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Adult dragonflies use vision as their primary means of assessing their environment. In this way, they are like us, and their behavior, as compared to that of many other, more secretive insects, is relatively easy to understand if we simply watch what they do. Many specific behaviors are characteristic of particular species or groups of species, so in making an identification, observing behavior is often as important as noting appearance.
Dragonfly behavior has evolved in response to a few simple needs:
The need to eat The need to avoid being eaten The need to reproduce The need to regulate body temperature (thermoregulation) The need to disperse
Some of the distinctive behaviors odonates have evolved to meet these needs are discussed in the following sections.
Dragonflies are voracious predators; they eat just about any animal they can catch and chew, including other dragonflies. Most prey of adult dragonflies are flying insects, taken on the wing. The two general types of aerial feeding used by dragonflies are hawking (the constant pursuit of flying insects) and sallying (darting out from a perch to capture prey and then return to the perch). Hawking dragonflies remind bird-watchers ofswifts or swallows and often feed in swarms as do those bird species, whereas salliers are reminiscent of flycatchers. Some species are hover-gleaners, picking prey from vegetation and other substrates while in flight. Most species use one of these foraging strategies predominantly but may occasionally use the other two as well.
Dragonflies that typically hawk for food include the darners, river cruisers (Macromia), baskettails (Tetragoneuria), emeralds (Cordulia and Somatochlora), spiketails, gliders (Pantala), and saddlebags (Tramea), among others. They are strong fliers of medium to large size. In genera such as the gliders and saddlebags, the hind wings are relatively broad based, allowing for almost effortless, gliding flight in light winds. Hawking dragonflies frequently fly back and forth along a set path or series of paths over open fields and meadows or along creeks, rivers, and even roads. They are on the wing for extended periods of time. Darners, gliders, saddlebags, and others that hawk may form feeding swarms of dozens to hundreds of individuals, most often at dusk in late summer and fall. When they eventually perch, they do so to rest, digest a meal, or avoid unfavorable weather conditions. In general, they are somewhat cryptically colored (dull earth tones predominate), tend to perch high or in the shade of dense vegetation rather than on low, exposed perches, and typically hang from a perch, their bodies oriented vertically.
Many species in the skimmer subfamily (Libellulinae) and many damselflies are salliers. From a perch on the ground, vegetation, fence post, or other surface, they alertly scan the sky for potential prey. Typically they sit in exposed situations that provide a wide field of view. When they spy a meal, they dash out to capture it and return to a perch, usually the one they just left, to finish eating. When perched and actively foraging, they tend to adopt a flight-ready position, the body oriented horizontally.
The third feeding technique used, especially by damselflies, is hover-gleaning, which involves flitting from spot to spot, picking food items off vegetation in rapid bursts, followed by a brief period of perching to chew up the prey. American bluets (Enallagma) are often seen feeding in this way. Other damselflies, such as the spreadwings and broad-winged damsels, sally out from a perch to fly catch or hover-glean a single prey item at a time, then perch again to finish eating.
Dragonflies typically avoid aerial predators-birds, bats, and insects such as robber flies, wasps, and even other dragonflies-by agile aerial maneuvering, as anyone who tries to net them can attest. Disturbed damselflies frequently dodge into nearby vegetation. If flight is a less viable option (e.g., at cold temperatures), perched damselflies sidle around perches such as grass stems or small branches, using the perch as a screen much as woodpeckers use tree trunks.
The patterns on the bodies or wings of some species may serve as cryptic coloration against certain backgrounds, and this may in part influence perch selection. The bright colors of some species, especially the blues of darners and many damselflies, fade to gray at cooler temperatures, when mobility is reduced. Dragonflies also seem to magically disappear from conspicuous perches when the sun goes behind a cloud, perhaps to avoid detection by predators when their activity levels drop.
Reproduction is the major goal of an adult dragonfly's existence. After a few days or weeks of prereproductive life, during which it must feed, grow, and mature, it begins a programmed series of activities focused on reproduction.
The first step is finding a mate. In most cases, mates are sought near or at the body of water in which eggs are to be laid. In a few cases, mates are sought away from water and then escorted there. Males typically arrive at rendezvous sites before females. Peak mating hours vary among species but are often in the late morning or early afternoon. Males commonly interact aggressively with other males in order to establish territories or otherwise secure advantageous positioning for attracting or finding mates. Males of some species seek females from a perch, whereas others, such as darners, patrol in search of mates.
Courtship is rare in dragonflies, and males usually quickly pounce on females that arrive at rendezvous sites or are otherwise encountered during mate searches, even knocking them to the ground in some cases. They then quickly proceed to the next step, which is formation of tandem linkage.
Tandem linkage is the physical link of the male's abdominal appendages to the rear of the head or the thorax of the female. This linkage often provides a close fit of species-specific body parts, which may inhibit interspecific mating attempts (and coincidentally makes these body parts useful to humans attempting to identify individuals as to species).
Next is copulation, which also tends to follow quickly. Dragonflies are unique among insects in that the secondary genitalia of males are housed in the undersurface of the second abdominal segment. Usually after linking with a female, but sometimes before, a male transfers sperm from near the tip of the abdomen to a storage area in his secondary genitalia. The female subsequently bends her abdomen forward to align her reproductive structures under the eighth abdominal segment with the male's secondary genitalia. This position, which involves two points of linkage, is called the wheel (fig. 3).
The elaborate complex of secondary genitalia in males not only stores and transfers sperm but is designed to remove any sperm placed by other males in prior mating attempts. Indeed, much of the time spent by a pair in the wheel (a few minutes to hours in some species) is taken up by sperm removal, followed by a relatively brief period of sperm transfer. Because the secondary genitalia also require a good fit, they, too, are useful for distinguishing a number of look-alike species.
After sperm transfer, the next step is oviposition. The female of some species (damselflies, darners, petaltails, and spiketails) uses her ovipositor, or vulvar lamina, to insert eggs into a substrate-usually some sort of vegetation. Other species use a variety of techniques, discussed in the species accounts, to drop or deposit their eggs in water, on vegetation, or on the ground.
The male may remain in tandem with the female during oviposition, apparently protecting his investment in the eggs being laid. In other species, the male hovers near the ovipositing female and chases off intruders (fig. 4), especially other males of the same species. The female of some species typically oviposits while alone. There is considerable variation within species in these modes of oviposition, however, and some species exhibit more than one type, depending on circumstances such as population density and habitat structure.
Dragonflies use a variety of behaviors in order to maintain an appropriate body temperature. Many of these movements and postures are designed to take advantage of solar radiation.
The most obvious of these is basking-perching on vegetation, fence wires, the ground, rocks, and other sites fully exposed to the sun, much as lizards do. The wings may be held down toward the sides to trap warm air close to the thorax. Many species that live in cooler climes, such as whitefaces (Leucorrhinia) and emeralds, have mostly blackish bodies, presumably to enhance absorption of solar radiation.
Some of the larger, hawking species, such as darners, can warm up by rapidly vibrating the large flight muscles in their thorax, either while perched or by flying.
Because dragonflies are most active in warm, sunny weather, they also have to worry about overheating. Simply seeking shade and reducing activity for a time can accomplish this. Some exposed perchers adopt a very distinctive position called the obelisk, in which the abdomen is pointed directly at the sun (nearly straight up at midday) to minimize the body surface area exposed to direct rays (fig. 5). The tip of the abdomen can also be pointed down (away from the sun) to achieve a similar effect; this posture is often adopted by saddlebags in flight, the dark patches on their hind wings shading the drooped abdomen.
As discussed in the section "Antipredator Behavior," the blue colors of many darners and damselflies and the red colors of some species such as meadowhawks (Sympetrum) are subject to reversible, temperature-induced changes, becoming bright at higher temperatures and dull at lower ones. The brighter color produced by higher ambient temperatures is also more reflective (absorbing less light), thus helping to reduce body temperature. Conversely, individuals at cooler temperatures increase their absorption of solar radiation via darker body color.
After emergence from the final larval stage, virtually all odonates disperse. For many, this involves flying a distance from a few feet to a few miles away. Such short-range dispersal probably serves a number of functions, including (1) occupation of good foraging areas, (2) avoidance of harassment by breeding adults, and (3) potential discovery of new breeding sites. Once they become sexually mature, adults return to breeding sites, from which they may commute between feeding and roosting sites.
A few species are capable of long-distance movements, although the exact nature and extent of these migrations is poorly known. Dragonflies believed to migrate in western North America are the Common Green Darner (Anax junius), Variegated Meadowhawk (Sympetrum corruptum), gliders, and saddlebags. Observations suggest that, in late winter and early spring, these species begin to emerge in large numbers in Mexico and the southern border states (including the warmer areas of California) and move north into the northern United States and southern Canada. They breed in summer and then die. A late summer and fall emergence resulting from this breeding activity typically produces large numbers of offspring that migrate back south to breed in fall and early winter, and their offspring in turn emerge in spring to repeat the cycle.
Life Cycles and Larvae of Dragonflies
Dragonflies are amphibians in the same general sense as frogs, toads, and many salamanders. The familiar, winged adults that are the primary focus of this guide are the final, reproductive stage in the odonate life cycle. But, like amphibians, they are preceded by an aquatic larval stage that, from hatching of the egg to emergence of the adult form, involves much of each dragonfly's total life span, growth, and development (fig. 6).
Although much less conspicuous than and markedly different in appearance from adults, odonate larvae (sometimes referred to as nymphs or naiads) are unique and fascinating in their way. Overall, larvae coloration is typically drab and designed to camouflage. Their eyes are smaller and their antennae are frequently more prominent than those of adults. Unlike adults, they use their legs for getting about, not for prey capture and handling. The abdomen is relatively short and sometimes armed with spines or knobs along the top and sides. In later larval stages, pads housing the developing wings lie on top of the front of the abdomen.
Like adults, larvae are high-level predators, feeding on a wide range of aquatic invertebrates, including other odonates. Large, active larvae are capable of capturing and subduing small fish and tadpoles. Their most distinctive feature, found only in the Odonata, is a double-hinged labium, or lower jaw. The labium consists of the postmentum, folded back under the front of the body; the prementum, hinged to the postmentum and, at rest, folded forward to cover it; and labial palps hinged to the front of the prementum and often covering the lower face. When potential prey draw within reach of this potent weapon, it is thrust forward at high speed. The movable palps at its tip, armed with hooks, teeth, and spiny hairs, capture and hold the prey. The labium is instantly retracted after capture, drawing the prey back to the chewing mouthparts. The structure of the labium varies among odonate families and is often useful for identifying larvae (fig. 7).
Other unique features of odonate larvae are the gills they use to extract oxygen from the water in which they live. In damselflies, this is accomplished primarily by three leaflike gills that extend from the tip of the abdomen. Typical dragonflies have internal rectal gills over which they are constantly pumping water. Both types of gill systems also aid in locomotion, although in different ways. Some zygopterans can use their external gills as a sort of tail fin, swished from side to side to help them swim along. Anisopterans can rapidly expel water out the rectum to jet forward when a quick getaway is called for.
The basic larval body plan is modified in different species that live in different habitats and have different lifestyles. For example, dragonfly larvae that clamber about in aquatic vegetation and actively stalk prey, such as the larvae of large darners, have smooth, streamlined bodies and large eyes facing to the side. Species whose larvae sprawl in bottom sediments, such as the Pacific Spiketail (Cordulegaster dorsalis), have hairy bodies to which camouflaging detritus can adhere and eyes raised above the medium in which their bodies are mostly hidden. Shallow burrowers, such as the clubtails, have somewhat flattened, hairy bodies and thickened or platelike antennae that rest at the surface to detect prey. The elongate tip of the abdomen is also raised up above the surface of the mud to allow respiration through the rectal gills. Sprawlers and burrowers are ambush predators that sit and wait for prey to wander into range.
Most of California's odonates have a single generation per year. Adults emerge, mature, and lay eggs in the warmer months, primarily April through October. Eggs hatch within a few days or weeks, and the larvae grow through a series of about 10 to 15 molts. The stages between molts are called instars. Larvae usually overwinter in a relatively late stage of development. In the spreadwings and many meadowhawks, which typically breed in temporary habitats late in the season, it is the eggs that usually overwinter, hatching in spring. Adults emerge again the following year to repeat the cycle.
There are exceptions, however. Some smaller damselflies, such as forktails (Ischnura) and bluets, have long flight seasons and may have two or three broods per year, the last brood of the season overwintering as larvae. On the other hand, some dragonflies, often those that live in streams or rivers, at high elevations, or in other more demanding habitats, may live as larvae for 2 to 4 years before emerging. Some darners, clubtails, spiketails, and emeralds are among these relatively long-lived species. No California odonate is known with certainty to survive winter as an adult and breed the following spring, although midwinter sightings of the Variegated Meadowhawk suggest that this species might be capable of doing so, at least in some years.
Excerpted from DRAGONFLIES AND DAMSELFLIES OF CALIFORNIA by TIM MANOLIS Excerpted by permission.
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|Broad-winged Damsels (Family Calopterygidae)||39|
|Spreadwings (Family Lestidae)||41|
|Pond Damsels (Family Coenagrionidae)||50|
|Petaltails (Family Petaluridae)||90|
|Darners (Family Aeshnidae)||92|
|Clubtails (Family Gomphidae)||106|
|Spiketails (Family Cordulegastridae)||121|
|Skimmers, Emeralds and Baskettails, and Cruisers (Family Libellulidae)||123|
|Checklist of California Dragonflies and Damselflies||181|
|Species of Hypothetical Occurrence||185|