Psalidothrips

Many of you may know thrips as small insects that infest buds and young shoots of garden plants, stymieing growth and causing malformed development. However, there is also a wide diversity of thrips species that feed on fungi, inhabiting leaf litter and other fallen vegetation. In tropical and subtropical regions of the world, one of the more numerous genera of such fungus-feeders is Psalidothrips.

Winged female (left) and wingless male of Psalidothrips comosus, from Zhao et al. (2018).

Close to fifty species of Psalidothrips have been described from various locations around the world (Wang et al. 2019). They are most commonly found among leaf litter and are believed to feed on fungal hyphae. Most Psalidothrips are relatively small, pale thrips, yellowish or light brown in coloration. As members of the family Phlaeothripidae, the last segment of the abdomen is modified into a tube ending in a ring of setae; in Psalidothrips, this tube is commonly short and the terminal setae are often longer than the tube.

As is common among thrips, the recognition of Psalidothrips and its constituent species is often complicated by within-species variation. Many species are known as both winged and wingless forms (Wang et al., 2019, note that Australian species seem particularly prone to winglessness). Wingless forms often show reductions in the sclerotisation of the thorax. It is difficult to name a single feature of the genus that does not find exception in some species or other. Most species are weakly sculpted. For the most part, the maxillary stylets are short and sit low and far apart in the head when retracted. The mouth-cone is similarly short and rounded. The head is often fairly short with rounded cheeks that do not bear strong setae. Setae on the anterior margin of the pronotum are often reduced. The wings, if present, are often more or less constricted at about mid-length. Many phlaeothripids possess a series of large setae on the abdomen that hold the wings in place when folded back; in individuals of Psalidothrips with such setae (obviously, they tend to disappear in wingless individuals), they are often relatively few in number and simply curved.

Many of these features are related to the thrips' litter-dwelling habits. The short mouthparts, for instance, presumably reflect how these thrips are gleaning fungi from the surface of leaves without needing to pierce the leaf's cuticle. As such, it will be interesting to see how the genus holds out as our understanding of thrips phylogeny improves. Is this a true evolutionarily coherent assemblage, or disparate travellers who are following a fashion?

REFERENCE

Wang, J., L. A. Mound & D. J. Tree. 2019. Leaf-litter thrips of the genus Psalidothrips (Thysanoptera, Phlaeothripidae) from Australia, with fifteen new species. Zootaxa 4686 (1): 53–73.

Rasahus albomaculatus, the White-Spotted Corsair

Though the Hemiptera began their long evolutionary history as plant-feeders, many of their subgroups later switched to a predatory lifestyle, their suctorial mouthparts being just as suited for stabbing flesh as vegetation. Among the most successful of the predatory bugs where the assassin bugs of the family Reduviidae.

Image copyright Jacob Gorneau.

This is Rasahus albomaculatus, a widespread assassin of the Neotropical region, found from Mexico to Argentina (Coscaron 1983). Though not one of the largest members of its genus, R. albomaculatus is a decent-sized bug, growing close to an inch in length. Rasahus is a genus of the reduviid subfamily Peiratinae, commonly known as corsairs for their fearsome aspect. Features distinguishing Rasahus from other genera of corsairs include their large eyes, a deep grove across the head in front of the ocelli, long procoxae, and well-developed spongy pads on the fore- and mid-tibiae. Rasahus albomaculatus is distinguished from other species of the genus by its colour pattern. The body is mostly black with white patterning on the wings. Stripes along the top of the wing and across the mid-length form a crude H-shape when the wings are closed, with separate spots towards the base of the wing and towards the tip. Other noteworthy features include a lack of granulation on the pronotum, and a rounded apex to the scutellum (Swanson 2018).

Corsairs are mostly predators of other insects and not often dangerous to humans (though their bite is supposed to be very painful). Indeed, they may be beneficial to humans as among their prey are believed to be other reduviids of the subfamily Triatominae, the blood-sucking "kissing bugs" that spread Chagas disease (contrary to the Wikipedia page on the western corsair R. thoracicus, corsairs do not spread Chagas themselves). Rasahus albomaculatus may provide its vertebrate co-habitants with far more comfortable living conditions.

REFERENCES

Coscarón, M. del C. 1983. Revision del genero Rasahus (Insecta, Heteroptera, Reduviidae). Revista del Museo de La Plata (nueva serie) (Zoologia) 13: 75–138.

Swanson, D. R. 2018. Three new species of Rasahus, with clarification on the identities of three other Neotropical corsairs (Heteroptera: Reduviidae: Peiratinae). Zootaxa 4471 (3): 446–472.

The Running of the Termites

I don't know how many people would profess to have a favourite genus of termites. Which is a shame, because there are some real stand-out examples. Snapping termites, magnetic termites, glue-spraying termites... For my own part, though, I have a particular fondness for the Australian harvester termites of the genus Drepanotermes.

Soldiers and workers of Drepanotermes perniger, copyright Jean Hort.

Nearly two dozen species of Drepanotermes are found on the Australian continent to which they are unique (Watson & Perry 1981). They are arid-environment specialists, being most diverse in the northern part of Australia. My reasons for being so fond of them are, I'll admit, decidedly prosaic. The worker caste of most termite species is very difficult if not impossible to identify taxonomically; one termite worker usually looks very much like another. Drepanotermes workers, however, are different. The name Drepanotermes can be translated as "running termite" and, as befits their name, Drepanotermes of all castes stand out for their distinctly long legs. Soldiers of Drepanotermes also have distinctively shaped mandibles which are sickle-shaped and have a single projecting tooth on the inner margin. They are similar to soldiers of the related genus Amitermes (of which Drepanotermes may represent a derived subclade) but the mandibles of Amitermes tend to be straighter and more robust.

The long legs of Drepanotermes reflect their active harvester lifestyles. Workers will emerge from the nest at night in search of food to carry back home. In the red centre of Australia they will primarily collect spinifex; they will also take fallen leaves, tree bark and the like. Soldiers keep guard while the workers forage. I've found them clustered around a nest entrance of an evening, just their heads poking out to snap at passers-by. Workers may wander up to about half a metre from the nest entrance as they forage. The concentrations of vegetable matter produced by Drepanotermes storing food sources in their nest may form a significant factor in the nutrient profile of areas where they are found.

Alate and soldiers of Drepanotermes rubriceps, copyright Jean Hort.

Depending on species and circumstance, the nests of Drepanotermes may be mounds or entirely subterranean with the latter being the majority option. They prefer compact soils such as clay though they may burrow through looser soils where there is a denser subsoil. Drepanotermes may construct their own nest or move into nests constructed by other termites. One aptly named species, D. invasor, seems to take over pre-existing nests more often than not. Subterranean nests are arranged as a series of chambers about five to ten centimetres in diameter connected by tunnels. These chambers may be arranged vertically, one below another, or they may form a rambling transverse network. Above ground, subterranean nests may be visible as an open circle devoid of vegetation. The ground in these circles is hard as concrete and may remain clear for decades after the actual nest has gone. Walsh et al. (2016) refer to the remains of nests protruding above ground along vehicle tracks after the soil around them has worn down. Local people have a long history of taking advantage of the open space offered by termite nests, such as to move more easily through scrub or as resting or working places.

The alate castes of Drepanotermes tend to be poorly known. Indications are that mature reproductives spend little time in the parent nest before leaving to breed. For most species, breeding flights take place in late summer. Alates may emerge either by day or night. The time of emergence seems to depend on the species; night-flying alates have distinctly larger eyes than day-fliers. Unfortunately, because alates have rarely been collected in association with a nest, we are largely still unable to tell which alates belong to which species.

REFERENCES

Walsh, F. J., A. D. Sparrow, P. Kendrick & J. Schofield. 2016. Fairy circles or ghosts of termitaria? Pavement termites as alternative causes of circular patterns in vegetation of desert Australia. Proceedings of the National Academy of Sciences of the USA 113 (37): E5365–E5367.

Watson, J. A. L., & D. H. Perry. 1981. The Australian harvester termites of the genus Drepanotermes (Isoptera: Termitinae). Australian Journal of Zoology, Supplementary Series 78: 1–153.

Booklice: The Cutest of Pests

Humans have a tendency to think of 'nature' and the 'environment' as something distinct from our own society. Environments unmodified by humans are seen as 'natural' whereas structures created by human activity, such as buildings, are not 'natural' and thought to be somehow outside the 'environment'. As such, people often react strongly to the idea of things associated with the 'environment', such as non-human wildlife, encroaching on their homes. But of course, human houses are as much an environment of their own as any other of the world's habitats, and many animals find them to be places where they can thrive. Among the animals that most regularly share our houses with us are booklice of the genus Liposcelis.

Liposcelis bostrychophila, copyright Andreas Eichler.

Representatives of Liposcelis can be found almost anywhere in the world except in the coldest of regions. About 130 species have been described in the genus to date (Yoshizawa & Lienhard 2010) with doubtless more yet to be discovered (by comparison, Broadhead's review of the genus in 1950 recognised only 22 species, with a six-fold increase since then). The family Liposcelididae, to which Liposcelis belongs, differ from other free-living members of the Psocodea (or 'Psocoptera') in their flattened body form, as well as being smaller than most other examples (Liposcelis grow little more than a millimetre in length). In the flattened habitus, they resemble the parasitic true lice of the Phthiraptera, and recent studies have agreed that the liposcelidids represent the closest relatives of true lice (Yoshizawa & Lienhard 2010). Liposcelis species are readily distinguished from other liposcelidids by the shape of the hind legs: an obtuse tubercle on the outer margin of the hind femur gives it a distinctly broad appearance* (indeed, the genus name Liposcelis translates into English as 'fat thigh'). Liposcelis are also distinctive in being invariably wingless; other liposcelidid species typically come in both winged and wingless forms. Though the genus as a whole is easily recognised, distinguishing individual species is often a far more challenging prospect requiring microscopic examination of fine features of the chaetotaxy (arrangement of bristles on the body) and cuticular sculpture. Authors have divided Liposcelis species between a number of diagnostic sections and subgroups based on these and other features but the monophyly or otherwise of these subdivisions is largely unstudied.

*This feature is also shared with a cave-dwelling species from Ascension Island currently placed in its own genus, Troglotroctes ashmoleorum, but it seems more than likely that this species is itself a derived offshoot of Liposcelis.

Liposcelis species can feed on a wide range of organic matter but, like other 'Psocoptera', their primary source of food is probably yeasts and fungal spores (their vernacular name has been attributed to their feeding on yeasts growing on the glue binding books, though I would note that they are also probably more likely to be seen crawling on the light background of a book's page than in other, less closely examined corners of the house). Turner (1994) provided a detailed review of the natural history of one of the most widespread domestic pest species in the genus, L. bostrychophila, and reports that he was able to maintain cultures on "'Weetabix'™, 'Shreddies'™, baby rice, soya granules, sage and onion stuffing mix, skimmed milk powder, 'Oat Krunchies'™, red lentils, and yellow split peas". Other stored foods from which complaints had been received of booklice included "sugar, bread, salt, bay leaves, gelatine powder, poppadoms, custard powder, dried yeast, instant potato, nuts, dried fruit, baby food, sauce mix, dried mushrooms, pasta, coconut, cocoa, milk powder, spices, glace cherries, garlic, baking powder, icecream mix, dried soup, cracked wheat, carob powder, maize meal, wheat germ, jellied sweets and bread crumbs". They have also been found on cured meat and may damage curated insect specimens. As well as obtaining moisture from their food, Liposcelis are also able to extract water directly from the atmosphere owing to the hygroscopic properties of their saliva. A booklouse will hold a drop of saliva inside its mouth, then swallow it when the ball has absorved enough water from the air.

Liposcelis sp. (possibly L. meridionalis?) from southern France, copyright Jessica Joachim.

Female Liposcelis bostrychophila generally reach maturity and begin producing eggs about two weeks after hatching and may produce two or three eggs a day. As each egg is about one-third the size of the adult, this means that a female at peak fecundity is producing her own body mass in eggs in a single day. Most Liposcelis species reproduce sexually but some are parthenogenetic. Domestic L. bostrychophila, for instance, seem to be entirely parthenogenetic with males of the species only known from isolated collections in Hawaii, Arizona and Senegal (Georgiev et al. 2020). Studies on an unnamed species of Liposcelis from Arizona found that sex determination seemed to be facultative, determined by the mother, with no evidence for differentiated sex chromosomes (Hodson et al. 2017). Females seemed to produce more males early in life and more females later. The same studies also established the occurrence of paternal genome elimination in this species, where chromosomes inherited from the father were inactivated in the offspring and not passed on to their own progeny (which raises the question that, if males are effectively a genetic dead end, why would a female produce male offspring at all?) Paternal genome elimination has also been found in the human louse Pediculus humanus, and may be characteristic of the broader clade encompassing these species, but other species remain unstudied. Liposcelis genomes are also remarkable in the occurrence of fragmentation of the mitochondrial genome. Whereas some Liposcelis species have only a single mitochondrial chromosome, as is standard for most other animals, some species have the mitochondrial genome divided between two, three, five or seven chromosomes (Feng et al. 2019). The functional significance, if any, of this feature remains unknown.

Though booklice may be found in houses and stores on the regular, they are mostly only minor pests, only causing distress when reaching large numbers (an exceptional case quoted by Turner, 1994, involved a house in New Jersey at the beginning of the 1900s that became so infested "'that a pinpoint could not have been put down without touching one or more of these bugs"). They are not believed to transmit pathogens, except perhaps incidentally by carrying microbes from one store to another. For the most part, these little beasties are just another part of the wildlife that shares our homes with us, whether we are aware of them or not.

REFERENCES

Feng, S., H. Li, F. Song, Y. Wang, V. Stejskal, W. Cai & Z. Li. 2019. A novel mitochondrial genome fragmentation pattern in Liposcelis brunnea, the type species of the genus Liposcelis (Psocodea: Liposcelididae). International Journal of Biological Macromolecules 132: 1296–1303.

Georgiev, D., A. Ostrovsky & C. Lienhard. 2020. A new species of Liposcelis (Insecta: Psocoptera: Liposcelididae) from Belarus. Ecologica Montenegrina 29: 41–46.

Hodson, C. N., P. T. Hamilton, D. Dilworth, C. J. Nelson, C. I. Curtis & S. J. Perlman. 2017. Paternal genome elimination in Liposcelis booklice (Insecta: Psocodea). Genetics 206: 1091–1100.

Turner, B. D. 1994. Liposcelis bostrychophila (Psocoptera: Liposcelididae), a stored food pest in the UK. International Journal of Pest Management 40 (2): 179–190.

Yoshizawa, K., & C. Lienhard. 2010. In search of the sister group of the true lice: a systematic review of booklice and their relatives, with an updated checklist of Liposcelididae (Insecta: Psocodea). Arthropod Systematics and Phylogeny 68 (2): 181–195.

Snakes and Lace

The holometabolous insects—that is, the clade containing most insects with a complex life cycle including differentiated larval and pupal stages—is one of the most extensive radiations of animals on this planet. Much of this diversity is assigned to four major orders: wasps, moths, beetles and flies. But there are also a number of smaller lineages making up the holometabolous insects. Among these are the lacewings and their relatives in the clade Neuropterida.

Female snakefly Puncha ratzeburgi, copyright Hectonichus.

Modern members of the Neuropterida are generally recognised as belonging to three orders—the lacewings and ant-lions in the Neuroptera, the snakeflies in the Raphidioptera, and the alderflies and dobsonflies in the Megaloptera—though go back a few decades and you may find texts referring to a single order Neuroptera. A number of authors have advocated for use of the name 'Planipennia' for the lacewing order to avoid confusion with the broader sense of Neuroptera but, while a case could certainly be made for this usage, it's just never really caught on. Most neuropteridans are fairly similar in overall appearance: long-bodied insects with well developed wings with numerous crossveins. Of the living holometabolous insects, they probably bear the greatest overall resemblance to the clade's ancestors and hence they are commonly thought of as 'relicts'. However, they do possess their own specialisations and are not primitive in every regard (for instance, the most primitive egg-laying apparatus among holometabolous insects belong to wasps). Species of Neuropterida are mostly predators as larvae. The larvae of the lacewing family Ithonidae may possibly feed on decaying plant matter though we don't know for certain (Grimaldi & Engel 2005). Adults are predators and/or pollen-feeders, or may not feed at all in some short-lived forms.

Male (above) and female dobsonflies Corydalus cornutus, copyright Didier Descouens.

The exact relationships between the neuropteridan orders have been debated over the years. Though most of their obvious similaities to each other represent shared ancestral features, there is a broad consensus that they do indeed form a clade. There has also been little, if any, question of the monophyly of the Raphidioptera and Neuroptera; the monophyly of Megaloptera has been more debated but seems more likely than not. Most recent studies have suggested that the Raphidioptera are the sister group to a clade of the other two orders (Engel et al. 2018). Raphidioptera are the least diverse of the generally recognised living orders of insects with about 250 known species. They are found in cooler regions of the Northern Hemisphere—in the temperate zone or at higher elevations in lower latitudes—and are completely absent from the Southern Hemisphere (Aspöck & Aspöck, 1991, refer to a failed attempt to introduce them to Australia and New Zealand but provide no details why such a thing was tried in the first place). They are characterised by a notably elongate prothorax (the first segment of the thorax) which explains the vernacular name of 'snakefly'. Larvae live under bark or in litter and moult into pupae with the onset of cold weather. The pupae of Raphidioptera and Megaloptera are primitive in aspect, with legs separate from the body wall, and are highly mobile. Engel et al. (2018) even refer to the pupae of Raphidioptera as 'active predators' but I've not been able to find corroborating details for that remarkable description.

The Megaloptera are often particularly large neuropteridans, reaching up to twenty centimetres in wingspan, and comprise a bit less than 400 species worldwide, mostly found in temperate regions. Larvae are aquatic, living under rocks and debris, and characterised by the presence of filamentous lateral gills on the abdomen. Adults are short-lived and feed little if at all. Male dobsonflies (of the subfamily Corydalinae) possess spectacularly large, curved mandibles of largely unknown purpose; certainly they do not seem to use them for biting.

Mantisfly Mantispa styriaca, a raptorial lacewing, copyright Gilles San Martin.

The largest of the three orders, by a considerable margin, is the Neuroptera with over 5700 known species. Needless to say, this level of species diversity is associated with a high diversity of appearances and lifestyles, too many to cover adequately here. The larvae of two families of Neuroptera, the Nevrorthidae and Sisyridae, are aquatic and there has been a long-running debate whether this aquatic habit is an ancestral feature of the order shared with the Megaloptera (Nevrorthidae larvae are generalist predators, Sisyridae are specialised feeders on freshwater sponges and bryozoans). However, recent phylogenetic studies (e.g. Vasilikopoulos et al. 2020) do not agree with earlier hypotheses that the Nevrorthidae represent the sister taxon of the remaining Neuroptera. Instead, Nevrorthidae and Sisyridae may form a clade with the Osmylidae, a family whose larvae are not aquatic but often inhabit damp stream banks. The aquatic Neuroptera probably entered the water independently of the alderflies. The current favourites for the sister clade of other neuropterans are the dustywings of the Coniopterygidae, a group of small neuropterans with reduced wing venation that have historically been difficult to place owing to their derived features.

An unidentified dustywing, Coniopterygidae, copyright Katja Schulz.

A fourth order has often been associated with the Neuropterida, the extinct Glosselytrodea. Glosselytrodeans are small insects known from the Late Permian to the Jurassic, characterised by wings bearing dense cross-veins of which the fore pair would have had a leathery appearance in life (not dissimilar in texture to the fore wings of grasshoppers and other Orthoptera). Other than the wings, the features of glosselytrodeans are poorly known: they seem to have been hypognathous (i.e. had the head directed downwards) with slender legs (Grimaldi & Engel 2005). Connections to Neuropterida are based on features of the wing venation but cannot be considered strongly supported. Other authors have regarded them as of uncertain position within the broader holometabolous clade, or even as more closely related to the Orthoptera than any Holometabola. Unless more complete remains should come to light, it seems likely that the question will remain open.

REFERENCES

Aspöck, H., & U. Aspöck. 1991. Raphidioptera (snake-flies, camelneck-flies). In: CSIRO. The Insects of Australia: A textbook for students and research workers 2^nd ed. vol. 1 pp. 521–524. Melbourne University Press.

Engel, M. S., S. L. Winterton & L. C. V. Breitkreuz. 2018. Phylogeny and evolution of Neuropterida: where have wings of lace taken us? Annual Review of Entomology 63: 531–551.

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Vasilikopoulos, A., B. Misof, K. Meusemann, D. Lieberz, T. Flouri, R. G. Beutel, O. Niehuis, T. Wappler, J. Rust, R. S. Peters, A. Donath, L. Podsiadlowski, C. Mayer, D. Bartel, A. Böhm, S. Liu, P. Kapli, C. Greve, J. E. Jepson, X. Liu, X. Zhou, H. Aspöck & U. Aspöck. 2020. An integrative phylogenomic approach to elucidate the evolutionary history and divergence times of Neuropterida (Insecta: Holometabola). BMC Evolutionary Biology 20: 64.

The Mirines

Every profession has its quirks, tricks of the trade that are difficult to learn and appreciate except through direct experience. One quirk of entomology is that specimens of each distinct type of insect will have their own nuances for the best method to preserve and present them. And there are some particular types of insect that can be particularly challenging in that regard. Which is a roundabout way of saying: I am not a great fan of mirids.

Green mirid Creontiades dilutus, copyright CSIRO.

Mirids are the largest recognised family of the true bugs in the Heteroptera, with over 11,000 species known worldwide and presumably many more remaining undescribed. They can be distinguished from most (though not, it should be stressed, all) other bug families by the presence of the cuneus, a distinct cross-fold near the outer tip of the hemelytron (the toughened basal part of the fore wing). Most mirids can be further recognised by the absence of ocelli. They are mostly smaller bugs, generally somewhat soft-bodied, and mostly plant feeders though there are some notable exceptions. They also (and this is the reason why they have sometimes been the object of my animus in the past) have a tendency to be what I can only describe as weirdly flimsy. Most insect specimens, at least while stil fresh and relaxed, hold together reasonably well when subject to basic handling. Mirids, on the other hand, will throw off legs if you so much as look at them too hard.

An ant-mimicking mirid, Dacerla inflata, copyright Judy Gallagher.

Mirids are divided between several subfamilies, with the type subfamily Mirinae including well over 4000 species (Kim & Jung 2019). Mirines tend to be relatively large compared to other mirids (up to a bit over half a centimetre in length) and are characterised by features of the genitalia, together with a pair of lamellate, divergent parempodia (fleshy structures that may help in gripping onto things) at the end of the legs between the claws. Other notable features (shared with the closely related Deraeocorinae) include a deeply punctate pronotum, and a relatively long beak that extends beyond the mid coxae at rest. Several species of Mirinae are notable pests. The green mirid Creontiades dilutus is one of the more significant bug pests of crops in Australia, attacking a wide range of hosts including cotton, stone fruit, potatoes, legumes and many more (Malipatil & Cassis 1997). It generally feeds from growing points, killing new buds and inhibiting the production of flowers and new growth. Other polyphagous pests causing similar damage include the tarnished plant bugs of the genus Lygus, whose vernacular name is somewhat self-explanatory, and the alfalfa bug Adelphocoris lineolatus.

Tarnished plant bug Lygus pratensis, copyright Hectonichus.

Six tribes have been recognised within the Mirinae, distinguished by their overall habitus. The Mirini, the largest tribe, have a more or less ovoid body shape with a distinct, raised pronotal collar and opaque hemelytra. The Hyalopeplini have a similar body shape to Mirini but transparent hemelytra. The Restheniini have a reduced evaporative area on the abdomen. The Stenodemini and Mecistoscelini are long and slender with long appendages, with the head directed forward in the Stenodemini. The Herdoniini are ant mimics, presumably for defence from predators. The appearance of an ant waist is achieved by a narrowing of the mirid's own body and wings, and/or an appropriately placed white triangular marking across the hemelytron. Despite the superficial distinctiveness of the tribes, however, a phylogenetic study of the Mirinae by Kim & Jung (2019) found at least two of them to be paraphyletic, with Mecistoscelini being nested within Stenodemini, and Hyalopeplini and Restheniini within Mirini. The affinities of the Herdoniini, unsampled by Kim & Jung, remain to be established.

REFERENCES

Kim, J., & S. Jung. 2019. Phylogeny of the plant bug subfamily Mirinae (Hemiptera: Heteroptera: Cimicomorpha: Miridae) based on total evidence analysis. Systematic Entomology 44: 686–698.

Malipatil, M. B., & G. Cassis. 1997. Taxonomic review of Creontiades Distant in Australia (Hemiptera: Miridae: Mirinae). Australian Journal of Entomology 36: 1–13.

The Origin of Hexapods

Insects have been described as the most evolutionarily successful group of animals in the modern world, and with good reason. Something like two-thirds of the currently known animal species are insects, and they are near-ubiquitous in the terrestrial and freshwater environments (for whatever reasons, they've never made that much of a go of it marine-wise). Nevertheless, the questions of how and when insects first came to be remains very much an open one.

The long-necked fungus beetle Diatelium wallacei, one of the countless weird oddballs in the insect world. Copyright Artour Anker.

Insects are usually recognised as including three main subgroups: the winged insects, silverfish and bristletails. They are readily united into a group known as the hexapods with a few less speciose assemblages: the springtails, the proturans and the diplurans. All living hexapods have the body divided into a head, thorax and abdomen, with three pairs of walking legs on the thorax and none on the abdomen. Though monophyly of the hexapods has been questioned in the past (which is why the springtails and the like are usually excluded from our concept of 'insect' these days despite having been included previously), the majority view is now firmly in favour of regarding them as a single, coherent lineage. How hexapods are related to other arthropods has been more vigorously debated. Earlier authors commonly associated them with the myriapods, the lineage including centipedes and millipedes. In more recent years, an increasing number of studies have instead associated insects with crustaceans. This realignment has primarily been pushed by molecular studies but there are also a number of interesting morphological features such as eye and brain structure that are more crustacean- than myriapod-like in insects. Indeed, it seems not unlikely that insects are not merely related to but are nested within crustaceans: for instance, a few recent studies have supported a relationship between hexapods and a rare group of crustaceans known as remipedes (Schwentner et al. 2017). The features previously seen as shared between insects and myriapods, such as tracheae and uniramous (unbranched) limbs, are then held to probably be convergent adaptations to a terrestrial lifestyle.

Whatever its relationships, it seems most likely that the immediate ancestor of the living hexapods was indeed terrestrial. Of the six basal hexapod lineages referred to above, five (all except winged insects) are almost exclusively terrestrial and were probably ancestrally so. The winged insects include a number of basal subgroups (such as mayflies and dragonflies) that are aquatic for at least the early part of their life cycle, but a terrestrial origin for winged insects as a whole remains credible.

Head of Rhyniella praecursor, from Dunlop & Garwood (2017).

From the perspective of the fossil record, the evidence related to hexapod origins is incredibly slight. The earliest fossil species that have been directly proposed as hexapod relatives are known from the Early Devonian and less than half a dozen such species have been mooted as such in recent years. The only named Devonian fossil whose status as a hexapod seems unimpeachable is Rhyniella praecursor, a springtail from the Rhynie chert of Scotland (Dunlop & Garwood 2017). The same deposit provided Rhyniognatha hirsti, a fragmentary fossil comprising a pair of mandibles and surrounding parts of the head capsule. Rhyniognatha has long been thought to be an insect, possibly even an early member of the winged insect lineage, but Haug & Haug (2017) recently argued that it could just as easily be the head of a centipede (a group already known from other fossils in the Rhynie chert).

Rhyniognatha hirsti, from the University of Aberdeen. Scale bar = 200 µm; m = mandible.

The Windyfield chert, a deposit of similar age and location to the Rhynie chert, has provided Leverhulmia mariae, originally described as a myriapod but reinterpreted as a hexapod relative by Fayers & Trewin (2005). Leverhulmia is a difficult beast to know what and how much to make of it. The original specimen is, speaking charitably, a bit of a mess: a flattened smear looking a bit like a sausage burst open after cooking for too long on the pan. The front and back ends of the animal both appear to be missing and the only features really distinguishable are a series of small jointed legs. Other specimens associated with this species by Fayers & Trewin (2005) are simply more legs detached from their original body. These legs, though, do preserve a reasonable amount of detail, including the presence of paired lateral claws at the ends of the tarsi like those of most insects (Leverhulmia also possesses a smaller median claw between the lateral claws, a feature not found in winged insects but present in silverfish and bristletails). In contrast, the legs of myriapods (as well as those of springtails and proturans) end in a single terminal claw.

Holotype specimen of Leverhulmia mariae, from Dunlop & Garwood (2017); the size of the scale bar was not specified but the entire specimen is about 12 mm long.

The overall appearance of Leverhulmia's legs might therefore be seen a suggestive of a relationship specifically to insects and not just to hexapods in general, but their number provides something of a barrier to accepting Leverhulmia as a bona fide insect. The train-wreck nature of Leverhulmia's preservation means we can't state confidently how many legs it had but there were at least five pairs: a couple more than the hexapods' standard-issue three. A number of structures on the abdomens of some living hexapods are potentially derived from modified legs, such as the springing furca of springtails and the ventral styli in hexapods other than springtails and winged insects, so some parallelism in appendage reduction is not out of the question. Nevertheless, unjointed styli are one thing; fully-jointed, functional walking legs are another. Supposed early members of the bristletail and silverfish lineages with jointed abdominal legs have been described from the Carboniferous by Kukalová-Peck (1987) but (as I've noted before) many of the more outlandish reconstructions of early insects by Kukalová-Peck have failed to stand up to subsequent scrutiny.

Similar interpretative difficulties surround Strudiella devonica, described as an early relative of the winged insects from the Late Devonian of Belgium. Though I was not unfavourable to this specimen when it was first described, Hörnschemeyer et al. (2013) would later argue against recognising it as an insect. The latter authors professed to be simply unable to discern many of the features cited by its original describers as evidence of insect affinity, and saw Strudiella as closer to a Rorschach blot than a dragonfly. Strudiella's status was defended by its original authors (Garrouste et al. 2013) but a number of subsequent authors seem to have taken Hörnschemeyer et al.'s caution to heart.

Close-up of the head of Strudiella devonica from Hörnschemeyer et al. (2013); the asterisk marks the base of a structure originally interpreted as an antenna.

The final candidate for stem-hexapod status worthy of consideration here is Wingertshellicus backesi from the Lower Devonian Hunsrück Slate of Germany. This marine fossil was interpreted as a stem-hexapod under the name Devonohexapodus bocksbergensis, with a thorax bearing three pairs of legs and an elongate abdomen with uniramous appendages. However, it was reinterpreted by Kühl & Rust (2009) who synonymised Devonohexapodus with the previously described Wingertshellicus, regarded the previously described 'thoracic legs' as appendages of the head, and did not accept the presence of differentiated thorax and abdomen. The appendages of the trunk (previously seen as the abdomen) were biramous rather than uniramous with a small endopod and a large flap-like exopod adapted for swimming, and the end of the body bore a pair of fluke-like appendages (comparable to the tail of a crayfish). Wingertshellicus thus lacked any resemblance to a hexapod, and Kühl & Rust doubted that it even belonged to the crown group of arthropods.

Laterally preserved specimen of Wingertshellicus backesi, from Kühl & Rust (2009); scale bar = 10 mm.

An attempt to estimate the age of divergence of hexapods from other arthropods using a molecular clock analysis by Schwentner et al. (2017) suggested that hexapods and remipedes went their separate ways in the late Cambrian or early Ordovician. This is up to 100 million years earlier than the fossils described above but we should be careful how much to read into this discrepancy. If most of the features associated with hexapods are related to adoption of a terrestrial lifestyle, then it might be difficult to recognise any early marine relatives if found. Conversely, while it is uncertain how much if any terrestrial vegetation was present prior to the Devonian, the only potential cover would have been low lichens, non-vascular plants or micro-algae. If stem-hexapods emerged onto land during this time, the environment would not be conducive to their preservation in the fossil record. Finally, not only are hexapods other than winged insects not found in the fossil record before the Devonian, they are barely found after it: after Rhyniella, none are known until the appearance of amber-producing trees during the Cretaceous. So if we can't find any sign of them for some 300 milion years that we know that they are around, then we obviously can't say too much about not finding them over the previous hundred million years. The stem-hexapods may have been around in this time but they remain in hiding.

REFERENCES

Dunlop, J. A., & R. J. Garwood. 2017. Terrestrial invertebrates in the Rhynie chert ecosystem. Philosophical Transactions of the Royal Society of London Series B—Biological Sciences 373: 20160493.

Fayers, S. R., & N. H. Trewin. 2005. A hexapod from the Early Devonian Windyfield Chert, Rhynie, Scotland. Palaeontology 48 (5): 1117-1130.

Garrouste, R., G. Clément, P. Nel, M. S. Engel, P. Grandcolas, C. D'Haese, L. Lagebro, J. Denayer, P. Gueriau, P. Lafaite, S. Olive, C. Prestianni & A. Nel. 2013. Is Strudiella a Devonian insect? Garrouste et al. reply. Nature 494: E4–E5.

Haug, C., & J. T. Haug. 2017. The presumed oldest flying insect: more likely a myriapod? PeerJ 5: e3402.

Hörnschemeyer, T., J. T. Haug, O. Bethoux, R. G. Beutel, S. Charbonnier, T. A. Hegna, M. Koch, J. Rust, S. Wedmann, S. Bradler & R. Willmann. 2013. Is Strudiella a Devonian insect? Nature 494: E3–E4.

Kühl, G., & J. Rust. 2009. Devonohexapodus bocksbergensis is a synonym of Wingertshellicus backesi (Euarthropoda)—no evidence for marine hexapods living in the Devonian Hunsrück Sea. Organisms, Diversity & Evolution 9: 215–231.

Schwentner, M., D. J. Combosch, J. P. Nelson & G. Giribet. 2017. A phylogenomic solution to the origin of insects by resolving crustacean-hexapod relationships. Current Biology 27: 1818–1824.

The Other Silver Fish

A couple of years ago, I presented a post about the Lepismatidae, the family including the familiar household silverfishes. In that post, I made an offhand reference to other, less well known families of the wingless insect order Zygentoma. The time has come to look at those families.

Squamatinia algharbica, a subterranean nicoletiid from Portugal, copyright S. Reboleira.

The Zygentoma are divided between five or six living families, depending on how you count them. The largest of these other than the Lepismatidae is the Nicoletiidae, representatives of which may be found in most parts of the world if you know where to look. And therein lies the rub: the eyeless nicoletiids are usually to be found in subterranean habitats, burrowed into soil or within caves. They are pale in coloration, usually white or golden. Nicoletiids have less flattened bodies than lepismatids and often lack the covering of scales found in the latter. One subfamily of nicoletiids, the Atelurinae, is sometimes treated as a separate family (hence the uncertainty above): not only do they have the covering of scales most other nicoletiids lack, they are generally less elongate and are oval or teardrop-shaped in form. Atelurines are inquilines of social insects, making their living in the nests of ants or termites. While some observations have been made of atelurines taking food directly from their hosts, it seems that they mostly live as scavengers on items dropped within the nest. They avoid capture and/or eviction by their hosts through their slipperiness and speed (Smith 2017).

An atelurine silverfish from Tasmania, copyright Zosterops.

The other three families are more localised in their distributions. The Protrinemuridae are found in scattered, disjunct locations including the Middle East, eastern Asia and Chile. They are similar in appearance to the Nicoletiidae, being eyeless, scaleless and subcylindrical, and were classified with that family until fairly recently. Differences between the Nicoletiidae and Protrinemuridae include the nature of the cuticular plates making up the underside of the abdomen. In nicoletiids, these are divided between median sternites and lateral coxites; in protrinemurids, a single undivided plate covers the underside of each abdominal segment. Maindronia is a genus of silverfish placed in its own family that resembles Lepismatidae in possessing eyes and a covering of scales. This genus also has a disjunct distribution, being known from Egypt, Saudi Arabia, Afghanistan and Chile.

Tricholepidion gertschi, copyright Samuel DeGrey.

The fifth family of Zygentoma includes a single living species Tricholepidion gertschi known from the coastal region of northern California, where it is found under decaying tree bark. Tricholepidion retains a number of plesiomorphic features for Zygentoma and is universally accepted as the most divergent member of the order: it lacks scales, it possesses ocelli as well as the compound eyes, it has five rather than four or fewer segments in the tarsi, and it possesses a greater number of ventral styli on the abdomen. Tricholepidion is also hypognathous (that is, it has the head oriented so that its mouth is directed downwards) whereas other Zygentoma are generally prognathous, with the mouthparts directed forwards (Engel 2006). Tricholepidion has usually been included in the Lepidotrichidae, a family originally described for a fossil species Lepidothrix pilifera from the Eocene Baltic amber, but Engel (2006) argued that Lepidothrix was in some ways more derived than Tricholepidion (specifically, it lacks ocelli) and that Tricholepidion should be placed in its own family. Indeed, Tricholepidion is divergent enough that some have even suggested it be excluded from the Zygentoma and regarded as the sister taxon to the broader clade uniting silverfish with the winged insects. This is certainly a minority view, however: most authors continue to regard it as a true, albeit highly unusual, zygentoman.

REFERENCES

Engel, M. S. 2006. A note on the relic silverfish Tricholepidion gertschi (Zygentoma). Transactions of the Kansas Academy of Science 109 (3–4): 236–238.

Smith, G. B. 2017. The Australian silverfish fauna (order Zygentoma)—abundant, diverse, ancient and largely ignored. Gen. Appl. Ent. 45: 9–58.

Libellulidae: On the Wing

Dragonflies of the order Odonata are unquestionably one of the more familiar groups of insects to the general public. They are large, visible and eye-catching, and may be quite colourful. Some have even taken to 'twitching' dragonflies in the same manner as bird species, identifying species observed on the wing and keeping a tally of how many they have seen.

And at the top of many people's list: the wandering glider Pantala flavescens, copyright Jeevan Jose, the world's most widespread dragonfly species.

Ecologically, in contrast, dragonflies may be called a relatively conservative group. All begin their lives as aquatic predators before emerging with adulthood as fast-moving aerial predators. All are generalists, feeding on whatever other insects may be unfortunate enough to fall into their grasp. All dragonflies conform to a fairly similar overall bauplan when compared to the diversity of forms that may be found in many other insect orders (for instance, there are no flightless dragonflies). Classification of dragonflies has often focused heavily on features of the wing venation, tracing its lines in their criss-crossing network.

Hind wing of a libellulid with the anal loop highlighted, from here.

The largest of the generally recognised families of dragonflies is the Libellulidae, containing over 1000 of the approximately 6000 known species of Odonata (Pilgrim & von Dohlen 2008). Characteristic features of the Libellulidae include the presence of the 'anal loop', an arrangement of veins in the hind wing forming what has been described as a boot shape. In the case of the genus Libellula, at least, the shape of the anal loop rather reminds me of one of the legs on the Manx flag. Members of the Libellulidae are commonly known as perchers or skimmers in reference to their hunting behaviours; others have similarly composed names such as darters or pondhawks. A number of members of the family have strikingly banded or coloured wings, leading to vernacular labels such as amberwings or pennants. Members of the genus Tramea are commonly known as saddlebags in reference to the dark patches at the base of their hind wings.

Common picturewing Rhyothemis variegata, copyright Tarique Sani.

Members of the Libellulidae have been divided between about a dozen subfamilies, again primarily defined on the basis of wing venation. However, distinctions between the subfamilies have always been vague with many subfamilies recognised by particular combinations of characters rather than characters unique to each subfamily alone. This vagueness has been underlined by recent molecular studies which have identified most subfamilies as polyphyletic. It seems likely that the defining features of these subfamilies are convergences related to similar ecologies. The 'Sympetrinae' include species with a preference for open watery habitats such as ponds and marshes where they spend a lot of time perched on exposed vegetation (Pilgrim & von Dohlen 2008). The 'Tetrathemistinae', with narrow wings with somewhat reduced venation, are found along forest streams (Fleck et al. 2008). The genera Tramea and Pantala, falsely united in the subfamily Trameinae by broadened bases on the hind wings, are specialised for long-distance flights spending extended periods on the wing (Pilgrim & von Dohlen 2008). Indeed, the wandering glider Pantala flavescens is the world's most widespread dragonfly species, being found in warmer regions of the entire globe and seemingly capable of migrations between separate continents.

The slightly freakish-looking larva of Orionothemis felixorioni, from Fleck et al. (2009).

So if we're going to have a stable classification for libellulids, we need to look past their wings. Intriguingly, larval features may prove more useful in this regard than adult characters. Fleck et al. (2008) examined a group of genera previously classified in the Tetrathemistinae but whose larvae were more similar to those found among members of the Libellulinae. A molecular phylogeny showed that, whereas the Tetrathemistinae as a whole were polyphyletic, these genera were indeed associated with the Libellulinae as their larvae indicated. With further research, we find that libellulid classification need not be all in vein.

REFERENCES

Fleck, G., M. Brenk & B. Misof. 2008. Larval and molecular characters help to solve phylogenetic puzzles in the highly diverse dragonfly family Libellulidae (Insecta: Odonata: Anisoptera): the Tetrathemistinae are a polyphyletic group. Organisms, Diversity & Evolution 8: 1–16.

Pilgrim, E. M., & C. D. von Dohlen. 2008. Phylogeny of the Sympetrinae (Odonata: Libellulidae): further evidence of the homoplasious nature of wing venation. Systematic Entomology 33: 159–174.

Mesopsocus unipunctatus: an Intriguing Barklouse

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?

REFERENCES

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.

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.