Nocturnal behavior and vision

All species of Megalopta are believed to be nocturnal or crepuscular.^[4] In general, Megalopta exhibit bimodal foraging patterns and are active for about 90 minutes after the sun sets and before the sun rises,^[8] regardless of food availability.^[9] Males and females are active at slightly different times and the females usually have higher overall rates of activity.^[8] This is hypothesized to be a niche shift to avoid competition from diurnal bees, or perhaps to avoid predation^[9]

Insects generally have a pair of compound eyes accompanied by a trio of single-lensed ocelli on the top of their head.^[10] While most nocturnal insects have superposition compound eyes, all hymenoptera have apposition eyes that are less sensitive to light than superposition eyes. Megalopta's apposition eyes are thirty times more sensitive to light than diurnal bees, but even this does not explain the accuracy of their vision.^[11] They may use their ocelli to supplement their night vision as well. Crepuscular bees have larger ocelli than diurnal bees, and nocturnal bees have the largest ocelli of all.^[12] Ocelli are not used for precise visual information, but can be used for other uses of visual information. Megalopta ocelli are highly adapted for sensitivity. They have large ocellar lenses and the percent area of the retina covered by the rhabdom is five times higher than diurnal bees.^[10] However, the adaptation to sensitivity most likely came at the cost of temporal resolution. One theory is they use neural spatial summation strategies to enhance their temporal resolution.^[13] Their rhabdom are elongated, with microvilli aligned in the same direction which potentially allows them to see the direction of polarized light.^[10] Megalopta ocelli have a dorsal rim, where the photoreceptors are sensitive to polarized light, such as from the rising or setting sun. Sunset and sunrise create unique polarized light patterns in the sky, and Megalopta may be using these light patterns to navigate.^[14] Despite having the superficial anatomy for polarized light directionality detection, the rhabdom are "worm-shaped" and arranged radially, which negates this ability.^[10] Meglopta also lack pigments in their ocelli that detect short-wavelength light, perhaps a result of their jungle habitat.^[10]

Nesting and the female hierarchy

Megalopta nest in dead branches, sticks, and vines. Their nests have a distinctive collar around the entrance made of chewed wood and consist of a large hollow tunnel and several cells.^[4] The internal cells are made of the same chewed wood as the entrance. Megalopta are facultatively eusocial.^[2] Their nests range from single female nests up to eleven female nests, but social nests usually contain 2-4 females.^[15] As the dry season progresses, single female nests gradually shift into multiple female nests, a result of females hatching and remaining in the nest.^[4] Usually, at least one female offspring stays in the nest and does not reproduce or grow ovaries. She instead maintains the nest and forages while the foundress continues to reproduce.^[16] Eusocial communities have an overlap of generations of nest-mates, meaning that two or more broods must be laid and hatch during the mating season.^[17] Eggs are usually laid during the dry season and hatch after 35 days.^[4][17] Eggs laid in a drier dry season usually produce numerically more, larger individuals that are involved in reproduction, as opposed to a comparatively wet dry season, which produces fewer, smaller workers.^[17] The association between reproduction and the dry season is likely a result of floral availability.^[1] The benefits of solitary versus eusocial lifestyles change with the seasons. In the dry season, eusocial lifestyles provide productivity benefits while in the wet season, the benefits are typically insurance-based.^[18]

In a multi-female hive, there is typically one dominant female and the rest are considered supernumerary.^[17] The dominant female usually is macrocephalic and the oldest female in the hive.^[3] The dominant female is analogous to the queen in honeybee hives, and can also be called the queen or the foundress. The supernumerary females fall into two roles: foragers and in-nest. The in-nest females generally stay in the nest and guard the entrance, as well as hygiene-oriented tasks.^[15] Halictinae are mass-provisional, meaning they cache the food necessary for larval growth before the eggs are laid and do not interact with larvae during development.^[16] However, experimentally, foundresses have displayed the ability to identify issues within a sealed cell. The foundresses opened the cell to assess the condition of the larvae and consequently either found something wrong, aborted the larvae, and reused the cell, or found nothing wrong and resealed the cell. The larvae in the resealed cells developed normally.^[16] Another parental behavior performed by foundresses is hygienic-based: the removal of faeces from cells that produced healthy adults.^[16] These deviations from mass-provisional behaviors are thought to be a precursor to eusociality.^[16]

Gynandromorphy

Gynandromorphy is the presence of both female and male characteristics in a single organism. Two species of Megalopta have exhibited gynandromorphy: Megalopta amoena and the heavily studied Megalopta genalis. Both specimens had a bilateral split with male characteristics on the left and female characteristics on the right. In comparison to male or female individuals who have bimodal foraging periods, the gynandromorph's activity was shifted significantly earlier in the day.^[8] It is important to note that this study was based on a single specimen and generalizations about gynandromorphy in Megalopta cannot be made. Furthermore, this mutation is extremely rare, with only two specimens having been found in the entire genus.

Interactions with other species

Macrosiagon gracilis is a parasite of M. genalis and M.ecuadoria.^[19] Megalopta have lower rates of brood parasitism than other solitary bees. Significant brood parasites include Lophostigma cincta, a mutilid wasp, and cleptoparasitic Megalopta species.^[9] In many dead nests, slits are found in the stick and the cells are destroyed. This is thought to be the work of the silky anteater, Cyclopes didactylus, but is unconfirmed.^[4] Females also often have large numbers of nematodes in their metasomal glands.^[4] The larvae of a parasitic fly, Fiebrigella sp., consume the pollen stores in Megalopta cells, causing the larvae inhabiting those cells to have a smaller body size.^[15] Parkia velutina is pollinated by Megalopta^[7]

Cleptoparasitism

Cogener cleptoparasitism has been observed in the subgenus Noctoraptor. This means that these bees parasitize other Megalopta species. M. byroni parasitizes M. genalis and M. ecuadoria, but most likely parasitizes other species^[4]. In the case of Noctoraptor, cleptoparasitism takes the form of brood parasitism. Noctoraptor do not have the physiology to build nests or cells, making them obligate parasites that invade the nests of the other Megalopta to lay eggs.

List of species

Species groups^[6]