Honey Bee Hormones and Metamorphosis

Introduction

Lovely Autumn days are here and the beech leaves are gold and brown. I hear the geese in a nearby field. Hundreds of them are flying in daily from the Arctic and when they’re disturbed, they rise up in black clouds with a deafening commotion and settle again quickly a little further away.  The temperatures are low now and, although pollen is still coming in thick and fast from Himalayan balsam and ivy, it’s too cold at 10°C for nectar secretion today, even when the sun is out.

All the colonies are dropping their brood nest temperatures as egg laying reduces. There are still probably lots of brood in the strongest colony which headed by a new queen and I marvel about how it all works inside a hive. Not just about how the bees themselves conduct business and shut down production for winter and batten down the hatches and seal things with propolis; the whole hidden dance of hormones and chemicals that controls a bee body, as they indeed do for us humans too, is intriguing.

Complete or Incomplete?

Have you ever wondered what makes a smooth fat white writhing honey bee larva transform into a hairy flying machine? It’s all to do with genes, hormones, brains and feedback loops. Honey bees, like all hymenoptera, and many other insects, undergo complete metamorphosis which is a series of developmental stages that completely transforms the animal so that a newly emerged mature bee (imago) looks nothing like it did at the start of life. The life stages are characterised by a larval stage when terms like maggot, grub or caterpillar might be used by some. Then there is a pupal stage. Larvae are just really eating machines that can pack away a lot of brood food and grow quickly, which they have to. Queen bees are a good example of fast growth since they emerge only 16 days after an egg is laid. A worker bee takes 21 days and a drone 24 from egg to emergence. If you are serious about managing honey bees and rearing queens you need to know all about development times. You also need to be able to answer exam questions if you decide to follow the education path. Numbers and I do not get along so this is what I made up to remember them when I started beekeeping― a queen-sweet 16, a worker- 21 key to the door, and a drone drones on for 24. Metamorphosis takes place in the pupal stage which we will examine soon.

Many other insects like termites, earwigs and dragonflies undergo incomplete metamorphosis which is a process in which the young resemble the adults but they are just a bit smaller and grow into each new body through moulting off the old one with the help of hormones.

Endocrine System

We discussed hormones in the article about winter bees some time ago and the role of juvenile hormone (JH) in adult bees. Now we look at JH and the endocrine system in larval development.

The endocrine system is very closely related to the nervous system and many of the endocrine organs and tissues are ductless glands comprising specialised nerve cells called neurosecretory cells that secrete hormones directly into the bloodstream. Exocrine glands on the other hand always secrete hormones and chemical signals to the outside of the body via ducts, and the venom gland is a good example of one.

With the help of AI, and Louise, I’ve made a chart to explain the dynamics of honey bee development for those who like visual aids to help grasp concepts. I’ve discovered that you need to understand what you want to produce before asking AI for any assistance otherwise what is created will probably be inaccurate.

 Before we get to the chart, there are a couple of things to explain. After a honey bee egg hatches around day three, the growing insect becomes a larva and has four moults or ecdysis when it sheds its ‘skin’ and there are very few visible external changes other than a larger larva. However, after the fourth larval moult there is a growth spurt and the larva gains 40% more weight before receiving its last meal around two days post-moult.  The cell is sealed and in this prepupal stage all is outwardly quiet and the larva lies still; but inside a lot is happening and the connection between the ventriculus and hind-gut opens up and the gut starts functioning. The kidneys (Malpighian tubules) open into the hind-gut and excretions are expelled into the cell. The larva spins a cocoon from secretions in the silk glands and its head faces the cell cap. A lot of remodelling goes on with changes to the head, mouthparts, thoracic segments, and there is some sting gland development. This change of form is called metamorphosis.

Then comes the fifth moult between prepupal and pupal stages when the pupa is shown with all the external features of an adult apart from wings which are just small bags held tightly against the sides of the thorax. The pupal stage lasts about 8 days in a worker and during this time the white pupa takes on colour gradually starting with the eyes which become pink then change to purple and eventually brown as the bee body slowly tans to a deepening brown and its adult colour.

Finally, there is the 6^th moult between the pupal and imago (formed adult bee) stages. You will come across the word ‘instar’ in your studies and this is simply the growth stage between the moults and so there are eight instars. There are slight variations in the length of instars between queen, worker and drone, and fluctuations in temperature can also cause variations in growth periods.

There are three hormones that we need to concern ourselves with for an understanding of the process of insect development and these are ecdysone, or moulting hormone, brain hormone or prothoracicotropic hormone (produced in the brain by neurosecretory cells) which promotes development by stimulating the prothoracic glands to produce ecdysone, and juvenile hormone (formerly called neotenin) which is secreted by the corpora allata (singular= allatum), which are a small pair of glands positioned just behind the brain.

The bee brain is tiny but powerful when you consider how essential it is for normal bee development. The insect inherits genes that govern the growth and characteristics of each stage; larva, pupa and imago, but it is the hormones that regulate the actions of these genes. The brain hormone is the conductor of this orchestra and the two ‘see-saw’ hormones are ecdysone and juvenile hormone; so, in simple terms, when one is high the other is generally lower and vice versa. In the early larval days, JH levels are high which keeps the larva at the growing stage between moults. Ecdysone levels are low at this time to prevent moulting till the time is right. The chart above should make it easier to understand what goes on and how the feedback loop works. When hormones levels are high the brain is inhibited and stops releasing brain hormone.

You can watch Annand’s amazing film of honey bee development here and I hope that this helps you if you are taking exams in November.  https://www.youtube.com/watch?v=f6mJ7e5YmnE