The Lithoglyphidae: Let's Get Fresh

Live individuals of Lithoglyphus naticoides, copyright Jan Steger.

In previous posts on this site (see here and here), I've introduced you to members of the Hydrobiidae, a diverse family of mostly freshwater gastropods. Hydrobiids have long been recognised as a tricky group to work with, because of their small size and general shortage of distinctive shell features. In recent years, an understanding has developed that the 'hydrobiids' may include a number of lineages that became independently adapted to fresh water, and a number of previously recognised subfamilies of the Hydrobiidae have come to be recognised as their own distinct families. One of these ascended subgroups is the Lithoglyphidae.

Flat pebblesnails Lepyrium showalteri with eggs, copyright Friends of the Cahaba River National Wildlife Refuge.

The Lithoglyphidae are a family of about 100 known species, mostly found in the Holarctic region (Strong et al. 2008), though they have also been recorded from South America. Most lithoglyphids have distinctively squat, relatively thick shells, and for a long time this was treated as one of the main defining features of the group. However, Thompson (1984) pointed out that the sturdy lithoglyphid shell was probably an adaptation to living in fast-flowing streams and rivers, and could also be found in other 'hydrobiid' groups. As well as reducing the shell profile, the lithoglyphid shell possesses a broad aperture that allows for a proportionately large foot, increasing the snail's clinging power. Thompson (1984) identified a number of other features characteristic of lithoglyphids, including a spirally sculptured protoconch and a simple, blade-like penis that lacks accessory lobes or glandular structures. As the soft anatomy of many 'hydrobiids' has not yet been described, it is still possible that some taxa currently identified as lithoglyphids are in fact impostors. Conversely, confirmed lithoglyphids now include some taxa more divergent in shell shape, such as the limpet-like Lepyrium showalteri from Alabama. This species is distinctive enough that when first described it was identified as a neritid, a member of a group of gastropods not even closely related to lithoglyphids (imagine a new species of rodent being identified as a ratfish). Sadly, Lepyrium is also now endangered, being extinct in one of the two river catchments it was historically known from (see here). Thompson (1984) notes that another North American lithoglyphid genus, Clappia, may be entirely extinct. For at least one species, the cause of extinction was pollution from coal mining; no cause was specified for the other species, but according to Wikipedia its native habitat in the Coosa River has been modified by the construction of hydroelectric dams.

Shells of Benedictia baicalensis, from Baikal.ru.

Also closely related to the lithoglyphids are the Benedictiinae, a group of 'hydrobiid' gastropods endemic to Lake Baikal in Russia. A single species of benedictiine has been described from Lake Hövsgöl in Mongolia, but has not been collected there since; it seems likely that its original location was an error (Sitnikova et al. 2006). Baikal is a remarkable place: one of the world's largest freshwater lakes (and easily the largest in terms of the volume of water it contains), it is basically a freshwater sea. While other large lakes such as the Rift Lakes of Africa are poorly oxygenated at deeper levels, effectively restricting most animal life to the surface layer, Baikal has oxygen-rich deeper waters allowing a rich deep-water animal community (this may also be related to the numerous hydrothermal vents in the depths of Baikal). Some of you may have heard of the endemic Baikal seal Phoca sibirica, but Baikal is also home to a wide diversity of endemic fish (including a dramatic radiation of sculpins), a remarkable array of endemic amphipods, and even its own endemic family of sponges. The Benedictiinae are currently classified as a separate subfamily of Lithoglyphidae, with the remainder of species in the Lithoglyphinae (Bouchet et al. 2005), but as the relationship between the two subfamilies has not yet been examined in detail it is possible that the lithoglyphines are paraphyletic to the benedictiines. The benedictiines generally have thinner shells the lithoglyphines, possibly related to the differences in their usual habitats.

REFERENCES

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Sitnikova, T., C. Goulden & D. Robinson. 2006. On gastropod mollusks from Lake Hövsgöl. In: Goulden, C. E., T. Sitnikova, J. Gelhaus & B. Boldgiv (eds) The Geology, Biodiversity and Ecology of Lake Hövsgöl (Mongolia), pp. 233-252. Backhuys Publishers: Leiden.

Strong, E. E., O. Gargominy, W. F. Ponder & P. Bouchet. 2008. Global diversity of gastropods (Gastropoda; Mollusca) in freshwater. Hydrobiologia 595: 149-166.

Thompson, F. G. 1984. North American freshwater snail genera of the hydrobiid subfamily Lithoglyphinae. Malacologia 25 (1): 109-141.

The Little Tike that is Tychus

Male (left) and female of Tychus niger, copyright Lech Borowiec.

Another brief beetle post for today. The species in the image above is the type species of Tychus, a genus of about 150 species of pselaphine beetles found in Eurasia and North America. Chandler (1988) regarded the North American species as a separate genus Hesperotychus but Kurbatov & Sabella (2008) felt that the differences between species from the two continents were not enough to warrant separation. There's probably still some work to be done here.

Like many other pselaphines, most of the work on Tychus has focused on the morphology of the various species, with relatively little having been said about its life habits. Heer (1841) described the habitat of T. niger as 'sub lapidibus et in graminosis' which I believe means 'under stones and among grass', and Kurbatov & Sabella (2008) recorded collecting a specimen of Atychodea pilicollis, a related species, in damp sand. The large, broad-ended palps (the appendages on the head behind the antennae) of Tychus species suggests that they are probably micropredators, like the pselaphine Bryaxis puncticollis whose hunting behaviour was described in an earlier post. Like most pselaphines, the small size of Tychus species means that they generally escape observation.

Tychus is the largest genus in the pselaphine tribe Tychini. Tychins are most diverse in the Holarctic region, with only a very few species found in southern and south-east Asia. Chandler (1988) characterised the Tychini by the shape of the third segment of the palp, which is invariably longer than wide, and by the antennae usually being inserted close together on a narrow rostrum, though this varies a lot in distinctiveness between species. Kurbatov & Sabella (2008) also identified a number of other features representing possible synapomorphies of the Tychini, and suggested that the Oriental genera Atychodea and Amorphodea represent the sister taxon of the remaining Holarctic genera. The genera of Tychini are all fairly similar in appearance; notable distinguishing features of Tychus include an asymmetrical aedeagus (the intromittent organ in the male genitalia) and the arrangement of foveae (hollows) on the elytra and sternites. Males of Tychus often have one of the antennal segments modified as in the male of T. niger shown above, with a median segment noticeably thicker and broader than those on either side. The purpose of this enlarged segment, as with so many other features of pselaphines, seems to be unknown.

REFERENCES

Chandler, D. S. 1988. A cladistic analysis of the world genera of Tychini (Coleoptera: Pselaphidae). Transactions of the American Entomological Society 114 (2): 147-165.

Heer, O. 1841. Fauna Coleopterorum Helvetica, pars 1. Impensis Orelii, Fuesslini et Sociorum: Turici.

Kurbatov, S. A., & G. Sabella. 2008. Revision of the genus Atychodea Reitter with a consideration of the relationships in the tribe Tychini (Coleoptera, Staphylinidae, Pselaphinae). Transactions of the American Entomological Society 134 (1-2): 23-68.

A Skinny Waist is Not That Important

Female Clitemnestra bipunctata carrying prey back to its nest. Copyright Bill Johnson.

It's time for another post on crabronid wasps! The hard-working individual in the photo above is Clitemnestra bipunctata, a species found in the United States of North America. This is the only species of this genus found north of Mexico; other species are found in South America and in Australasia (Australia, New Guinea and New Caledonia). Clitemnestra belongs to the Gorytini, a closely related tribe to the Bembicini featured in an earlier post, though C. bipunctata is smaller than the average bembicin. You can find a good description of the biology of C. bipunctata at Bug Eric's website.

When Bohart & Menke (1976) prepared their revision of the Sphecidae (which then included the current Crabronidae), they included Clitemnestra in the genus Ochleroptera. Ochleroptera was recognised as closely related to Clitemnestra, but the two genera were distinguished by the shape of the metasoma (the effective abdomen). In Ochleroptera, the first segment of the metasoma is relatively long and narrow, and there is a distinct constriction between it and the remainder of the metasoma (you can see this clearly in the picture above). In Clitemnestra, this segment is shorter and broader, and it is not so divided from the other segments. Because the latter arrangement is the more primitive, Bohart & Menke suggested that Ochleroptera was descended from Clitemnestra, and because both genera were found in both South America and Australasia, they suggested that the two genera had both originated in Australasia and dispersed independently to South America.

Habitus of Clitemnestra noumeae, from Ohl (2002).

However, it was soon realised that the distinction between the two genera was not as clear as had been thought. A number of species were identified in South America in which the segment shape was intermediate: a bit long and narrow for 'Clitemnestra', but not narrow enough for 'Ochleroptera'. As there were no other significant features distinguishing the genera, they were eventually synonymised. Subsequently, Ohl (2002) described another species, Clitemnestra noumeae, from New Caledonia that also has an intermediate metasomal form. Though no formal analysis has yet been done to confirm things one way or another, it seems likely that 'Clitemnestra' and 'Ochleroptera' do not represent independent dispersals between Australasia and the Americas. Instead, 'Ocheroptera' species have probably arisen independently within Clitemnestra on more than one occasion.

To date, Clitemnestra bipunctata is the best studied species in the genus natural history-wise; other species have only been described on sporadic occasions. Clitemnestra species nest in burrows in vertical banks, which they primarily stock with various species of leafhoppers. In the case of one Australian species, C. plomleyi, it was suggested that the burrows it was seen using were not dug by the wasp itself, but had been left behind by beetles or other wasps (Evans & O'Neill 2007). It remains to be seen whether this is typical behaviour for the species, or it may have represented opportunistic behaviour by one enterprising individual.

REFERENCES

Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World: a generic revision. University of California Press.

Evans, H. E., & K. M. O'Neill. 2007. Sand Wasps: Natural History and Behavior. Harvard University Press.

Ohl, M. 2002. A new species of the wasp genus Clitemnestra Spinola, 1851 from New Caledonia (Hymenoptera, Apoidea, Crabronidae, Bembicinae). Deutsche Entomologische Zeitschrift 49 (2): 275-278.

Hunter Balls: The Hydrachna Water-Mites

Hydrachna sp., copyright J. C. Schou.

Hydrachnae are among the most rapacious of living animals, bold, fierce and cruel, the natural and inveterate enemies of all their congenera; they are no less hostile to each other, against which is waged a permanent war of extermination. Neither do they hesitate in attacking such animals as are suitable to their appetites, though double the size of their assailant.

This lurid description was applied to the water-mites by John Graham Dalyell in his 1851 book, The Powers of the Creator displayed in the Creation, or observations on life amidst the various forms of the humbler tribes of animated nature. The water-mites are a diverse group found mostly in fresh waters around the world; Dalyell probably intended the name Hydrachna to cover all water-mites, but modern authors recognise a large number of genera and families in addition to Hydrachna (which is distinctive enough that it is placed in its own family. Whether Hydrachna proper deserves the full force of Dalyell's description may be debatable, but there is no denying that they are predators.

As explained in an earlier post, water-mites belong to a group of mites, variously referred as Parasitengonae, Parasitengonina or some variation thereof (depending where you look), that is characterised by a life cycle including parasitic larvae and predatory adults. In the case of Hydrachna, the adults, which are nearly spherical in shape and bright red in colouration, feed on the eggs of aquatic bugs such as water boatmen or backswimmers that they find attached to submerged plants (so for all his charcterisation of Hydrachna as 'bold, fierce and cruel', Dalyell probably committed no less horrific an act of cruelty to chickens when he sat down to a fried breakfast). Despite their aquatic habits, Hydrachna are only clumsy swimmers themselves. After all, it does not require any great athleticism to hunt down an egg.

Larvae of Hydrachna, copyright Pfliegler Walter.

As well as finding their food on submerged plants, female Hydrachna lay their own eggs in them. They have needle-like chelicerae that they use to cut into the plant's stem, and then lay their eggs in air spaces within the plant cells (up to 1500 at a time: Walter et al. 2009). When the eggs hatch, the emerging larvae (which are kind of rugby ball-shaped) swim in search of a suitable host. Usually, this is a water-bug of much the same sort whose eggs were being devoured by the larvae's parents, though some Hydrachna species have also been recorded parasitising aquatic beetles. While some water-mites are quite picky about where exactly they choose to attach to a host, Hydrachna are not so: they may attach pretty much anywhere. They also do not exclude each other: a single host insect may end up with a large number of Hydrachna larvae attached to it (enough to have a serious impact on the host's health). The palps on either side of the chelicerae are used to initially hold on to the host before the larva cuts into the host cuticle with its chelicerae; once its hold with the chelicerae is firm, the palps are folded out of the way (Redmond & Lanciani 1982). Once attached, feeding on the host's haemolymph may not commence immediately: if the larva has found itself on a host that has not yet reached maturity, it will often wait until after the host has moulted. This is because the feeding larva becomes massively engorged and may swell up to hundreds of times its original size. In this swollen state, it obviously becomes immobile (one cannot walk if one's legs no longer touch the ground); should the host shed its cuticle with the engorged larva attached, the larva would be unable to reattach itself to the host.

After about two weeks of feeding, the larva is ready to mature, but this does not necessarily mean leaving the host. In parasitengonines, the first nymphal instar (the protonymph) after the larval stage is dormant as the mite metamorphoses into something closer to its adult form, the first of the two 'pupal' stages that the mite will go through in its life (the second comes between the active deutonymphal instar and maturity, but involves less of a radical change in morphology). Hydrachna passes this 'pupal' stage while still attached to the host, only detaching when it becomes an active deutonymph. As well as saving the larva the inconvenience (and danger) of dropping off the host while in an engorged state, this helps ensure that the deutonymph emerges in a suitable habitat. Hydrachna prefers still waters, such as ponds and lakes. Some species of Hydrachna prefer to breed in temporary seasonal pools, and may remain attached to the host for several months while their home pools are dry. Somehow they can tell the difference between the temporary pools and the more permanent waters in which the hosts spend the rest of their time.

REFERENCES

Redmond, B. L., & C. A. Lanciani. 1982. Attachment and engorgement of a water mite, Hydrachna virella (Acari: Parasitengona), parasitic on Buenoa scimitra (Hemiptera: Notonectidae). Transactions of the American Microscopical Society 101 (4): 388-394.

Smith, I. M., D. R. Cook & B. P. Smith. 2010. Water mites (Hydrachnidiae) and other arachnids. In: Thorp, J. T., & A. P. Covich (eds) Ecology and Classification of North American Freshwater Invertebrates, pp. 485-586. Academic Press.

Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 233-420. Texas Tech University Press.