Thorax of a Bee

Muscles and Movement: The Midsection of Insects

The thorax contains structures essential for locomotion and respiration, namely the six legs, the four wings, flight muscles and (part of) the tracheae. It is divided into three segments, each associated with a pair of legs: prothorax (closer to the head), mesothorax (middle), and metathorax (closer to the abdomen). A wing pair is located on the mesothorax and metathorax, and this region is enlarged in bees (relative to other insects) to accommodate their large flight muscles.

A non-Technical Summary of the Honey Bee Thorax

The thoracic structure is largely unchanged between the three honey bee castes. A summary is provided below.

The Honey Bee Legs are for Movement and Manipulation

Adult bees, as with other insects, have three pairs of legs, and each pair has further specialised functions. In general, all legs perform the basic functions of body movement and weight support, as well as holding objects.

• The front legs of honey bees contain notches filled with stiff hairs that serve as antennae cleaners. As pollen accumulates on the bee’s body, forelegs brush it away from the eyes and antennae, whilst the middle and hind legs pack that pollen into the pollen baskets (Amador et al., 2017).

• The middle legs located on the mesothorax have no specialized functions – it contributes to movement, holding objects and supporting the bee’s weight.

• The hind legs are enlarged to accommodate the pollen basket for collecting pollen (elaborated below).

A Pollen Basket on the Hind Leg

The corbiculae are designed for carrying pollen that adheres to the worker bees’ hairy body while foraging (a mechanism thought to be mediated by static electricity generated between the pollen grains and body hairs; Koh & Robert, 2020). The front and middle legs comb the pollen grains towards the hind legs, where it is mixed with a small amount of nectar to create a moist pellet. This mixing makes the pollen stickier and easier to pack into the pollen baskets so it does not fall during flight.

Drone and queen bees do not have pollen baskets, for they do not participate in foraging activities.

Other Methods of Pollen Collection

Certain native & solitary bees carry pollen grains on their abdomen, or internally in their stomach. These adaptations can assist bees in several ways, with one such example being from Mountcastle & colleagues, showing that a bumblebees pollen load in their crop, they are more stable in flying than a nectar load (Mountcastle et al., 2015).

Female Megachilidae (leafcutter) bees carry pollen on their abdomen instead of their legs (Muller & Bansac provide an example using Megachile pilicrus; Muller & Bansac, 2003 ). These solitary bees would harvest pollen from Asteraceae flowers, and then brush the pollen from their legs into the abdomen.

Yellow-faced bees from the genus Hylaeus lack the hairs (scopa) that distinguish bees from other insects. They ingest the pollen from the flowers and transport it in their stomach, as other bees would with nectar (Muller, 2023). The pollen is regurgitated at the nest and added to the stockpile of larvae food.

How Much Nectar and Pollen do Worker Bees Carry?

Bees are selective in their foraging and will visit the same variety of flowers during a trip. Flowers are selected on the basis of cues like colour, scent, and nutritional content of pollen (Di Pasquale et al., 2013). For a 100 milligram worker bee, the pollen load ranges at 20-30% of its mass, (20-30 milligrams; Feuerbacher et al., 2003), with a maximum nectar and pollen load of 80% (80 milligrams). Up to 50 milligrams of nectar can be carried in the honey bee stomach. Overall, the fully loaded forager carries 180 milligrams, 80% of its body weight… not bad at all!

More Details on the Honey Bee Legs

The individual honey bee leg has five segments:

• Coxa: The first segment attaching the leg to the thorax.
• Trochanter: The second segment, acting as a joint between the coxa and femur.
• Femur: The third segment, which is thick and strong.
• Tibia: The fourth segment, featuring antennae cleaners in the front legs.
  • In the hind leg, the tibia is enlarged and flattened to form the pollen basket with long, curved hairs for brushing pollen into them.
• Tarsus: The final segment, divided into five subsegments (tarsomeres) ending in claws and a pad (arolium).
  • The claws enable bees to grip things, whereas the arolium is a pad located between them for adhering to smooth surfaces (leaves, hive walls or glass; ).
    • The two structures enable bees to walk and balance on any surface without sacrificing function (eg: loss of claws and reduced ability to manipulate objects).

Four Wings for Flight

The forewings and hindwings are attached to the mesothorax and metathorax respectively. They are powered by indirect and direct flight muscles, which control the fine and fast movements during flight. The indirect muscles alter the shape of the thorax, causing the entire wing to move, while the direct flight muscles control the wing angle, enabling them to maintain the desired coordination (Iwamoto, 2011).

There are two primary indirect muscles:

• Dorsal longitudinal muscle, deforms the thorax and produces the downstroke of the wings.
• The Dorsal ventral muscle does the opposite, generating the upstroke movement of wings with no thoracic deformation.

The direct flight muscles are numerous and attach at the wing base, controlling the fine movement of the wings, allowing bees to conduct actions like hovering (Iwamoto, 2011).

The wings are highly vascularized. Veins running across their structure contain trachea, blood vessels and nerve fibers. All insects are thought to share the same framework of eight “core veins”, and classifications between similar-looking species have been made by looking at the differences in wing vein patterns (indeed, some beekeepers use the wing structure to differentiate the European honey bee, Apis mellifera, from the Asian honey bee, Apis cerana)

How the Wings Cause Flight

For honey bees (and flying insects in general), a single wingbeat comprises of three cycles (from Gullan & Cranston “The Insects”, 2nd edition):

1. A downward forward motion followed by an upward, backward motion.
2. A rotation (partial or complete) of the wing around its base.
3. Various flexion across the wing in response to the local air pressure.

The flying insect makes constant adjustments during wing beats as the air ahead is thrown backward and downwards, causing the insect to travel upwards and forwards. To go higher, they direct more air downwards to increase lift. To turn mid-air, the wings on the inside of the turn are reduced in power by reducing the wingstroke, rotating the insect in the desired direction. This movement of air around the bee and through its wings generates vibrations, which we hear as a “buzzing” sounds. Now you can answer the question “Why do bees buzz?”.

Also present in the Hymenoptera family are two pairs of wings that are coupled to work together. Bee wings are mechanically linked via “hamuli” on the base of the hindwing that latch onto the forewing during flight, as well as a forewing posterior rolled margin, “coupling” them to work as a single aerodynamic unit in place of “two pairs working independently”. Both structures are made up of a hardened regions of their exoskeleton (“sclerotized cuticle”), and this hinge-like connection allows a large angle variation between the wings (135°–235°) providing increased lift and drag during flight (Ma et al., 2019).

The Honey Bee Nervous System

Within the thorax, the thoracic ganglia (a cluster of nerve cells) serve as the main center for controlling movement and are connected to the brain by the ventral nerve cord. The ventral nerve cord refers to the chain of thoracic and abdominal on the floor of the body cavity (hence, “ventral”). The ganglia sends signals to the muscles, controlling the coordinated movement of the legs and wings. The central nervous system runs from the bee head to its abdomen, and is divided by a series of ganglia connected by nerve cords.

The honey bee brain is a fusion of three ganglia in the head:

• Protocerebrum, associated with the eyes and vision centers.
• Deutocerebrum, associated with the antennae.
• Tritocerebrum, associated with sensing internal stimulus – eg from basic bee body functions.

The insect body has a sympathetic nervous system (visceral nervous system) which provides innervation to the gut, hormonal organs (the corpora allata makes Juvenile Hormone in honey bees which is important in larval growth; Bomtorn et al., 2014), reproductive organs and the tracheal systems, including spiracles. The thoracic ganglia belongs to the peripheral nervous system, and helps bees coordinate their flight.

Muscles of the Honey Bee Thorax

Adapted from Gullan & Cranston “The Insects”, 2nd edition

As with most insects, bees only have striated muscles (and no smooth muscles, unlike mammals), divided into two groups: those that contract asynchronously, with multiple contractions per impulse (flight muscles) and those that contract synchronously, with one contraction cycle (contract-relax) per nerve impulse (most other muscles).

The direct flight muscles are connected to the wings and produce the upward stroke by contracting the muscles attached to the wing base, whereas the downward stroke is by contracting the muscles extending from the insect sternum to the wing base.

The indirect flight muscles are not connected to the bee’s wings, and instead deform their thorax, producing structural changes that move wings indirectly. This causes the main part of the wing to move in an upward stroke, and assists the direct muscles in setting flight (it is harder to fly upwards than downwards).

Muscles for Walking

Bees contract and simultaneously relax the opposing leg’s muscles to provide a forward motion. For example, a worker bee may contract her left front leg and relax the right front, then contract the right middle leg while relaxing the left one, and contract the left hind leg and relax the rear. Make sense? No? Diagram below.

The moving muscles are located close together in the thorax, and enable continuous contact with the ground by providing a “tripod” stability structure. In the diagram below, the legs highlighted black provide the forward stability (left side front and hind legs, right side mid-legs), whilst the grey thrust backward. The aforementioned claws and arolium provide stability on the walking surface.

To move faster, the frequency of the rearward retraction period is decreased (relaxation period; legs highlighted grey). This makes the tripod structure less stable and at times, no legs may be in contact with the surface, making the bee “hover” without the use of its wings.

How Insect Larvae Move

Honey bee larvae do not move in the “walking” sense. They stay in their cells until the adult bee emerges and begins moving. However, the larvae of other insects like small hive beetles and wax moths move by crawling. This movement is triggered by contracting the muscles in one part of their body, which relaxes in another part, producing movement via its hydrostatic skeleton (a structure that utilizes water to make up an internal body fluid system). These waves of contraction run from head to tail, detaching the legs from the current surface and moving it forward onto the next one, producing motion.

In legless larvae, bands of adhesive hooks grip and detach from the surface to propel it forward in lieu of legs.

Oxygen and Gas Exchange in Honey Bees

The thorax houses spiracles, which are hollow tubes connected to a network of tracheae and tracheoles, delivering oxygen directly to the tissues. Both the thorax and abdomen contain spiracles, but their primary functions differ:

• In the thorax, they support the high metabolic demands of flight and deliver oxygen to the flight muscles (Suarez 2000). There are three pairs of spiracles in the thorax.
• In the abdomen, they support the metabolic needs of the honey bee digestive system (including honey stomach), reproduction, and waste elimination. There are seven pairs of spiracles in the abdomen.

The internal air-filled tracheae run throughout the insect body and are higher in tissues that require more oxygen, i.e. the two pairs of wings. As air enters from from the spiracles it passes into the target tissue via a combination of ventilation and diffusion across the concentration gradient (from a high concentration in the external air, to a low concentration within the target issue). As oxygen goes in, carbon dioxide and water vapor goes out, completing the respiration cycle. The system can be thought as a compromise between oxygen intake and reducing water vapor; spiracles are kept closed during periods of inactivity, and only open periodically (Kovac et al., 2007).

How do Larvae Breathe?

Honey bee (and most insect) larvae do not have air sacs, so they breathe through diffusion of gases in the trachea. Oxygen moves in from the air into the trachea and carbon dioxide movies out (Rozen 2017). When the trachea is enlarged, it can store more oxygen and act as an internal reserve when spiracles are closed.

Honey Bee Circulatory System

Honey bees have an open circulatory system; muscular pumps move haemolymph (bee blood) around the insect body. Haemolymph enters through openings in the diaphragm and first moves into the dorsal vessel of the abdomen (“heart”). From there, it is pumped towards another dorsal vessel near the head (“aorta”). The circulation cycle first supplies haemolymph to the head and thorax, and then the abdomen. A series of contractions coordinates the activities between the various pumps and vessels to ensure efficient haemolymph delivery.

The Thorax’s Role in Honey Bee Anatomy

The thorax is the center for locomotion, allowing the bee to fly, walk, and perform tasks related to foraging and communication. Its structures work in tandem with the sensory organs in the head and the many organs of the abdomen, enabling the bee to complete its daily activities.

Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.