Microstigmus

The thin-waisted social wasps (genus Microstigmus), which typically resides in neotropic regions from Central to South America, are a small genus of wasps that build nests and live in colonies ranging in size from 1 to 18 members.^[1] Microstigmus is widely considered to be the only true eusocial species within the Crabronidae family.^[2] Like all Hymenoptera, Microstigmus has an interesting sex determination pattern. Females are 2n (diploid), spawning from eggs that have been fertilized, while males are 1n (haploid) and spawn from unfertilized eggs. Female wasps contain the eggs within their egg sac and have the option of fertilizing them or not, thus having complete control on the gender of the offspring.^[3] This wasp genus, part of the suborder Apocrita, is distinguished physically by the narrow waist (petiole) between the end segment of the thorax (mesosoma) and the beginning of the abdomen (metasoma). Specifically, it is in the subgroup of "apoid wasps", those that exhibit social behavior in nesting and foraging. Evidence does exist that in some species social altruism is visible; however, studies on these topics have been limited. The small size and enclosed nest structure makes it difficult to obtain significant data. Although it was originally classified under the Sphecidae family due to its elongated petiole, Microstigmus has been reclassified into the new Crabronidae family.

Biology

Like all insects, the wasps of this genus have three body segments, with an exoskeleton covering the internal organs in each of them. The species of Microstigmus ranges in length from 2.5 to 5.5 millimetres (0.098 to 0.217 in).^[4] Being within the family Crabronidae, Microstigmus has a thread like waist connected between the thorax and abdomen. Unlike other Hymenoptera, the head and thorax (mesosoma) lack branched or plumose hairs.^[5] The wasps also have their posterior metatarsus modified into a cleaning mechanism.^[6] Many of the different species in this family have varying color schemes that serve to warn predators (some have venomous stings) or to provide camouflage.

Sphecid wasps also have a silk gland that is important in many functions, such as nest building. The glands are formed initially with one multinuclear terminal cell from which a canal (pathway from gland to environment) is exposed to the outside environment via spinnerets. This epidermal gland evolves independently for most species under the Pemphredoninae subfamily.^[7]

Diet

As with most wasp species, those in the Microstigmus genus are predators. Prey for the wasps can vary from flies to aphids. Most of the time, the kill is performed with the powerful mandibles literally biting through the head of the prey causing either death or immobilization.^[3] Social wasps will kill the prey and then return to the nest to provide for the young. This can be done in several ways. One is for the wasp to suck out the inner bodily fluids of the prey and carry the hemolymph back to the nest where the adults will regurgitate some of the fluids to the larva wherein the adult retains some of the hemolymph.^[8] This trophallaxis between adults and larva may have played a role in social wasp evolution in three primary ways. Adults who provide nourishment for larva and get food in return may reduce the need to forage for food for themselves. Therefore, flights from the nests may be more efficient where flights are only performed for larval food foraging and nest construction materials over species that do not practice trophallaxis.^[8] The larval-adult saliva exchange may also increase reproductive fitness in females. An example would be a period of inclement weather where food is hard to come by. The reproducing females would still have a source of nutrition and be able to continue reproductive cycles which are crucial in eusocial colonies. There might also be a situation where the trophallaxis benefits the adults but also hinders the larva in form of nutritional castration where the lack of food results in underdeveloped reproductive organs thus leading to the creation of a worker class.^[8]

Nesting

Nests generally have a rough external appearance with a brownish-gray hue although some species have a whitish appearance with smooth texture. Species usually form communal nests that hang from a strong support such as a tree branch, vine, or overhanging rock formation via a petiole which is produced from a root tip rather than silk. The nests are typically built with substrate surface material such as bark, lichens, and sand grains interlaced with silky secretion produced by the female wasps.^[9] The nests are developed into conical structures approximately 12 mm in length. Most of the infant holding cells are the near the bottom and the adults reside in the upper hollow section of the next close to the entrance. The interior of the entrance is covered by a translucent coating.^[10] In most instances the beginnings of the nest are created all by a single queen wasp that decides to spawn more workers once the nest has reached a certain size (generally the size of a walnut). While most social insects build their nests over time gradually increasing in size, the Microstigmus construct the entire confines of the nest in one effort and thus the workers and queen are confined to the nest until it is fully completed.^[11] As such all of the construction materials are gathered prior to the building of the nest as seen with Microstigmus comes.^[9]

Behavior

Larva interaction

As Microstigmus is a social genus that lives together in a colony, larval care is provided by adults. In the Microstigmus thysanoptera, the adults form a compact mass of prey provisions and silk fibers which, upon hatching, the larva consumes.^[9] The fecal matter produced by the larva that have molted is removed from the nest along with other trash by the adults performing housekeeping duties.^[9] Larval incubation habits indicate the sociality among wasps, as females will routinely bring food to the same incubation cell in a cooperative effort.^[10] Social divisions also reveal the specialization of roles and thus how each individual contributes to the overall colony. No two eggs are at the same stage of development and only one egg is available for care at any given time. This is supported by the fact that there is only one female with significantly larger ovaries over the other females at any given time and this select female is considered the primary egg layer.^[11]

The biology/anatomy of the larva has unique characteristics (associated only with Microstigmus) that reveal evolutionary features. One is the concial supranal process which is potentially an adaptation for obtaining the prey provisions stuck on the walls of a nursery cell. During the larval stage, there also is a complete lack of spinnerets which indicate that a pupal stage with a cocoon is not present in some species of Microstigmus (M. xylicola nests have been found with pupa present).^[4] The spinulose lobe of the mandible is not like most and is a possible modification for ingestion of collembolans.^[12] Other aspects of Microstigmus biology are shared with other genera in the Crabronidae family such as antennal papillae, lack of galeae, and tridentate mandibles which indicate close relation to Spilomena and distant from Ammoplanus. While these characteristics presently differentiate the genera, they may change in future as more specimens are being collected for study.^[12]

Adult/nest interaction

A typical worker female exhibits repetition of select behaviors, mainly with a purpose of sustaining the colony. They would forage for food for the young which can range from decaying dead insects and bodily fluids of paralyzed insects to decaying fruit. Adults usually feed off nectar as their primary source of nourishment but in some colonies such as M. nigrophthalmus the adults feed off the secretion of the larva (a form of defecation). Adults deliver the food to the young through a process called trophallaxis. Adults also inspect the nest, clean by ejecting excess material, and silk which is reinforcement of the nest structure through adding silk to the petiole. More rarely, individuals would partake in nest defense, usually from small arthropods, and mate where a female and a male would exit the nest joined at the genitalia.^[3]

Before leaving the nest, the adults generally make an inspection walk around the nest and will also perform the same task upon returning, generally to make sure the nest is in good order and not damaged in any way.^[9] Another potential reason for the inspection routine could be to check for parasites around the nest. Along with the inspections, adults also perform the maintenance routes where the wasps remove particles and waste and check the integrity of the nest. Females occasionally will apply silk to reinforce the nest surface, pedicel and attachment point between the leaf and pedical.^[9]

Eusociality

In a social context, in colonies of wasps with more than one adult, one adult would almost always remain at the nest site while the others would go out and forage for food or construction materials.^[10] Nest defense are also done together with most of the females congregating around the nest during an attack.^[10]

Continuing the idea of eusociality, it is important for social insects to be able to discriminate against non-nest members. However, in the Thripoctenus species, tolerance of non-related individuals was observed, and may have some social benefit. One hypothesis and is that in a solitary nest where more members can provide more work, the additional help is kept around in order to benefit the overall fitness of the colony. A second hypothesis is that additional members would increase the longevity of the nest as it is theorized that each wasp is only capable of so much silk production which is the key building block in nest construction.^[9]

Kin selection based on Hamiliton’s Rule provides an explanation to the population genetics framework and specifies the patterns of local and family genetic makeup that results in the most favorable outcome. The key to this is that the genetic structure spawns nonrandom genotypes which coupled with differential reproduction makes kin selection possible.^[13] Inter-nest relatedness was high resulting in few independent haploid genomes which makes possible the development of a social hierarchy.^[13]

Relatedness

Between the females within a nest, relatedness was very high and contributes to altruistic behavior in an effort to support the survivability of the brood regardless whether the brood is one’s own offspring or not. Additionally, the nest has a group effort toward raising brood members due to the high cost of constructing where females have to invest, time, effort, and energy in order to produce and maintain a nest. Relatedness between individuals drastically decreases over a few generations and as such, mating in Microstigmus colonies tend to be low to the point where it is not uncommon to see a nest with only one female with fully functioning ovaries. Reproduction within Microstigmus nests from various subspecies where some has a queen-worker divide and the queen's sole purpose is to reproduce, while others have select females rear offspring.^[3]

References