or a subgenital plate, depending upon the species. Both structures are for laying eggs and extend over segment 9 and possibly beyond. Back to the top

The above text from: Dragonflies of the North Woods, by Kurt Mead, with permission of Kollath Stensaas Publishing.
Dragonfly 101 Dragonfly Biology Dragonfly Behavior Dragonfly Life Cycle Surveying Species of Interest Collecting Specimens Preserving Specimens

Stream Cruiser - Didymops Transversa

Elegant Spreadwing - Lestes inaeqalis

Riverine Clubtail - Stylurus amnicola