Order VIII. Thysanoptera.
Small Insects, with a palpigerous mouth placed on the under side of the head and apposed to the sternum so as to be concealed. With four slender wings, fringed with long hairs on one or both margins, or with rudiments of wings, or entirely apterous. Tarsi of one or two joints, terminated by a vesicular structure. The young resemble the adult in general form, but there is a pupal stadium in which the Insect is quiescent and takes no food.
The tiny Insects called Thrips are extremely abundant and may often be found in profusion in flowers. Their size is only from 1⁄50 to ⅓ of an inch in length; those of the latter magnitude are in fact giant species, and so far as we know at present are found only in Australia (Fig. 253). As regards the extent of the Order it would appear that Thysanoptera are insignificant, as less than 150 species are known. Thrips have been, however, very much neglected by entomologists, so it will not be a matter for surprise if there should prove to be several thousand species. These Insects present several points of interest; their mouth-organs are unique in structure; besides this, they exhibit so many points of dissimilarity from other Insects that it is impossible to treat them as subdivisions of any other Order. They have, however, been considered by some to be aberrant Pseudoneuroptera (cf. Vol. V.), while others have associated them with Hemiptera. Both Brauer and Packard have treated Thysanoptera as a separate Order, and there can be no doubt that this is correct. Thysanoptera have recently been monographed by Uzel in a work that is, unfortunately for most of us, in the Bohemian language.[[456]]
Fig. 253—Idolothrips spectrum. Australia.
The antennae are never very long, and are 6 to 9-jointed. The head varies much, being sometimes elongate and tubular, but sometimes short; it has, however, always the peculiarity that the antennae are placed quite on its front part, and that the mouth appears to be absent, owing to its parts being thrust against the under side of the thorax and concealed. Their most remarkable peculiarity is that some of them are asymmetrical: Uzel looks on the peculiar structure, the "Mundstachel," m, m (Fig. 254) found on the left side of the body, as probably an enormous development of the epipharynx. Previous to the appearance of Uzel's work, Garman had, however, correctly described the structure of the mouth;[[457]] he puts a different interpretation on the parts; he points out that the mandibles (j), so-called by Uzel, are attached to the maxillae, and he considers that they are really jointed, and that they are lobes thereof; while the Mundstachel or piercer is, he considers, the left mandible; the corresponding structure of the other side being nearly entirely absent. He points out that the labrum and endocranium are also asymmetrical. We think Garman's view a reasonable one, and may remark that dissimilarity of the mandibles of the two sides is usual in Insects, and that the mandibles may be hollow for sucking, as is shown by the larvae of Hemerobiides. There are usually three ocelli, but they are absent in the entirely apterous forms.
Fig. 254—Face (with base of the antennae) of Aeolothrips fasciata. (After Uzel.) a, Labrum; b, maxilla with its palp (c); bl, terminal part of vertex near attachment of month-parts; d, membrane between maxilla and mentum; e, mentum ending in a point near f; g, membrane of attachment of the labial palp h; i, ligula; j, j the bristle-like mandibles; k, the thicker base of mandible; l, chitinous lever; m, mouth-spine, with its thick basal part n, and o, its connection with the forehead, r, r; p, foramen of muscle; s and t, points of infolding of vertex; u, a prolongation of the gena.
The wings appear to spring from the dorsal surface of the body, not from the sides; the anterior pair is always quite separated from the posterior; the wings are always slender, sometimes very slender; in other respects they exhibit considerable variety; sometimes the front pair are different in colour and consistence from the other pair. The abdomen has ten segments, the last of which is often tubular in form. The peculiar vesicular structures by which the feet are terminated are, during movement, alternately distended and emptied, and have two hooks or claws on the sides. The stigmata are extremely peculiar, there being four pairs, the first being the mesothoracic, 2nd metathoracic, 3rd on the second abdominal segment, 4th on the eighth abdominal segment.[[458]] There are four Malpighian tubes, and two or three pairs of salivary glands. The dorsal vessel is said to be a short sack placed in the 7th and 8th abdominal segments. The abdominal ganglia of the ventral chain are concentrated in a single mass, placed in, or close to, the thorax; the thorax has two other approximated ganglia, as well as an anterior one that appears to be the infra-oesophageal.
The metamorphosis is also peculiar; the larva does not differ greatly in appearance from the adult, and has similar mouth-organs and food-habits. The wings are developed outside the body at the sides, and appear first, according to Heeger, after the third moult. The nymph-condition is like that of a pupa inasmuch as no nourishment is taken, and the parts of the body are enclosed in a skin: in some species there is power of movement to a slight degree, but other species are quite motionless. In some cases the body is entirely bright red, though subsequently there is no trace of this colour. Jordan distinguishes two nymphal periods, the first of which he calls the pronymphal; in it the Insect appears to be in a condition intermediate between that of the larva and that of the true nymph; the old cuticle being retained, though the hypodermis is detached from it and forms a fresh cuticle beneath it. This condition, as Jordan remarks, seems parallel to that of the male Coccid, and approaches closely to complete metamorphosis; indeed the only characters by which the two can be distinguished appear to be (1) that the young has not a special form; (2) that the wings are developed outside the body.