Homoptera

(invertebrate zoology) An order of the class Insecta including a large number of sucking insects of diverse forms.

An order of the class Insecta related to the or-der Hemiptera. This is a major group of sucking insects, with more than 30,000 species, even though in Asia and Africa the number of undiscovered species probably still exceeds the discovered ones. Common examples are the cicadas, aphids, and leafhoppers. The group is difficult to characterize because of the large number and diverse forms of the species it contains.

The head of these insects is hypognathous or opisthog-nathous, the beak appearing to arise from the ventral posterior margin of the head or even from the prosternum. The gula is membranous or absent. As in the Heteroptera, the beak consists of two pairs of stylets, formed by the maxillae and the mandibles, ensheathed in the labium. The maxillary stylets fit together to form a double tube, one channel serving for the passage of food and the other for saliva.

Most winged species have four wings, but male scale insects have only two. In most forms both pairs of wings are membranous and transparent, but in some, the forewings are somewhat thickened and may then be either coriaceous and translucent, or opaque, and with or without an apical membranous area. When the insects are at rest, the forewings are usually held, rooflike, over the dorsum, with the apex of one of them slightly overlapping the apex of its complement ( Fig. 1).

Fig. 1. A cicadellid, dorsal view.

In most species metamorphosis is gradual, but in a few it is practically holometabolous. Adults and nymphs of most species are terrestrial, but a few species are subterranean in all stages and others are subterranean only in immature stages. A number of species are vectors of virus diseases of plants.

Series Coleorrhyncha

This group is characterized by the origin of the beak, formed at the anteroventral extremity of the face, and by the fact that the propleura form a sheath for the base of the beak. The hindwings are absent, and the forewings are held flat over the abdomen in repose. The flight function has been lost. They occur in Tasmania, New Zealand, and South America.

Series Auchenorrhyncha

This series and the Sternorrhyncha are the major groups of the Homoptera. In the Auchenorrhyncha the beak arises at the anteroventral extremity of the face and is not sheathed by the propleura. The Auchenorrhyncha includes a large number of species. A number of classifications have been proposed; the classification adopted here is a common one. It divides the series into the superfamily Fulgoroidea and the families Cicadidae, Cercopidae, Membracidae, and Cicadellidae. These families are not subordinate to the superfamily Fulgoroidea.

Superfamily Fulgoroidea includes insects commonly known as lantern flies. This group is subdivided into 20 families and includes many species which are important because of the economic damage they do while feeding or because they carry virus diseases of plants.

Included in the family Cicadidae are the cicadas, harvest flies, and jar flies. The insects in this family are probably better known to the layperson than any other homopterous family because of their large size and the strident songs of the males. Adults are usually found on trees or shrubs. At least in some species, the songs of the males assemble local populations.

Spittle bugs and froghoppers are common examples of the family Cercopidae. Insects in this group attract attention in the immature stages, during which they surround themselves with a mass of froth or spittle. One species, the meadow spittle bug, is very common in the temperate portion of the Northern Hemisphere, and its masses of spittle are familiar sights. Some species of Australia and the East Indies live within a calcium carbonate tube attached to stems or leaves.

The treehoppers (family Membracidae) are small to medium in size and seldom attract attention ( Fig. 2a). Most of them feed on woody plants and are found on the stems in sunny locations. Frequently a number of specimens are arranged in a vertical row on the stem, all with their heads downward.

Membracids on stems. (a) Adult. (b) Nymphs. (Courtesy of C. H. Hanson)

The greatest number of species occurs in the warmer regions of the world. In many species the enlarged pronotum has adornments, excrescences, and processes which are astonishing in appearance, some of them nearly as large as the remainder of the insect. The nymphs ( Fig. 2b) leave the trees and feed on herbaceous plants, often occurring in great numbers in pastures. The adults return to woody plants before oviposition.

The leafhoppers are included in the large family Cicadellidae. These usually small insects are known to many people by sight but not by name, because of their common occurrence in great numbers at night near lights ( Fig. 1). Probably the greatest number of species occurs in tropical areas, but the majority of these have not been described. Leafhoppers occasionally bite humans, but apparently they have never been seen taking blood. Several species have been found to be vectors of virus diseases of plants.

Series Sternorrhyncha

In this group of families, the beak appears to arise either between the fore coxae or behind them. The antennae are usually long, filamentous, and have no well-differentiated terminal setae. Wingless forms are common.

The Sternorrhyncha includes a large number of species, many of them of great economic importance. The winged forms are not strong flyers, but they are so light that they may be borne considerable distances by air currents.

The family Psyllidae are known as jumping plant lice. Its representatives resemble cicadas in appearance, but are much smaller. About 1000 species are known. Some psyllids produce severe damage to their food plants. This damage may result from the mere feeding by tremendous numbers of individuals, from the resulting yellowing or rolling of leaves, or from galls produced on the leaves. Indirect damage may result from the growth of fungi on leaves which have become coated with the sugary excrement, the honeydew, of the psyllids.

The whiteflies (family Aleyrodidae) are 0.28 in. (7 mm) or less in length and usually lightly covered with a white, powdery, waxy material which has led to their common name. Whiteflies directly damage plants by their feeding. Indirectly, damage results from spotting at the feeding site, growth of fungus on the excreted honeydew, or from increased susceptibility of leaves to winter damage.

Members of the large superfamily Aphidoidea have four wings, or none. The wings are usually membranous or whitish and opaque. The forewings are much larger than the hindwings. Honeydew is usually produced. This superfamily includes the families Aphididae and Chermidae. The true aphids (Aphididae) are very attractive to ants, and colonies of very small aphid species which otherwise might escape notice can often be located by observing the attending ants. In a few species, this relationship has progressed to the point where ants are necessary for the survival of the aphid species, as in the corn root aphid. Many aphid species are important because of damage done in feeding.

Chermidae is a small family of minute insects, the adelgids and phylloxeriids. Both winged and wingless forms occur. The grape phylloxera has been a severe pest of cultivated grapes and once threatened the entire wine industry of France.

The scale insects and mealy bugs (superfamily Coccoidea) are usually small. More than 4000 species have been described. In the males, the hindwings are reduced to clublike halteres. The wings are usually held flat over the back in repose, and the venation is greatly reduced. The females are wingless.

Scale insects injure the host plant by their feeding on leaves, stems, or roots, and a number of species are very important economically. Most species produce large quantities of honeydew. A few species have been shown to be vectors of virus diseases of cucumbers, tobacco, and cacao. See also Entomology, economic; Insecta.