Psocoptera (Psocids, topiclice) (Insects)

Psocoptera (Corrodentia, Copeognatha) constitutes an order of neopterous, exopterygote insects commonly called pso-cids, barklice, or topiclice. Their closest relatives are the Phthiraptera or true lice. Psocids are small and soft-bodied and therefore have received little attention from collectors. Only recently, when several dozens of species have been found in stored products, and the tropical forests have proved to harbor a highly diverse fauna, has greater interest been focused on this group.
Psocid, Graphopsocus cruciatus.
FIGURE 1 Psocid, Graphopsocus cruciatus.


Adult psocids range from about 1 to 10 mm in body length. Most adults are fully winged with the forewings longer and more complexly veined than their hind wings (Fig. 1). The forewings at rest generally exceed the tip of the abdomen. Antennae are long and slender, in the larger forms tending to be longer than the body. The head is rounded with compound eyes often large and bulging but sometimes greatly reduced. The postclypeus (Fig. 2) is usually swollen to accommodate the well-developed cibarial pump muscles. Often the postclypeus bears pigmented chevron marks (Fig. 2) indicating the attachment points of these muscles. Mouthparts are of the chewing type with large, well-developed mandibles. The maxilla contains an elongate, slender lacinia. The hypopharynx includes structures unique to these insects that are important in maintaining water balance. In fully winged forms the prothorax is frequently reduced while the meso- and metanota are swollen to accommodate the flight muscles. Wing venation tends to be relatively simple. Legs tend to be slender, with the hind femur somewhat swollen in some forms, allowing short hops or initiation of flight by jumping. In the small topiclice of the genus Liposcelis ( Fig. 3 ), the greatly swollen hind femora are thought to facilitate the ability to change course rapidly and crawl backward. In many winged adult psocids, the hind coxae each bear a rasp and tympanum; the two structures together are called the coxal organ. This is thought to be a stridulatory organ, although as yet no sound has been detected from it.
Face of a psocid, Aaroniella badonneli, showing prominent postclypeus (PC) with chevron marks. Scale: 0.2

FIGURE 2 Face of a psocid, Aaroniella badonneli, showing prominent postclypeus (PC) with chevron marks. Scale: 0.2
Habitus of a topiclouse, Liposcelis corrodens, showing enlarged hind femur (F) and neotenic features: absence of wings and ocelli and reduced compound eyes. Scale: 0.5 mm.
FIGURE 3 Habitus of a topiclouse, Liposcelis corrodens, showing enlarged hind femur (F) and neotenic features: absence of wings and ocelli and reduced compound eyes. Scale: 0.5 mm.
The abdomen consists of 11 segments. The first 7 are usually membranous, although the terga of the first 2 may be sclerotized and fused together. The clunium, formed by fused segments 8- 10, is the bearer of external genitalic structures. Distal to the clunium are three semimembranous flaps guarding the anus—the dorsal epiproct and the lateral paraprocts. These may be modified in various ways and probably are involved with copulation.


Most psocids are oviparous. In various taxa, eggs may be laid singly or in groups and either bare or with a covering of encrusting material, webbing, or both. Eclosion (emergence of the pronymph from the egg) is facilitated by means of an oviruptor, a bladelike or sawlike structure on the head of the pronymph. By means of a slight rocking motion, imbibing of air, and hemolymphatic pressure on the head, the insect pushes its oviruptor against the thin egg cuticle, creating a slit through which it exits. Immediately it molts again, casting off the pronymphal exuviae (the cast cuticle), which remain partially caught in the exit slit of the egg.
Eggs of some species undergo a winter diapause and in the Northern Hemisphere this is brought about by the mother’s perception of decreasing day length in late summer.
Nymphs undergo five to six instars, requiring 4-6 weeks to reach maturity. Adults remain teneral for a day or two prior to engaging in sexual activity. Females of some sexual species produce a sex pheromone that, at a short distance (1-2 cm), brings about immediate, rapid search behavior in mature males—i nvolving them running about and vibrating their wings. This reaction may be elicited by a piece of substrate on which an “advertising” female has been standing or even by the tips of a pair of forceps that have just held such a piece of substrate. Behavior after contact is tremendously varied. Frequently a male “antennates” a female, that is, touches his antennae to hers and then crawls over her back and forces himself under her body from in front. The copulatory structures then grasp together, after which the male may stay under the female or move—180° to face opposite the female (suborder Trogiomorpha) or back on top of the female (family Lachesillidae). Copulation lasts only a few seconds in many groups of the suborder Psocomorpha, although in the genus Lachesilla it generally requires some 35 min. In the suborder Trogiomorpha, it may last up to 2 h during which individual sperm pass through the slender spermathecal duct, then clump together, and become enclosed in “spermatophores” in the spermathecal sac. This is probably the sequence of events in any form in which the spermathecal duct is slender and spermatophores are seen in the spermathecal sac.
Oviposition may start within a day or 2 of copulation. Under favorable conditions, oviposition may continue over a period of 2 months.


Although most adult psocids are fully winged, a variety of levels of wing reduction are seen throughout the order. Associated with wing reduction are reductions in several other structures: compound eyes, ocelli, paraproctal sensorium, and ctenidia (comb-based setae) of the basal hind tarsomere. The resulting adult appears nymph-like in these characters, and these changes can thus be regarded as neotenic. Several patterns of change are seen, which can be summarized as completely wingless, Liposcelis (Fig. 3); with very short scale-like or button-like wings, Trogium, Cerobassi; wing development variable in both sexes, Rhyopsocus, Lachesilla (some species); with sexual dimorphism in wing development and males wingless or nearly so, females variable in wing development, Embidopsocus, Psoquilla, Archipsocus; sexual dimorphism and males fully winged, females variable in wing development, Peripsocus (some species), Valenzuela (some species); and sexual dimorphism with males fully winged, females wingless or nearly so, Camelopsocus, Mesopsocus (most species), Reuterella, Lachesilla (some species; Fig. 4). Except for the first two categories, these reductions have occurred repeatedly in the evolution of the order. Thus, they appear to offer advantages in energy conservation and keeping the organism closely associated with its habitat.


Psocids live in a great variety of habitats, including living leaves, especially of monocotyledonous plants and conifers; dead foliage of all plants, both hanging and in ground litter; trunks and branches of trees and shrubs (open surfaces and under bark); rock surfaces; and human dwellings. In general, psocids feed on algae, fungi, lichens,
 Male (A) and female (B) of a psocid, Lachesilla pal-lida, showing extreme sexual dimorphism in wing development.
FIGURE 4 Male (A) and female (B) of a psocid, Lachesilla pal-lida, showing extreme sexual dimorphism in wing development.
small eggs of insects, particles of organic debris, and dead bodies of insects. Some seem to be strict alga feeders (some Psocidae, Myopsocidae, Peripsocidae), and it is likely that they are somewhat specific in their choice of algae. There are also some strict lichen feeders, and some specificity has been observed among them. Some of the (micro)fungus feeders are not so specific. In culture, the corti-colous lepidopsocid Echmepteryx hageni takes lichens, algae, yeasts, and pollen and occasionally nibbles on its own eggs when these are not well concealed. The species of leaf dwellers are primarily micro-fungus feeders, whereas the species living on open surfaces of tree trunks and branches and on rock outcrops are primarily lichen and alga feeders.


The topiclice, genus Liposcelis, are frequently household pests and may be among the causative agents of asthmatic reactions. Of greater monetary importance is their tendency to enter and reproduce rapidly in food storage and food processing facilities, where they may render the products unfit for human consumption. Control methods emphasize sanitation and reduction of relative humidity.

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