Early life for bees: egg and larva
A queen bee indulges in many strange behaviors, not the least of which is choosing the sex of her offspring. Of course, this surreal proclivity is shared throughout the Hymenoptera, a fact that doesn’t make it any less mysterious.
But even more perplexing than the deciding is implementing. How does the queen fulfill her desire to lay a fertilized or unfertilized egg? We mere mortals have no idea what that might feel like. Does she squirm one way or another to make it happen? Or does she click her tarsal claws and make a wish? Perhaps, as some have speculated, the queen simply tucks her abdomen into a certain position to get the desired result.
Since the question of implementation is above my pay grade, I will skip it entirely and describe the first days of a worker bee’s life beginning immediately after the sex decision is made, at the point where the fertilized egg leaves the queen’s body to reside within a sweet-smelling waxen chamber. The queen carefully centers her egg in a brood cell and glues the end to the floor, so the entire egg stands as vertical as the Washington Monument.
Honey bee queens use an assortment of pheromones to achieve colony goals, and “egg-marking signal” illustrates her control. As an egg leaves her body, the queen coats it with a compound that lets her workers know that she alone laid that particular egg. The coating contains chemical odors that laying workers cannot produce, thus enabling police bees to distinguish between genuine queen-laid eggs and imposter eggs that laying workers sometimes deposit in the brood nest.
Once they are discovered, renegade worker eggs are eaten by the egg police, an act which conserves nutrients within the colony and assures that workers — not drones — develop in worker cells. Furthermore, it assures that any drones that are raised carry the queen’s genetics exclusively. Once her scent-marked egg is placed, the queen wanders off to the next available cell, confident her message is clear.
Many tiny eggs
Honey bee eggs are often described as rice-like, but they are much smaller than typical grains of rice, measuring anywhere from 1.2 to 1.8 mm long by 0.4 mm wide, and weighing from 0.12 to 0.22 mg. Some of the size distribution is due to genetic differences among queens, and some are just normal same-queen variation. Compared to bee eggs in general, honey bee eggs are unusually small, a situation made possible by a staff of nurse bees that continually feed the larvae as they grow.
The worker bee egg is soft and covered with a flexible covering called the chorion. The egg contains the original fertilized ovum along with a yolk. Inside the egg, the cells begin to divide and differentiate into various tissue types, including the embryo. As the embryo develops, the egg begins to curve and sag, eventually tipping over and coming to rest on its side. Just before hatching time, the shell layers become more transparent, and a tracheal network becomes visible as white lines against a graying background.
The shell dissolves
While most insects rupture the egg's outer covering to emerge, honey bees do not. In a process called eclosion, the chorion is dissolved and absorbed by the developing bee, meaning you will never find a pile of eggshells on the floor of a brood cell. Immediately before hatch, fluid oozes from splits along the larva's dorsal midline and coats the outside of the egg. The fluid begins to dissolve the chorion and, as it disappears, the larval segments become apparent. This type of system, where nutrients are preserved and reused whenever possible, is typical of honey bee colonies.
Hatching generally occurs after three days, but embryo development within the egg is temperature-dependent, so the actual time until hatch can be anywhere from 48 to 144 hours. During that time, the egg loses about 30% of its initial weight.
Small yolks and progressive feeding
Because the newly-hatched honey bee larva will be fed continuously and intensively until pupation, it doesn’t need to be very big at hatching time, which means the yolk doesn’t need to provide large quantities of nourishment. This type of feeding regimen is called progressive feeding and is similar to the way we feed our children: constantly.
In comparison to honey bees, most solitary bee larvae are fed only once. The mother bee collects pollen, mixes in a bit of nectar, and shapes it into a “pollen loaf ball.” On top of the fresh pollen loaf, she lays an egg and then splits, leaving the offspring to fend for itself. This type of feeding is called mass provisioning. Because the lonely pollen loaf is the only food the larval bee will receive between hatch and pupation, a head start in the form of a large and nutrient-rich yolk is important to survival.
Throughout the bee families, egg size is often inversely proportional to the number of eggs a female bee lays. For example, most female carpenter bees lay very large eggs — some over a half-inch long — but she may only lay ten of these monsters in her lifetime. In comparison, a honey bee queen may lay 2000 eggs per day in peak season.
The open larval stage
The second stage of honey bee development begins as the chorion melts away to expose the white, legless grub known as a larva. The larval stage is the eating stage, the only stage before adulthood where food is taken in from outside its body. In contrast, during the egg and pupal stages, the bee lives on stored nutrients in the yolk and in fat bodies respectively, but nothing new is brought in from the outside.
Eating is what larvae do best. Like stand-alone digestive machines, a larva’s entire body is designed to consume and process vast quantities of food. Their bodies consist of a mouth, salivary glands, a mid- and hindgut, and an intestine with no exit. They have a few other parts, including spiracles for breathing and silk glands for spinning a cocoon, but otherwise, they are one-trick ponies that do nothing but munch.
Feeding the kids
During the first two days of a larva’s life, it eats a rich, two-part diet tendered by the nurse bees. The little white crescent-shaped grubs literally float in an ever-replenished pool of brood food. Most of the meal comprises a clear liquid secreted by the nurse bee’s hypopharyngeal glands. The second component is a milky-looking liquid from the mandibular glands.
Depending on conditions, workers may visit each brood cell from many hundreds to over a thousand times per day. The worker inspects the food supply to see if more is required. If all is well, the worker moves on. If not, she tops it off with secretions from her glands.
By the third day, the mandibular portion is dropped from larvae's diet destined to be workers. After that, around day five, the clear hypopharyngeal portion of their diet is supplemented with pollen and honey. Throughout the entire larval stage, the eating doesn’t stop, and the tremendous amount of food needed to grow a larva keeps the workers as busy as, well, bees.
Molts and instars
It is difficult to visualize the amount of growth that occurs during the larval stage. Within the space of about five-and-a-half days, the bee grows from a barely visible C-shaped shadow lying in a pool of royal jelly, to fat and healthy grub. Some entomologists have measured a 1500-fold weight increase during the larval stage. In comparison, if your eight-pound newborn baby girl gained weight at the same rate, she would weigh 12,000 pounds just six days later. Imagine the diapers!
To accommodate its rapidly expanding girth, the developing bee undergoes a series of molts, shedding its skin six times before emerging as an adult. The first molt occurs at hatch (eclosion) as the bee absorbs her protective coating, the chorion. The next three molts occur almost daily as the larva grows at an alarming rate.
The final two molts occur a bit later. The fifth molt is postponed until after the cell is capped, and the bee is completing the prepupal stage at about day eleven. The sixth and final molt occurs when the bee emerges as an adult worker, somewhere around 20 or 21 days.
The stretch of time between molts is called an instar, so just after the bee hatches, she begins her first instar of development. Since a honey bee has a total of six molts, she will have five instars before she emerges as an adult worker, of which four are larval instars. Both queens and drones also have six molts and five instars, although the instars differ in length from those of the worker bees. On average, queens require only 16 days to mature, while drones have a more leisurely 24-day schedule.
Timing and terminology
It is generally accepted that worker bee development requires approximately 20 to 21 days. However, this is an average, and the actual length of time depends on genetics, temperature, nutrition, and perhaps other environmental factors. The eating stage lasts 5.5 days or six days, but a convenient memory device, especially for beginners, is doubling the three-day egg stage to get the 6-day larval stage and doubling that to get the 12-day pupal stage. Add those three together (3+6+12) and you get 21. It’s a nice neat package that’s very close to reality.
However, what we can see and what is occurring biologically are two different things. The six days a larva spends in an open brood cell is only part of the larval phase. The complete larval stage spans about eight days. The last 2 to 2.5 days, often called the prepupal stage occurs under a capping.
Normally, we think of the prepupal phase as part of the pupal stage, but the word “prepupal,” meaning “before the pupal,” tells you otherwise. Since there is nothing directly before the pupal stage other than the larval stage, the prepupa is actually a larva.
In The Biology of the Honey Bee, Mark Winston writes, “This last larval stage is referred to as the prepupal stage . . . The development time for larvae is usually considered the duration of the unsealed larval period since this is easiest to observe.” Technically, the larval stage ends at the fifth molt when the larva metamorphoses into a pupa. This fifth molt occurs at the end of the 11th day after three days spent as an egg and about eight spent as a larva.
Sometimes, in our zeal to make things sound simple, we obfuscate the science. Unfortunately, misnaming stuff because they are the “easiest to observe” can be confusing. The important thing for beekeepers to understand is that the larval stage does not end when the brood cell is capped. Instead, it makes sense to think of the six-day eating period that occurs between hatching and capping as the “open larval” stage and the two-day capped prepupal stage as the “capped larval” stage.
By adding the open larval and capped larval stages together, you get something closer to 8 days, which gives you approximately 3+8+10 for a 21 day cycle, or perhaps a bit shorter, closer to 20, when you consider normal variations in development time.
The intestine as a closed system
If you recall, the larval digestive system has a closed intestine. Because of that, all the waste the larva generates is stored in the intestine with no place to go. Although it seems odd and perhaps a bit uncomfortable, this biological system is vital to honey bee health.
Because larvae have no legs and feet, they cannot travel from their natal cell to find facilities. But any excrement they passed at home would simply remain in the cell with them, contaminating their food and living quarters. But larval bees never contaminate their homes — either on purpose or by accident — because they can’t. Until the end of the larval period, the part of the digestive system that eats is not connected to the part that excretes.
Imagine the digestive system as a series of tubes, each one connected to the next. In a fully-formed adult bee, the mouth connects to the alimentary canal which connects to the midgut where most digestion and nutrient absorption occur. From there, the waste travels to the hindgut where it collects before being excreted.
But because the connection between the midgut and the hindgut is missing in the larval stage, the feces merely accumulates in an ever-expanding midgut. Then, during the capped larval (or prepupal) stage just before the cocoon is spun, the two sections of the digestive tract grow together, and suddenly the waste can travel the rest of the way through the bee.
Once the feces is expelled, the bee enters the “defecated larval” stage and cocoon spinning begins. The honey bee waste is captured within the cocoon's strands and woven into it, a system that will keep the feces from contaminating the pupating bee. And because the cell is already capped, no more food can come in.
A failsafe system
This clever method of sanitation is not unique to honey bees. Similar systems are used by nearly all bee species, including those that live in tubes or in underground cavities. Since the food supply must be kept clean and nutritious, preventing contact with biological waste is an essential part of life in a cell.
The last-step larva
To spin a cocoon, the capped larva completes a series of somersaults while secreting silken fibers from her spinneret, an opening found between the two larval maxillae. Over and over she tumbles until she is completely encased. Once finished, she stretches out the full length of her cell, ending with her head at the top so she will be in position for emergence as an adult.
Inside the cocoon, the bee develops by using part of the fat she stored during the eating phase and, except for the wings, recognizable parts of an adult bee gradually become visible. At the end of the fourth larval instar — at about day 11 — the worker bee will molt for the fifth time and become a fully-formed pupa.
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This article first appeared in American Bee Journal, Volume 160 No 5, May 2020, pp. 509-511.