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THYSANOPTERA 249
Some species in crawling also curve the abdomen up over the back - a pose
assumed by the young as well as by the adults.
Thrips are to be found generally upon all types of vegetation and are perhaps
most abundant in flowers and on the leaves of the host plants, but they may
also occur on the fruits and twigs. Some species appear to be scavengers upon
dead vegetable matter, sap, and fungi while a considerable number are predacious
upon mites, aphids, other thrips, and various minute forms of animal
life and the eggs and young of larger insects. Certain Australian species belonging
to the genera Kladothrips, Choleothrips, Haplothrips, and Eothrips
produce true galls or pseudogalls on the leaves of trees in which the young are
reared (Moulton, 1927). Thrips injure plants by destroying the cells of the
living leaf and fruit tissues, by causing sterility, and also by disseminating
bacterial viruses and fungus diseases: Bailey (1935) has listed four species of
Thrips and three species of Frankliniella as carriers of plant diseases. Many
other genera are also suspected of being vectors of such diseases. Thrips are
thought to be beneficial by pollinating flowers. They often appear in great
swarms and not only destroy growing crops but cause considerable annoyance
to human beings by getting into the eyes, ears, nose, mouth, clothing, and
even by biting to some extent. Farm laborers are often quite severely bitten
during hot dry weather, apparently in an attempt on the part of the insects
to secure moisture rather than to suck blood (Bailey, 1936). Migrations of
adults of several species have been observed, and I have seen the tobacco or
onion thrips, Thrips tabaci Lind., fairlY fill the air over a bean field in California
on a warm late summer afternoon just before sunset.
Thrips are normally bisexual, the sexes being quite similar except that the
males are somewhat smaller and the females. in the TERE13RANTIA, have an
ovipositor. Both sexes or either may be wingless, brachypterous, or winged, and
in a few species all of these conditions occur.
Parthenogenesis is not uncommon, and in a number of species, notably the
greenhouse thrips, Heliothrips hremorrhoidalis Bouche, and the pear thrips,
Tamiothrips inconsequens (Uzel). males are exceedingly rare or unknown.
Members of the genera Actinothrips, Diceratothrips, Elaphrothrips, and Zeuglothrips
give birth to living young.
Reproduction by oviparity and ovoviviparity. The eggs vary in shape from
elongate-oval to reniform. They are laid at random or in groups on the exterior
or in cracks, crevices, galls, and under bark or debris by the TUBULIFERA
or they may be inserted within the tissues of the plants by means of the sharp
sickle-like ovipositor of the female by the TEREBRANTIA. The eggs are
small, some scarcely visible to the unaided eye and may be deposited in great
numbers. Those inserted in plant tissues may be exposed to view with the aid
of fine instruments and a good hand lens or a microscope. Their location is
evidenced by the puncture and swelling of the surrounding tissues.
The eggs are laid in the early spring and throughout the summer and hatch
in a few days or a week or more. The young literally push out through the
punctures made by the female in deposition or they escape freely from those