Insect identification > Thysanoptera

Thysanoptera


The Thysanoptera - sometimes called Physapoda - are very small insects, peculiar in many ways. The common name for members of this group is thrips, unchanged in spelling whether one or many are referred to.

As a whole these insects appear to have some affinities with the hemipteroid groups (Anoplura, Hemiptera and Homoptera), yet to be considered, but are generally looked upon as forming an order by themselves, though in some regards they seem to have certain relations to the Corrodentia and Mallophaga.

It is not improbable that they form a group originating not far from the common trunk of all the above-named orders.

Thrips vary from one-fiftieth to one-third of an inch or more in length. Their mouth-parts form in part a short, stout cone attached far back on the underside of the head, composed of the labrum, a portion of the maxillae, and the labium. Within this cone are three bristles consisting of the lobes of the maxillae and one mandible, the other not being developed.

The animals are sucking insects. Four wings are usually present, rather long and narrow, with few veins, and fringed behind and generally in front also, with slender hairs, longer than the breadth of the wing itself. When at rest the wings lie flat on the top of the abdomen.

In some cases they are greatly reduced in size or may even be wanting entirely. The tarsi are composed either of one or two segments, usually the latter; at the tip is a bladder-like portion which can be drawn into the segment or pushed out. The abdomen consists of 10 segments, the last either conical or tubular in form.

Summarizing these facts, the adult Thysanoptera may be described as:
Small insects with greatly modified mouth-parts forming a cone attached to the back part of the head beneath and used for sucking. Wings four, generally present, long, narrow, with few veins, and fringed behind (usually in front also) with long hairs. Tarsi of one or two segments, the tip with a bladder-like swelling capable of being drawn into the tarsus. Abdomen of 10 segments, the last either conical or tubular. Metamorphosis incomplete but approaching completeness.

Thrips feed on plant juices, puncturing the tissues and extracting the sap, leaving white marks or streaks where the cells without their juices have dried. They attack stems, leaves and blossoms, in the last case often blighting them and preventing the setting of fruit. On leaves of plants the under surface appears in most cases to be the preferred place of attack and the insects do not move about much. With grasses and cereals the stems as well as the leaves suffer, thus checking the growth of the top, and in some cases the kernels of growing grain are also fed upon. Some species live under loose bark and a few have been reported as feeding upon other insects. In many cases the injury caused by these insects is very serious. Over sixteen hundred species of thrips are now known.

In one section (suborder Terebrantia) the female has an ovipositor with which she saws slits in the epidermis of plants, placing an egg in each slit. In the other section (suborder Tubulifera) there is no ovipositor and the eggs are laid upon the surface of the food material.

The nymphs considerably resemble the adult. After from two to four molts they leave their food to find some more protected place and there molt again, at which time wing stubs appear and other changes can be seen. Another molt and now the insect becomes quiet unless disturbed, not feeding, and marked changes become evident, bringing it more nearly like the adult. The completion of these changes is followed by a molt which produces the adult itself. This is more than a typical incomplete metamorphosis, yet not entirely comparable with a complete one. It may be regarded therefore as intermediate between the two.

In some cases parthenogenesis, i.e., the production of the next generation by unfertilized females, occurs. This is perhaps to some extent determined by weather conditions in this group.

Parthenogenesis is frequently present here and there among insects and will be considered more fully elsewhere. Driving rains are very destructive to all kinds of thrips. Ladybeetles and other insects of several species feed freely upon them.