Chrysopidae, with its 1200 recognized species, is one of the two large families of Neuroptera, second only to the Myrmeleontidae. The larvae of many chrysopid species feed on insect and mite pests of agricultural crops or horticultural plantings and because of their value in biological control, chrysopids are the most frequently studied of the Neuroptera. Adults are medium-sized to large, delicate insects with four subequal wings (forewing length 6 – 35 mm) and relatively long, filiform antennae.
In most species the adults
are green with large golden eyes, but some species have black, brown, or
reddish adults). Larvae vary in shape and habits; some are voracious, active,
and more-or-less generalist predators, with sleek, fusiform bodies (thus the name“
aphislions”). Others are slow-moving, cryptic, trash-carrying predators with
bulbous bodies, elaborate tubercles, and long, hooked setae; they are usually
associated with specific types of ant-tended prey.
Still others live in ant
nests where they feed on the inhabitants; they have rotund, bulbous bodies,
greatly shortened appendages, and a dense covering of stiff, hooked setae that
hold protective trash on the body. Currently, the Chrysopidae comprises three
subfamilies (Nothochrysinae, Apochrysinae, and Chrysopinae); all three are only
weakly supported by molecular data and only the first is well defined on the
basis of adult and larval characters. Systematic and comparative biological
studies are needed to clarify the taxonomy and phylogenetic relationships of
the chrysopidtaxa and also to facilitate their use in biological control. Given
the wide range of morphological and behavioral variation among chrysopid
larvae, it is clear that inclusion of all life stages is crucial for advancing
the systematics of the family.
Recent studies of previously unknown larvae have
led to changes in the tribal assignments and the recognition of new Neotropical
genera. However, except for the European and Japanese faunae where larvae of
approximately 80% of the species are described, the world’s chrysopid larvae are
poorly known. The Nothochrysinae includes only nine extant genera; it is
believed to be the basal chrysopid lineage, but molecular data have not confi
rmed this opinion.
Defining characteristics occur in the adult and larval
stages; however, larvae from very few genera are known. Apochrysinae may be
monophyletic, but more supporting data are needed. The larvae of one
apochrysine species have been described, but distinguishing subfamilial traits
were not apparent.
The subfamily contains the largest and visually most spectacular
green lacewings; its 13 genera are based largely on somewhat variable
characters in wing venation. Biological studies are needed. The large subfamily
Chrysopinae encompasses over 97% of the known chrysopid species; it includes 60 genera distributed among four tribes, at least two of which are poorly defined
and probably not monophyletic. The tribe Chrysopini is the largest and least
well known; it contains almost all of the lacewings of economic importance.
As
a group, the Chrysopidae is cosmopolitan; similarly, all of the subfamilies are
widely distributed. Nevertheless, many of the genera have limited geographic
distributions. For example, among the Apochrysinae, two genera occur only in
Africa, four in the Neotropics, six in the Oriental region or Australia, and
one in the eastern Palearctic. Most genera of Nothochrysinae are endemic to
small geographic ranges; many species are known solely from a very few
specimens.
The genera within Chrysopinae range from cosmopolitan to narrowly
endemic. Typically, chrysopid eggs are laid at the end of long stalks, either
singly, in groups, or in clusters with the stalks loosely or tightly
intertwined. The egg stalks can be naked or they may bear oily droplets; the
droplets contain nutrients or defensive substances that protect the egg or the
newly hatched larva from natural enemies.
Larvae of some chrysopid species have
fairly large prey ranges; they may feed on homopterans, lepidopteran eggs or
larvae, and a variety of soft-bodied arthropods. But, contrary to popular lore,
some species have evolved a very strong association with a particular type of
prey. In Chrysopa , prey specialization can be restricted to a single species
of prey and is based on a suite of intrinsic and extrinsic factors, including
maternal oviposition behavior, egg size, larval morphology and behavior,
phenotypic plasticity in life-history traits, responses to natural enemies that
are associated with specific prey, and phenology.
Studies indicate that prey
association, such as that in Chrysopa , and also habitat association, as shown
in Chrysoperla , can evolve in a manner that is very similar to the evolution
of host specifi city in phytophagous insects; there is good evidence that both
can be involved in speciation. Adults of most chrysopid genera feed on honeydew
and pollen; in these lacewings, the dorsal crop diverticulum has numerous
tracheae and is filled with symbiotic yeast.
These symbiotes provide essential
nutrients that are deficient in the diet. Adults in a few genera are
predacious. In some species, adults emit foul-smelling defensive odors when
they are disturbed. Some chrysopid species are multivoltine, others are
univoltine; most enter diapause and undergo dormancy (hibernation, aestivation)
during unfavorable (e.g., cold, hot, or dry) seasons.
The diapausing stage
(free-living larva, prepupa, or adult) varies among lacewings and is a
characteristic of the genus. Some chrysopids that diapause as adults undergo
seasonal color changes that appear to refl ect the background color of their
habitat during the unfavorable season. Although lacewings are not considered
especially strong flyers, they can move considerable distances with the wind.
In species that diapauses as adults, there is a seasonal pattern to movement
between habitats. Photoperiod often provides very important cues for timing
lacewing dormancy and seasonal movement; temperature, moisture, and food can
also be signifi cant factors. The genetic basis for lacewing responses to
seasonal cues has been demonstrated; some exhibit geographical variability and
epistasis. Chrysopine lacewings have two modes of hearing.
The “ear” (tympanal
organ) is at the base of the radial vein in each forewing. It is the smallest
tympanal organ known, and it receives the ultrasonic signals of insectivorous
bats. Ultrasonic signals at low rates (1–50 pulses per second) cause the
lacewing to cease flight and to fall. As the bat continues to approach, its
signal increases in frequency; the high-frequency signal causes the lacewing to
flip its wings open quickly and fl y, thus aiding its escape.
The second type
of hearing, the perception of low-frequency, substrate-borne sounds that are
emitted during courtship, is accomplished through scolopidial organs in the
legs. Such sounds are an integral part of courtship in Chrysoperla species;
variation in the production and perception of these sounds may have a role in
speciation.
The endemic complex of green lacewings on the Hawaiian Islands,
belonging to the genus Anomalochrysa , has evolved several unique
characteristics and exhibits an extraordinary range of variation in morphology
and behavior. For example, unlike any other known chrysopids, Anomalochrysa
females lay sessile (unstalked) eggs, either singly or in batches. Larval body
shapes range from fusiform with greatly reduced lateral tubercles and few,
short setae, to flattened with well developed lateral tubercles and numerous,
long, robust setae.
In continental lineages, such broad variation is found only
among genera. In some species, adults or larvae are very bright and colorful;
in others they are dull or resemble bird feces. Males and females may produce
conspicuously loud clicking sounds during courtship and mating; how these
sounds are produced and perceived is unknown.
Some species in the genus
Chrysoperla are mass-reared for release in the biological control of
agricultural and horticultural pests. Among those in North America are
Chrysoperla carnea and Chrysoperla rufilabris. These species possess characteristics
that are advantageous for mass-rearing. For example, adults do not require
prey, but will reproduce when fed artificial diets; they can be stored for long
periods without significant loss of reproductive potential; and larvae can
develop when fed artifi cial or factitious prey.
Larvae of Ceraeochrysa
species, which are trash-carriers, share many of the above traits that subserve
mass production. They have the added advantage of being camouflaged and thus
protected from their own natural enemies, for example, ants. The role of
lacewings in pest management, whether naturally occurring or augmentative, is
far from fully exploited.
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