1911 Encyclopædia Britannica/Thysanoptera

by George Herbert Carpenter

(The illustrations in this article are all after H. Uzel, Monographie der Ordnung Thysanoptera, and are used with his permission.)

Fig. 1.—Limothrips denticornis, female, Europe.

THYSANOPTERA (θύσανος, a fringe, and πτερόν, a wing), a term used in zoological classification for a small order of the class Hexapoda (q.v.). The minute insects included in it, which haunt blossoms and leaves, are fairly well known to gardeners by the name Thrips, a generic term used by Linnaeus for the four species of the group which he had examined and relegated to the order Hemiptera. The term Thysanoptera was first used by the Irish entomologist A. H. Haliday (1836), who made a careful study of the British species and recognized that their structural peculiarities required ordinal separation. H. Burmeister in 1838 also considered that these insects should form a distinct order, for which he proposed the name Physopoda, with reference to the bladder-like outgrowths (fig. 3) on the feet. Since then various authors have incorporated the Thysanoptera with one of the large orders, some, following Linnaeus, regarding them as Hemiptera, others grouping with them the Orthoptera or “Pseudoneuroptera.” But all recent students agree with Haliday and Burmeister in allowing the Thysanoptera to rank as a distinct order, showing affinities on the one hand with the Corrodentia (book-lice and biting lice) and on the other with the Hemiptera (cicads, bugs, &c.).

(After H. Uzel.)
Fig. 2.—Head of Aeolothrips fasciata, face view, showing eyes, bases of feelers and jaws.
a,	Clypeus.
b,	Membrane between clypeus and labrum.
c,	Labrum.
d,	Mandible.
e,	Unpaired piercer (? inner lobe of left maxilla).
f,	Maxilla.
g,	Its palp.
h,	Second maxillae, forming labium.
i,	Labial palp.

Characters.—The Thysanoptera, small insects with firmly chitinized cuticle, are recognized by the combination of imperfectly suctorial jaws—the mandibles acting as piercers and maxillae retaining their palps (see fig. 2)—with the presence of two pairs of excessively narrow wings (fig. 1), which are partly or completely surrounded by elongate delicate bristles forming a fringe. Other important structural features are mentioned below. In their life-history the Thysanoptera belong to the Exopterygote division of the Hexapoda (q.v.). The newly hatched insect closely resembles the parent, and the wing-rudiments appear externally on the second and third thoracic segments; but before the final moult the nymph remains quiescent, taking no food. Its condition thus recalls the pupal instar of the higher (Endopterygote) Hexapoda; and the Thysanoptera, though few in number, are seen to be of great interest to the student, exhibiting at once transition between the biting and the suctorial mouth, and the passage from “incomplete” to “complete” metamorphosis.

Structure.—The head is usually quadrangular in form with small but prominent compound eyes (fig. 2), whose facets are relatively large and convex; three ocelli may also be present on the vertex. The feelers are inserted close together (fig. 2) on the extreme front of the head; they exceed the head in length, but they are composed of only from six to nine segments, which are beset with prominent spines, some of the latter appearing to be organs of special sense. The mouth, with its jaws, forms a conical outgrowth which projects backwards, so that its apex lies beneath the prothorax. The labrum (fig. 2 c), which encloses the cone in front, is irregularly triangular in shape. Behind the mouth the two maxillae of the second pair are intimately associated to form the labium (fig. 2 h), whose appendicular nature is shown only by a median furrow and by short, cylindrical palps (fig. 2 i) with two or four segments. The maxillae of the first pair (fig. 2 f) enclose the mouth at either side. They are broad at the base, but taper towards the tip and carry palps (fig. 2 g) with two or three segments. Within the mouth lie a pair of slender piercers (fig. 2 d), while a single piercer (fig. 2 e) is situated asymmetrically on the left side. The nature of these structures has been much disputed. H. Uzel, with the majority of students, regards the paired organs as mandibles and the unpaired as an epipharynx. H. Garman and W. E. Hinds believe that the paired piercers are the inner lobes of the maxillae, and the unpaired piercer the left mandible, the right mandible being absent. C. Börner has stated that the unpaired piercer is attached directly to the base of the left maxilla. He therefore regards it as the inner lobe (lacinia) of that maxilla, comparing it with the remarkable “pick” of the maxilla of a book-louse (see Copeognatha in article Neuroptera). The paired piercers, connected by muscles with the base of the maxillae, but attached directly to the head skeleton, into which they can be withdrawn, are regarded by Börner as true mandibles.

(After H. Uzel.)

Fig. 3.—Foot of young Trichothrips (under surface).

A, With “bladder,” b, protruded; B, retracted, a, Claws.

Turning to the thorax we find that the first segment (prothorax) is distinct and free, with a wide dorsal sclerite. The mesothorax and metathorax are rather intimately fused together. Most remarkable in this order is the structure of the feet; there are never more than two tarsal segments, and the claws, usually so conspicuous in insect feet, are reduced (fig. 3 a) or absent. But the legs carry peculiar spines, and the terminal tarsal segment is cup-shaped at the end; from this hollow a delicate bladder (fig. 3 b) can be protruded, apparently by the force of the blood-pressure, and by means of this bladder—acting as a sucker—the insect obtains firm hold on any surface which the foot touches. The narrow, delicate, fringed wings have already been described; each wing may be surrounded by a nervure and traversed by two longitudinal nervures, or the nervuration may be altogether degenerated. A fair number of species are wingless (fig. 5), either in one or both sexes, and the occurrence of winged females with wingless males is noteworthy.

(After H. Uzel.)

Fig. 4.—Ovipositor (side view) of Physopus.

8, 9, 10, Abdominal segments.

a, Anterior; b, posterior process of ovipositor.

Ten segments are recognizable in the abdomen, which is elongated and tapers at the hinder end. In two of the three families of Thysanoptera the female has a conspicuous ovipositor (fig. 4) with serrate processes, projecting from the ventral surface of the abdomen between the eighth and ninth segments. The number of spiracles is greatly reduced; in the adult a pair is present on the mesothorax, sometimes also a pair on the metathorax, and there is always a pair on the first and another pair on the eighth abdominal segment. These spiracles, according to Hinds, are remarkable honeycomb-like structures, and perforations to the tracheal tubes have not been demonstrated. The internal structure of the Thysanoptera has been studied by K. Jordan. The possess a long, tubular gullet and a highly concentrated nervous system; in addition to the suboesophageal ganglion, there are two thoracic ganglia and a single abdominal nerve-mass which is situated far forward. In this condensation of the nervous system and in the presence of four Malpighian (excretory) tubes the Thysanoptera resemble the Hemiptera.

Development and Habits.—Many species of Thysanoptera are known to be habitually parthenogenetic. The eggs are laid on the food-plant, those females possessed of an ovipositor cutting through the epidermis and placing their eggs singly within the plant-tissues; a single female may take five or six weeks to deposit all her eggs. The young insect resembles its parent in most points, but the head is disproportionately large; the anterior abdominal spiracles are on the second segment instead of on the first, and the foot has only a single segment. At first the eyes consist of a few distinct facets on either side of the head; they increase in number as growth proceeds, and become aggregated to form the curious compound eye of the adult. From two to four moults occur, after which the “pronymph” stage is reached, which in the insect is moderately active and possesses wing-rudiments reaching to the second abdominal segment. After another moult the insect passes into the passive nymphal or “pupal” stage, during which it takes no food and rests in some safe hiding-place, such as the soil at the base of its food-plant or the hollow of a leaf-stalk. During this stage the cuticle draws away from the imaginal cuticle which is forming beneath, ultimately becoming separated as a thin transparent pellicle through which the form of the adult can be seen.

Thysanoptera are found on the leaves and in the blossoms of plants. According to Hinds they feed chiefly on the green tissues, which “are punctured by the piercing mouth-parts and the sap withdrawn by suction. The traces of their feeding are left in irregular streaks of dry, whitened cells.” It has been stated that when present in blossoms they feed on nectar, but it is more probable that there—as on the green parts—they suck sap. In any case, their presence in apple blossoms has been known to prevent the formation of fruit through injury to the essential organs of the flower, and some species do considerable damage to ears of corn. Some Thysanoptera habitually dwell on the under-surface of leaves, and others periodically migrate to roots. While the majority of the Thysanoptera are thus vegetarian in their diet, and are frequently injurious in farm and garden, some species, at least occasionally, adopt a predaceous habit, killing aphids and small mites (so-called “red-spiders”) and sucking their juices. There are even records of an Anaphothrips, when cut off from its normal vegetable food-supply, becoming cannibalistic and feeding on its own species. The usual variations in habit that characterize plant-feeding insects are exhibited by the Thysanoptera some species being found only on one particular food-plant, while others thrive indifferently on a large assortment. Some members of this order spend the winter in the adult state, others in the “larval” or “pupal” condition. They shelter in crevices of the bark of trees, in the dried stems of herbaceous plants, or among moss and fallen leaves on the ground. Hinds states that the hibernating individuals live for more than six months. During summer there may be eight or nine successive generations when conditions are favourable and food abundant.

Distribution and Fossil History.—The Thysanoptera are probably world-wide in their range, but they have hardly been studied outside Europe and North America. Fossil insects referable to the order have been found in Tertiary beds as old as the White River Oligocene of North America, and the Baltic amber, but nothing is known as to the previous history of the group.

Classification.—Only about 150 species of Thysanoptera are known; the European species with a few exotic forms have been described by Uzel, the North American by Hinds. These writers both follow the classification of Haliday, who divided the order into two groups or sub-orders.

(After H. Uzel.)

Fig. 5.—Megalothrips lativentris, female, Europe.

1. Terebrantia: In this division (figs. 1, 4) the abdomen is cylindrical, the female is provided with a ventral ovipositor and has the terminal abdominal segment conical; the corresponding segment in the male is usually bluntly rounded. The fore wings have at least a single longitudinal nervure—often two—reaching from base to tip of the wing. The maxillary palps have usually three, the labial either two or four segments. There are two families of Terebrantia: (a) the Aeolothripidae, whose feelers have nine segments; whose wings, relatively broad and rounded at the tip, have a few cross nervules, and whose ovipositor is curved backwards; and (b) the Thripidae, whose feelers have six to eight segments, whose narrow acuminate wings have no cross nervules, and whose ovipositor (fig. 4) is curved downwards. This latter family contains the great majority of the order.

2. Tubulifera: This division comprises but a single family—the Phloeothripidae; the species are not numerous, but some of them are of large size for Thysanoptera, as much as 8 mm. (one-third inch) in length. These insects have the abdomen flattened, with its terminal segment (see fig. 5) narrow and cylindrical. The palps, both maxillary and labial, have two segments. There is no ovipositor, and the wings are either without nervures or have only a single degraded longitudinal nervure which does not reach to the tip. While the Terebrantia are often rapid in their movements, the Tubulifera are slow and deliberate. According to Hinds, “they never run or spring.”

Bibliography.—The number of important writings on the Thysanoptera is not large. A. H. Haliday's papers in Entom. Mag., 1836—1837, vols. iii. and iv., are still valuable and contain nearly all that is known of the fifty British species. K. Jordan's anatomical studies (Zeits. wiss. Zool., 1888, vol. xlvii.) are valuable and the descriptions of the jaws by H. Garman (Amer. Nat., 1896, vol., xxx.) and C. Börner (Zool. Anz., 1904, vol. xxvii.) should also be consulted. Indispensable to the student are H. Uzel's Monographie der Ordnung Thysanoptera (Königgratz, 1895; in the Czech language, but with a German summary), and W. E. Hinds's Monograph of the North American species (Proc. U. S. Nat. Mus., 1903, vol. xxvi.).

(G. H. C.)