Mesopsocus immunis, photographed by Dioctria (David).

Belongs within: Homilopsocidea.
Contains: Peripsocidae, Philotarsidae.

The Peripsocoidea are a clade of bark or stone-surface dwelling barklice, females of which have a strongly sclerotised and square-shaped dorsal valve to the gonapophyses (Yoshizawa 2002). Members include the Mesopsocidae, bark-dwelling species characterised by long, thirteen-segmented antennae and glabrous wings (at most with a few minute marginal setae). Bryopsocus is a distinctive moss-associated genus found in wet forests of New Zealand (New & Lienhard 2007).

Mesopsocus unipunctatus: an intriguing barklouse
Published 14 May 2018

I’ve maintained before that barklice or Psocoptera/Psocodea are the cutest of all insects, an opinion that I still stand by. Nevertheless, their small size and inoffensive habits mean that they don’t get the attention that they deserve.

Female Mesopsocus unipunctatus, copyright Tom Murray.

Mesopsocus unipunctatus is a widespread barklouse species in Europe and North America (and possibly in Asia as well where a lack of records may reflect a lack of people looking). It is a relatively large species as barklice go, growing up to about half a centimetre in length. Mature males are fully winged but females have the wings reduced to rudiments and are flightless. Mesopsocus unipunctatus are found living on the bark of trees, primarily on branches rather than on the trunk, and their diet is predominantly made up of the micro-alga Pleurococcus and fungal spores. They are active in early summer: populations in Yorkshire had the first nymphs hatching during April and numbers of individuals reached a peak in late June to early July. The population survived over winter as eggs, laid in clusters of five to eight and covered with a protective layer of hard faecal matter (Broadhead & Wapshere 1966).

Mesopsocus unipunctatus shares much of its range with a closely related species, M. immunis, and the two are often found in association (Broadhead & Wapshere 1966). Differences between the two are slight: M. immunis tends to be paler in coloration but the two species are best distinguished by features of their terminalia. They both feed on the same diet and are active around the same time of year (conversely, other ecologically similar barklice species found in Yorkshire by Broadhead & Wapshere, 1966, were active later in the summer). So how do the two manage to persist without one excluding the other? As it turns out, they differ in oviposition behaviour. Mesopsocus unipunctatus prefers to lay its eggs right at the tips of tree branches whereas M. immunis mostly lays about 25 to 50 cm back from the tip. Mesopsocus immunis also covers its egg masses with a layer of silk in addition to the layer of faecal matter used by both species. These behaviours mean that M. immunis egg masses are better protected from one of their major threats, a mymarid wasp that parasitises them. However, M. unipunctatus compensates for its higher vulnerability to parasitoids through a greater resistance to cold, meaning that a higher proportion of its unparasitised eggs survive the winter. The greater cold resistance of M. unipunctatus means that it may also be found at altitudes and latitudes beyond the range of M. immunis.

Male Mesopsocus unipunctatus, copyright Ken Schneider.

Another feature of M. unipunctatus worth mentioning is that it shows variation in coloration attributed to industrial melanism. This phenomenon is better known in Lepidoptera: you may have heard of one of the most famous animals supposed to exhibit it, the peppered moth Biston betularia. Individuals of M. unipunctatus in England vary in the degree of dark markings on the abdomen, from some that are almost entirely dark through those with a mottled pattern of dark patches and stripes to some in which the dark markings are restricted to the primary transverse stripe on the fourth abdominal segment. The head and thorax are also darker in some individuals than others though it is notable that not all individuals with darkened abdomens also have darkened heads and thoraces (Popescu et al. 1978). Industrial melanism is so-called because this variation in colour pattern is supposed to be related to industrial pollution. It is supposed that the original paler, broken coloration provided camouflage on lichen-covered bark but selection came to favour darker color patterns as trees became blackened with soot. Studies on melanism in M. unipunctatus did indeed find a correlation between the number of dark individuals in a population and the degree of pollution in the environment (Popescu 1979). However, aviary studies of predation rates on M. unipunctatus individuals released into simulated habitats were a bit more equivocable: survival rates of light-coloured individuals were better among branches taken from rural locations but neither morph was definitely favoured among branches from urban environments. Also, darker individuals exhibited faster growth rates in polluted environments than lighter individuals, perhaps due to better absorption of heat despite sunlight being blocked by smog. Are there more dark-coloured individuals in industrial locations because they die less, or because they live more? Another question I don’t know the answer to: has M. unipunctatus also reflected Biston betularia in seeing a drop in melanistic individuals with the reduction of smog levels in England in recent decades?

Systematics of Peripsocoidea

Characters (from Yoshizawa 2002): Small to large in size, about 2–5 mm in length. Body whitish in ground colour with blackish-brown markings or wholly blackish-brown. Head with rounded vertex; postclypeus well bulged; epicranial suture with (Mesopsocidae) or without broad internal ridge; anteclypeus sclerotized or not (Mesopsocidae). Forewing coloration, venation, and ciliation variable. Hindwing ciliation variable; Rs and M + Cu separate (most Mesopsocidae), or fused for short (Philotarsidae) or long (Peripsocidae) distance basally; Rs and M fused. Tarsi 2- (Peripsocidae) or 3-segmented; claws with preapical tooth; pulvillus narrow. Abdomen without eversible vesicles ventrally. Male genitalia: hypandrium usually simple except Philotarsus. Phallosome variable; aedeagus rounded (most of Mesopsocidae and Philotarsidae) or pointed apically; phallobase united and rounded apically; endophallus with (Bryopsocidae, Peripsocidae, and some Philotarsidae) or without sclerites. Female genitalia: epiproct usually rectangular in dorsal aspect except rounded in Bryopsocidae. Subgenital plate with 1-lobed egg guide extended from dorsal margin. Gonapophyses complete; ventral valve and dorsal valve strongly united, forming ovipositor; dorsal valve strongly sclerotized, somewhat rectangular in lateral aspect, with or without (Peripsocidae) subapical process; external valve variable.

    |--Bryopsocus Thornton, Wong & Smithers 1977SN04 [BryopsocidaeY02]
    |    |--*B. townsendi (Smithers 1969)NL07, SN04 [=Austropsocus townsendiNL07]
    |    `--B. angulatus (Smithers 1969)SN04 [=Zelandopsocus angulatusNL07]
               |  i. s.: Idatenopsocus orientalisY02
               |         Hexacyrtoma Enderlein 1908SN04
               |         Newipsocus Badonnel & Lienhard 1988SN04
               |         Mesopsocopsis Badonnel & Lienhard 1988SN04
               |         Acmomesopsocus Li 2002L02b
               |           `--*A. tibeticus Li 2002L02b
               |         Aphanomesopsocus Li 2002L02b
               |           |--*A. fuscus Li 2002L02b
               |           |--A. bipunctatus Li 2002L02b
               |           `--A. furvus Li 2002L02b
               |         Oegomesopsocus Li 2002L02b
               |           `--*O. guangxiensis Li 2002L02b
               |         Conomesopsocus Li 2002L02b
               |           |--*C. melanostigmus Li 2002L02b
               |           `--C. meniscatus Li 2001L03
               |         Microtrichopsocus Badonnel & Lienhard 1988L02b
               |         Rhinopsocus Badonnel & Lienhard 1987L98
               |           `--R. cincinnatus Lienhard 1988L98
               |         Cyrtopsochus Costa 1885 [=Cyrtopsocus; incl. Gibbopsocus Badonnel 1975]L98
               |           |--*C. irroratus Costa 1885L98
               |           |--C. canariensis (Meinander 1973) [=Mesopsocus canariensis, Cyrtopsocus canariensis]L98
               |           |--C. gibbosus Lienhard 1988 [=Cyrtopsocus gibbosus]L98
               |           |--C. nasutus (Enderlein 1907) [=Mesopsocus nasutus; incl. M. dromedarius Ball 1937]L98
               |           |--C. pustulatus (Badonnel 1975) [=Gibbopsocus pustulatus; incl. G. meridionalis Badonnel 1975]L98
               |           `--C. truncatus Lienhard 1996L98
               |--+--Psoculus Roesler 1954 [Psoculidae]SN04
               |  |    `--*P. neglectus (Roesler 1935) [=Reuterella neglecta]L98
               |  `--+--Psoculidus Badonnel & Lienhard 1988SN04
               |     `--Palmicola Mockford 1955SN04
               |          |--*P. aphrodite Mockford 1955SN04
               |          `--P. solitaria Mockford 1955M93
               `--Mesopsocus Kolbe 1880SN04 (see below for synonymy)
                    |--*M. unipunctatus (Müller 1764)L98 (see below for synonymy)
                    |--M. apterus Kaplin 1990L98
                    |--M. atlasicus Badonnel 1945L98
                    |--M. badhysi Kaplin 1990L98
                    |--M. blancae Baz 1988 [incl. M. andalusicus Baz 1989]L98
                    |--M. boops (Hagen 1859) [=Psocus boops, Elipsocus boops]SN04
                    |--M. brachyonematus Li 2002L02b
                    |--M. carthaginensis Lienhard 1988L98
                    |--M. corniculatus Li 2002 [=M. corniculata]L03
                    |--M. curvimarginatus Li 2002 [=L. curvimangiratus]L02b
                    |--M. dichotomus Li 2002L02b
                    |--M. diopsis Enderlein 1902E07 [=*Lobocoria diopsisNL07]
                    |--M. dislobus Yoshizawa 1998SN04
                    |--M. duboscqui Badonnel 1938L98
                    |--M. fuscifrons Meinander 1966L98
                    |--M. giganteus Lienhard 1995L98
                    |--M. graecus Lienhard 1981L98
                    |--M. helvsticus Lienhard 1977L02b [=M. duboscqui helveticusL98]
                    |--M. hiemalis Marikovsky 1957L98
                    |--M. hongkongensis Thornton 1959SN04
                    |--M. immunis (Stephens 1836) [=Psocus immunis; incl. M. lusitanus Lienhard 1981]M93
                    |--M. jiensis Li 2002L02b
                    |--M. jinicus Li 2002L02b
                    |--M. kopetdaghensis Kaplin 1992L98
                    |--M. laricolus Li 2002L02b [=M. laricisL02a]
                    |--M. laterimaculatus Ball 1937 [incl. M. africanus Baz 1989, M. maroccanus Badonnel 1945]L98
                    |--M. laticeps (Kolbe 1880)SN04 (see below for synonymy)
                    |--M. montinus Enderlein 1907E07
                    |--M. neimongolicus Li 2002L02b
                    |--M. nigrimaculatus Li 2002L02b
                    |--M. phaeodematus Li 2002L02b
                    |--M. salignus Li 2002L02b
                    |--M. stenoperus Li 2002 [=M. stenopterou]L02b
                    |--M. strongylotus Li 2002L02b
                    |--M. troodos Lienhard 1995L98
                    |--M. vernus Lienhard 1977L98
                    |--M. wardi Meinander 1973L98
                    |--M. yemenitus Lienhard 1988L98
                    |--M. yeni New 1991SN04
                    `--M. ypsilon Ball 1937 [incl. M. fuscilabrum Baz 1989]L98

Mesopsocus Kolbe 1880SN04 [incl. Holoneura Tetens 1891L02b, Lobocoria Enderlein 1910L02b, Trocticus Bertkau 1883L02b]

Mesopsocus laticeps (Kolbe 1880)SN04 [=Elipsocus laticepsNL07, *Holoneura laticepsNL07; incl. Mesopsocus laticeps ab. pedunculata Enderlein 1901L98]

*Mesopsocus unipunctatus (Müller 1764)L98 [=Hemerobius unipunctatusNL07, Elipsocus unipunctatusM93, Holoneura unipunctatusM93; incl. Hemerobius aphidioides Schrank 1781L98, Elipsocus aphidioidesM93, Mesopsocus unipunctatus var. bifasciatus Enderlein 1906L98, M. unipunctatus var. borealis Enderlein 1910L98, M. unipunctatus var. fasciatus Enderlein 1906L98, *Trocticus gibbulus Bertkau 1883NL07, L98, Psocus longicornis Stephens 1836 non Hemerobius longicornis Fabricius 1777 (not preoc. if dif. gen.)L98, P. naso Rambur 1842L98, P. obliteratus Zetterstedt 1840L98, P. oculatus Zetterstedt 1840L98, P. signatus Hagen 1861L98, Elipsocus signatusM93, Mesopsocus unipunctatus var. subfuscus Enderlein 1906L98, Caecilius vitripennis Curtis 1837L98]

*Type species of generic name indicated


Broadhead, E., & A. J. Wapshere. 1966. Mesopsocus population on larch in England—the distribution and dynamics of two closely-related coexisting species of Psocoptera sharing the same food resource. Ecological Monographs 36 (4): 327–388.

[E07] Enderlein, G. 1907. Wissenschaftliche Ergebnisse der Schwedischen Zoologischen Expedition nach dem Kilimandjaro, dem Meru und den Umgebenden Massaisteppen Deutsch-Ostafrikas 1905–1906 vol. 15. Corrodentia pt 2. Copeognatha. Almqvist & Wiksells Boktryckeri-A. B.: Uppsala.

[L02a] Li F. 2002a. Psocoptera of China vol. 1. Science Press: Beijing.

[L02b] Li F. 2002b. Psocoptera of China vol. 2. Science Press: Beijing.

[L98] Lienhard, C. 1998. Faune de France. France et Régions Limitrophes. 83. Psocoptères Euro-Méditerranéens. Fédération Française des Sociétés de Sciences Naturelles: Paris.

[L03] Lienhard, C. 2003. Nomenclatural amendments concerning Chinese Psocoptera (Insecta), with remarks on species richness. Revue Suisse de Zoologie 110 (4): 695–721.

[M93] Mockford, E. L. 1993. North American Psocoptera (Insecta). Sandhill Crane Press, Inc.

[NL07] New, T. R., & C. Lienhard. 2007. The Psocoptera of Tropical South-east Asia. Brill: Leiden.

Popescu, C. 1979. Natural selection in the industrial melanic psocid Mesopsocus unipunctatus (Müll.) (Insecta: Psocoptera) in northern England. Heredity 42 (2): 133–142.

Popescu, C., E. Broadhead & B. Shorrocks. 1978. Industrial melanism in Mesopsocus unipunctatus (Müll.) (Psocoptera) in northern England. Ecological Entomology 3: 209–219.

[SN04] Schmidt, E. R., & T. R. New. 2004. A systematic and phylogenetic revision of the family Elipsocidae (Insecta: Psocoptera), with the erection of two new families: Lesneiidae and Sabulopsocidae. Invertebrate Systematics 18: 157–213.

[Y02] Yoshizawa, K. 2002. Phylogeny and higher classification of suborder Psocomorpha (Insecta: Psocodea: ‘Psocoptera’). Zoological Journal of the Linnean Society 136: 371–400.

Leave a comment

Your email address will not be published. Required fields are marked *