Small Carpenters

It's time for another dip into the wide diversity of bees. The small carpenter bees of the tribe Ceratinini are small (often less than a centimetre in length), slender bees found on all continents except Antarctica, though their toehold in Australia is a very tenuous one indeed with only a single known species there. Though diverse, with hundreds of known species, the difficulty of breaking the tribe into clearly defined, monophyletic groups has lead recent authors to recognise a single genus Ceratina (Michener 2007). Distinctive subgroups previously treated as separate genera, such as the relatively large Megaceratina and the heavily punctate Ctenoceratina of Africa, and the both bright metallic and heavily punctate Pithitis of Africa and southern and eastern Asia, are now treated as subgenera. There are a lot of recognised subgenera, over twenty at last count, but there are also a lot of species not yet assigned to subgenus. Phylogenetic analysis of the ceratinins supports monophyly of most subgenera and a likely African origin for the clade as a whole, with multiple dispersals into Eurasia followed by a single dispersal to the Americas (Rehan & Schwarz 2015).

Ceratina sp., possibly C. smaragdula, copyright Vengolis.

Distinctive features of Ceratina compared to other bees include the absence of a pygidial plate, a flattened and hardened patch on the tip of the abdomen in females. As members of the family Apidae, Ceratina are long-tongued bees with a scopa (cluster of pollen-carrying hairs) on the hind legs, though the scopa does not enclose a bare patch for carrying a shaped pollen ball as in some other apids (for instance, the familiar honey bees). The scopa is less extensive in small carpenter bees than it is in other apids and the hairs on the body as a whole are rather short, so Ceratina look much shinier and less fuzzy than other bees. Ceratina are black or metallic green in colour (on rare occasions, the metasoma is red) and usually have yellow patches, particularly on the face.

Ceratina nest in a fennel stem, copyright Gideon Pisanty.

The name 'carpenter bee' refers to their practice of nesting in hollow stems or twigs, entered at broken ends. The absence of the pygidial plate is probably related to this manner of nesting: it is normally used by ground-nesting bees to tamp down soil when closing the nest. Most of the time, Ceratina are solitary nesters but two or more females may sometimes work on a nest together. In these cases, they adopt a proto-eusocial division of labour with one female laying eggs while the others act as 'workers' (I have no idea how they decide who gets to do what). Though a reduction in hairiness in bees is often associated with kleptoparasitism, no Ceratina species are known to behave in that manner (though some kleptoparasites are known among the members of the closely related and very similar tribe Allodapini). The reduction of the scopa may instead be associated with the bees carrying food supplies for the nest in their crop as well as on the legs. Cells are lined up in the nest stem with only simple partitions between them. These partitions are made from loose particles, mostly the pith of the stem, with no obvious adhesive holding them together. In at least some species, females will return to the nest after completion, dissembling and reassembling cell walls in order to clean out dead larvae and faeces that are then incorporated into the partitions. As such, while small carpenter bees are not directly on the evolutionary line leading to the more integrated colonies of the social bees, they do provide us with a model of what one stage in their evolution may have looked like.

REFERENCES

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.

Rehan, S., & M. Schwarz. 2015. A few steps forward and no steps back: long-distance dispersal patterns in small carpenter bees suggest major barriers to back-dispersal. Journal of Biogeography 42: 485–494.

Mooching Off the Relatives

Something I've referred to before but only (I think) in passing is that, among the enormous diversity of bees that inhabit this world, there are a large number of species that act as cleptoparasites*. That is, instead of constructing and provisioning their own nests, they lay their eggs in the nests of other bee species. When the eggs hatch, the emerging larvae feed on the provisions that the constructing bee intended for her own offspring. One lineage of these cleptoparasites is the megachilid genus Coelioxys.

*Depending on the source, you may see this term spelt as either 'cleptoparasite' or 'kleptoparasite'. Personally, I've never been able to decide just which I should be using.

Coelioxys sodalis, copyright jgibbs.

Coelioxys is a diverse, cosmopolitan genus with nearly 500 known species, closely related to the even more diverse leafcutter bees and resin bees of the genus Megachile. Species vary in size from half a centimetre to nearly an inch in length. They are fairly similar to species of Megachile in overall appearance, the most obvious difference being that (as with most cleptoparasitic bees) their covering of hair is greatly reduced. In particular, the dense scopa of hairs that covers the underside of the metasoma in female Megachile is absent. The primary function of the hairs in bees is to carry pollen; with no nest of their own to worry about, cleptoparasitic bees have no need for such dense hairs. Coelioxys females also differ from Megachile in the shape of the metasoma which is tapering and ends in a narrow tip. More on that in a moment.

As might be expected for such a large genus, Coelioxys has been divided between a number of subgenera. Until recently, the definitions of a number of these subgenera was somewhat uncertain. The biggest problem was that most revisions of the genus had been done on a regional level so (for instance) North American taxa were more finely subdivided than in the Old World. However, a recent phylogenetic analysis of the genus by da Rocha Filho & Packer (2017) redefined a number of subgenera and adjusted their definitions. For instance, the type subgenus Coelioxys, recognised as subcosmopolitan by Michener (2007), became restricted to just two species, the European C. quadridentata and the North American C. sodalis. Whereas Michener's concept of Coelioxys was essentially recognised by lacking the specialised features of other subgenera, the restricted Coelioxys sensu stricto can be recognised by having the outer margin of the pronotal lobe conspicuously rounded, as well as having the pilosity on the mesosoma suberect, long and thin, without spots of appressed hairs (da Rocha Filho & Packer 2017).

For the most part, Coelioxys species are cleptoparasites of Megachile though some have also been found mooching off Apidae species. Coelioxys quadridentata, for instance, has been found in association with nests of both Megachile and Anthophora. In most cases, a female Coelioxys will lay into a host nest before it is closed, while the constructor is away foraging for supplies. The narrow metasoma allows the Coelioxys to reach into the cavity containing the nest and insert her eggs into the nest wall where the host will not notice it. Often, multiple eggs will be laid in a single nest. After the nest is closed, the eggs hatch into larvae that look fairly unremarkable for their first one or two instars: like other bee larvae, not doing much more than sit there and eat. But upon reaching the second or third instar, the Coelioxys larva develops greatly enlarged mandibles that it uses to stir through the nest's food mass and execute any other larvae and eggs contained therein. Both the original host larva and any other Coelioxys larvae the nest may contain are dealt with in this manner (presumably the process of finding an appropriate host nest is difficult enough that the waste of eggs is still worth it for the parent Coelioxys to increase the chance that at least one reaches maturity). Its competitors thus removed, the larva them moults back to a more average form with nothing more agin to do but eat until the time to mature is reached.

REFERENCES

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.

Rocha Filho, L. C. da, & L. Packer. 2017. Phylogeny of the cleptoparasitic Megachilini genera Coelioxys and Radoszkowskiana, with the description of six new subgenera in Coelioxys (Hymenoptera: Megachilidae). Zoological Journal of the Linnean Society 180: 354–413.

The Colletinae: Going to Ground

In a recent post, I considered one of the families of short-tongued bees, the Halictidae. In this post, I'll turn my attention to members of one of the other short-tongued bee families, the Colletidae. Specifically, I'm looking at members of the subfamily Colletinae.

Mating ball of male ivy bees Colletes hederae, copyright Charles J. Sharp.

Members of the Colletidae differ from other bee families in that their glossa, the 'tongue' at the end of the proboscis, is apically bilobed or bifurcate. They are also distinctive in lining their nests with a plasticky, cellophane-like material. It has been thought that this material was made from dry saliva but the bulk of it is now known to come from a large gland in the abdomen that opens near the base of the sting (Almeida 2008). A nesting female will swallow droplets of the glandular secretion from her partially protruded sting then regurgitate it as she licks the wall of the nest cell. This waterproof lining both protects the cell from outside elements while preventing the loss of moisture from within. Many colletids, including colletines, leave the cell food provisions in a semi-liquid state; other bees whose nests are less watertight will dry and compact the provisions, presumably because the bulk of them would otherwise be lost before the larva hatched. In colletines, the egg is attached to the cell lining when laid, suspended above the provisions for the hatching larva to swandive into upon emergence. All colletids are solitary nesters with species nesting either in burrows in the ground or in hollows in vegetation; the majority of colletines are ground nesters*.

*One species, Colletes daviesianus, has apparently taken in Germany to boring its nests in the sandstone and mortar used in building construction.

Female Colletes daviesianus, copyright Donald Hobern.

In his 2007 edition of The Bees of the World, Charles Michener recognised five subfamilies within the Colletidae. The Colletinae were distinguished from three of these subfamilies by their retention of a covering of dense hair over the body (from the last subfamily, the Diphaglossinae, they differ in features of the glossa and wing venation). The hind leg of the female bears a well-developed scopa (dense arrangement of hairs for the carrying of pollen) on the femur and tibia with a corbicula (bare patch within the scopa where a ball of compacted pollen may be carried) on the underside of the femur. Hairiness is an ancestral characteristic for bees and phylogenetic studies have established that the Colletinae as recognised by Michener is a paraphyletic grouping (e.g. Almeida & Danforth 2009). As a result, it has been further subdivided with the name 'Colletinae' now restricted to what Michener had recognised as the tribe Colletini. As such, the Colletinae now includes just two genera of moderate-sized bees (seven to sixteen millimetres in length). The larger of these, Colletes, is found in temperate and tropical regions around the world except for the Indo-Australian region where it is notably absent. The other genus, Maurecolletes, is restricted to South America. One of the most distinctive features of Colletinae in the strict sense compared to other ex-colletines is the lack of the basitibial and pygidial plates, flattened and hardened plates possessed by other hairy colletids at the base of the hind tibia and at the end of the abdomen.

The absence of these plates is intriguing in light of the ground-nesting habits that seem to be the norm for Colletes (the nesting habits of Maurecolletes seem to be unknown). In other ground-nesting bees, the basitibial and pygidial plates are used to press the soil of the nest walls and opening into place. One would think this would mitigate against their loss. An explanation may be provided by the fact that some South American Colletes nest in the hollows of dead, pithy plant stems instead of in the ground, a characteristic shared with members of the less hairy colletid subfamilies belonging to the sister group of the colletines (Almeida & Danforth 2009). Ground-nesting Colletes species also bear noteworthy resemblances to stem-nesting colletids. Nest cells are closed with a layer of the cellophane-like wall membrane rather than the earthen plugs used by other ground-nesting bees. In many species, cells are lined up in a burrow divided by transverse partitions rather than placed in their own individual side branches. The possibility has been suggested that stem-nesting arose within the common ancestors of modern colletines and less hairy colletids. Ground-nesting in Colletes would then represent a secondary reversion by these species to the previous habit. When they did so, they retained the adaptations and habits that had originally been associated with their time in the twigs.

REFERENCES

Almeida, E. A. B. 2008. Colletidae nesting biology (Hymenoptera: Apoidea). Apidologie 39: 16–29.

Almeida, E. A. B., & B. N. Danforth. 2009. Phylogeny of colletid bees (Hymenoptera: Colletidae) inferred from four nuclear genes. Molecular Phylogenetics and Evolution 50: 290–309.

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.

The Halictidae: Short Tongues and Waxy Chambers

In an earlier post, I introduced you to the diverse group of bees known as the Halictinae. In this post, I'm going to take a step back and consider the family of bees to which the halictines belong, the Halictidae.

Nomia sp. feeding at a flower, copyright Graham Wise.

The Halictidae are one of the families of what are known as 'short-tongued bees' (the other short-tongued families recognised by Michener, 2007, are the Andrenidae, Colletidae and Stenotritidae). Bees have their mouthparts modified compared to those of other wasps to form a mobile proboscis. The tongue works in three main sections from base to tip. The first two sections work like the upper and lower parts of your arm, or of the arm of a crane, to extend and fold back the proboscis against the underside of the head. The third section beyond these two includes a flexible structure, the glossa, that may be thought of as working like the tongue proper to collect nectar and pollen from the inside of flowers. Somewhat self-explanatorily, this glossa is extremely long and slender in the families of 'long-tongued bees' (the Apidae and Megachilidae) but relatively shorter and broader in short-tongued bees. Naturally, these differences in tongue structure may be reflected in differences in which types of flowers the different types of bees chose to visit. Just to confuse matters, some species of Halictidae may have relatively long proboscides overall, but in this case the extra length is achieved by extending the length of the middle 'arm' section rather than of the glossa itself. The primary features separating Halictidae from the other families of short-tongued bees relate to the structure of particular sclerites incorporated into the proboscis that I'm not going to go into here, but notable points include that the glossa of Halictidae is pointed at the tip and hairs on it are usually branched or bifid at the tips.

Male Halictus tetrazonianellus with proboscis extended (the glossa is the orange structure at the end of the proboscis), copyright Gideon Pisanty.

For the most part, halictids are moderately built bees: neither remarkably slender nor particularly robust. Halictids vary extensively in size: many are small, even minute, but some may be relatively large by bee standards. Coloration is similarly variable, with both metallic and non-metallic species belonging to the family. Members of the genus Nomia (which tend to be relatively large for halictids) often bear contrasting bright bands across the back of the metasoma. Michener (2007) recognised four subfamilies within the halictids: the Rophitinae, Nomiinae, Nomioidinae and Halictinae, with the Halictinae being considerably more diverse species-wise than the other three. Nomioidines have sometimes been included by other authors within the Halictinae but, as there is a general agreement that nomioidines form the sister lineage of the halictines in the strict sense, the question of whether to combine them or not is purely a matter of semantics. Rophitines differ from other halictids in having a relatively large labrum whose tip remains visible between the mandibles when they are closed (other subfamilies have the labrum hidden by the closed mandibles). Rophitines, as well as kleptoparasitic halictines, also have the tip of the labrum simply truncate or rounded; in other subfamilies, the tip of the labrum in females is produced into a distinct process. Rophitines also have the scopa (the array of long pollen-carrying hairs on the hind leg) less developed on the trochanter and femur than on the tibia whereas other subfamilies (excluding, again, kleptoparasitic forms in which the scopa is reduced) generally have the longest scopal hairs on the femur. Nomiines commonly have the third submarginal cell on the wing (if present) as long as the first submarginal cell or at least more than twice the length of the second. In nomioidines and halictines, the third submarginal cell is much shorter. Another notable feature of the last two subfamilies is that the basal vein (the upper of the three veins radiating from the basal midline of the wing) is much more strongly curved near the base than in other bee families; this feature may or may not be discernable in rophitines and nomiines.

Just to show that bees can sometimes get insane: a male of the Colombian species Chlerogella anchicaya, from Engel et al. (2014).

For the most part, halictids construct their nests in burrows in the ground (some halictines nest in rotting wood). Cells of the burrows are generally lined with a wax-like membrane secreted by the parent bee. The membrane is duller and less watertight in Rophitinae than in other subfamilies; one rophitine genus, the southwest North American Protodufourea, appears to not produce such a membrane. Most non-halictine halictids are solitary nesters though some nomiines are known to work communally, and may even show low levels of division of labour. Kleptoparasitism is not known outside the Halictinae.

REFERENCE

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.

The Osmiin Mason Bees

As I'm sure I must have had cause to say before, the world of solitary bees is a spectacularly diverse. Literally tens of thousands of species have been described to date, and no doubt many more remain. The classification of bees was reviewed by in great detail by Charles Michener (2007) in his monumental Bees of the World, and it was there that I turned to learn about the subject of today's post, the osmiins.

Female Osmia ferruginea, copyright Gideon Pisanty.

The Osmiini are currently recognised as a tribe of the Megachilidae, one of the two families of long-tongued bees (the other is the Apidae, including, among others, the majority of social bees). Megachilids are most easily characterised by the position of the scopa, a dense array of hairs used by bees for carrying pollen. In most bees possessing a scopa (it tends to be reduced or lost in kleptoparasitic forms), it is located on the hind legs but in megachilids it covers the underside of the metasoma. Osmiins are distinguished from other megachilids by the combination of a well developed sting, elongate stigma on the fore wing, arolia between the claws, and the lack of a pygidial plate. They are often smaller bees, less than a centimetre in length, though the largest osmiins grow close to two centimetres. Some osmiins are also more or less metallic in coloration, an unusual condition for megachilids. No feature has been identified that is unique to osmiins as a whole and their monophyly relative to other megachilid tribes (particularly the Megachilini) has long been called into question. A number of authors have recognised a division of living osmiins between two subtribes, the Osmiina and Heriadina. Osmiina have generally been distinguished from Heriadina by features such as a smaller stigma in the fore wing, a mesopleuron (a plate forming much of the side of the mesosoma) that is shorter ventrally than dorsally, and a propodeum that generally slopes downward from the base (rather than being initially flat). Again, however, the validity of this division has been questioned as no one feature uniformly distinguishes the two groups. A phylogenetic analysis of the Megachilidae by Gonzalez et al. (2012) did not support monophyly for Osmiini or either of its subtribes, but a proper revision of the group's higher classification remains to be done.

Female Hoplitis parana, copyright Gideon Pisanty.

Like other solitary bees, osmiins nest in cavities (a handful are kleptoparasites that do not construct their own nests). They often do not construct these cavities themselves but occupy pre-existing ones such as abandoned beetle burrows and hollows in wood, or crevices between rocks. Some species of Osmia have a predilection for nesting in empty snail shells. Cells are most commonly demarcated in the nest by walls constructed of chewed leaves, often held together with a sticky substance such as mud, resin or (more rarely) nectar. In some cases, the amount of leaf material used is reduced or abandoned, so the cell walls are made entirely of mud or resin. In some European species of Hoplitis, the cells are lined with petals; the species H. papaveris, for instance, lines its cells with bright red poppy petals. Osmia brevicornis, a species found in southern Europe and central Asia, is unusual in that its nest is not divided into cells. Instead, the nest cavity (an abandoned beetle burrow) is uniformly packed with pollen, with eggs being progressively inserted into the pollen mass as it is laid down. The larvae feed on the pollen around them after they hatch, and cocoons end up randomly scattered through the remains of the mass as they mature.

REFERENCES

Gonzalez, V. H., T. Griswold, C. J. Praz & B. N. Danforth. 2012. Phylogeny of the bee family Megachilidae (Hymenoptera: Apoidea) based on adult morphology. Systematic Entomology 37: 261–286.

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.